Guidance

Screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome

Updated 16 December 2024

Applies to England

For naming conventions used throughout this guidance and other general information, please see the handbook overview.

1. The combined test

The combined test assesses the chance of the baby having trisomy 21 (T21) (see section 4.1 in the handbook overview), trisomy 18 (T18) (see section 4.2 in the handbook overview) or trisomy 13 (T13) (see section 4.3 in the handbook overview) by using:

• maternal age (see section 4.1 below)
• biochemical markers – free beta human chorionic gonadotropin (bhCG) and pregnancy associated plasma protein-A (PAPP-A) (see section 4.2 below)
• ultrasound measurements – nuchal translucency (NT) and crown rump length (CRL) (see section 4.3 below)

The NT measurement must be used in combination with a maternal blood sample for calculating the chance result. It must not be used alone.

Assessment techniques and biometric charts used for ultrasound measurements must meet nationally agreed standards, as outlined in the British Medical Ultrasound Society (BMUS) guidelines ‘Fetal size and dating: charts recommended for clinical obstetric practice’ (2009) and ‘A practical solution to combining dating and screening for Down’s syndrome’ (2011) (both available at BMUS: fetal measurements).

The eligibility criteria for the combined test is when the CRL is between 45.0mm and 84.0mm.

If the CRL is less than 45.0mm, the woman is recalled for a further ultrasound scan to measure the NT and remeasure the CRL.

If the CRL is greater than 84.0mm, the gestational age is calculated using the head circumference (HC). The quadruple test (see section 2) is offered if the criteria is met.

If it is not possible to measure the NT, at least one more attempt is offered (‘twice on the couch’). This may be on the same day or at a later date according to local guidelines. The CRL and NT measurement must be taken on the same day.

If it is not possible to measure the NT despite ‘twice on the couch’, further attempts do not need to be offered. The woman must be offered the quadruple test.

The combined test gives the woman more time to consider screening options. In practice, there are 2 ways the combined test can be performed and this determines how the woman receives the chance result. The maternal blood sample can be:

• taken at the time of the ultrasound scan
• taken before the ultrasound scan (from 10 weeks onwards)

If the sample is taken at the time of the ultrasound scan and sent to the screening laboratory, the chance result report from the combined test is usually available within a few days of the result being authorised by the laboratory.

If the sample is taken immediately before the ultrasound scan (from 10 weeks onwards), the screening laboratory makes the biochemistry results available to the ultrasound department; the chance result is calculated in the ultrasound/maternity department.

The laboratory is responsible for the chance calculation software and the chance result.

Ultrasound/maternity departments must have a process in place to share ultrasound measurements and final screening results with the laboratory. It is recommended that the laboratory take responsibility for auditing all chance results.

Chance results can only be recalculated in exceptional circumstances. For example, incorrect data on the screening request form that affects the chance calculation.

Every woman who accepts a pregnancy dating scan needs to be informed that unexpected findings may be identified. This must include any woman who declines screening for T21 and/or T18 and T13.

The NHS FASP does not provide guidance regarding the clinical care of a woman who has declined the combined test and has an unexpected finding on the ultrasound scan. This may include an NT of greater than or equal to 3.5mm. Local guidance for the care and management of these women should be in place.

The Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway requirements specification outlines failsafe systems providers must have in place. For example, follow-up of attendance at appointments.

2. The quadruple test

The quadruple test screens for T21 only.

The quadruple test is offered when the:

• NT measurement cannot be obtained
• CRL measurement is greater than 84.0mm and the HC measurement is between 101.0mm and 172.0mm

The quadruple test uses maternal age and the following 4 biochemical markers measured from 14⁺² to 20⁺⁰ weeks:

• alpha-fetoprotein (AFP)
• human chorionic gonadotropin (hCG) or free bhCG
• inhibin-A
• unconjugated oestriol (uE3)

This combination of markers has a lower detection rate (DR) and a higher screen positive rate (SPR) than the combined test. For a woman presenting in the second trimester, an ultrasound scan is required to measure the HC to date the pregnancy and complete the quadruple test.

The quadruple test is performed when the HC is between 101.0mm and 172.0mm.

If the blood sample is taken on the same day as the ultrasound scan the HC must be between 101.0mm and 172.0mm.

If the HC is measured on a different day to the blood sample, the gestational age when the blood sample is taken should be between 14⁺² and 20⁺⁰ weeks.

If the HC is equal to 172.0mm the blood sample must be taken on the same day as the ultrasound scan.

If the HC is more than 172.0mm, the quadruple test must not be offered. The woman is offered the 20-week screening scan.

If the HC is less than 101.0mm and the CRL is greater than 84.0mm, there are 2 options.

2.1 Option 1

Use the HC measurement to calculate the dates that the woman will be between 14⁺² and 20⁺⁰ weeks. Offer an appointment during this time for a blood sample to be taken for the quadruple test.

2.2 Option 2

Use the HC measurement to calculate the date that the woman will be over 14⁺² weeks. Rebook the woman for a further ultrasound scan where the HC can be remeasured and a blood sample taken for the quadruple test.

The Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway requirements specification outlines failsafe systems providers must have in place. For example, follow-up of attendance at appointments.

2.3 More information

More information is available regarding practical solutions to combining dating and screening requirements at the ultrasound scan for the combined or quadruple test in the BMUS ‘A practical solution to combining dating and screening for Down’s syndrome’ (2011) guidelines (available at BMUS: fetal measurements).

The NHS FASP does not provide guidance regarding the clinical care of a woman who has declined the quadruple test and has an unexpected finding on the pregnancy dating scan. Local guidance for the care and management of these women should be in place.

3. Non-invasive prenatal testing (NIPT)

NIPT screening for T21, T18 and T13 is offered following a higher chance result (between 1 in 2 and 1 in 150) from either the NHS combined or the quadruple test in both singleton and twin pregnancies.

The UK National Screening Committee (UK NSC) recommended introducing NIPT as an evaluative rollout. This is so any necessary changes to the screening pathway can be made.

4. Markers used in the combined and quadruple tests

There are a number of markers that are used to calculate the chance result.

4.1 Maternal age

All pregnant women have a chance of having a baby with T21, T18 or T13. This chance increases with age. The older the woman, the greater the chance the baby will have one of these conditions.

The graph below shows the chance of a woman having a baby with T21, T18 or T13 (y axis) according to maternal age (x axis).

The graph shows that:

• for the same maternal age the chance of T21 is higher than that of T18, and the chance of T18 is higher than that of T13
• the chance of a woman having a baby with one of the conditions increases as maternal age increases
• the chance does not increase much between 15 to 30 years of age, but after 30 years of age the chance increases more rapidly with age

The table below shows the chance of a woman having a baby with T21 at 16 weeks of pregnancy.

Maternal age (years)	Chance of woman having a baby with T21	Probability of woman having a baby with T21 (%)
20	1 in 1,500	0.07
30	1 in 900	0.1
40	1 in 100	1.0

4.2 Biochemical markers

Screening laboratories must use the recommended combination of biochemical markers for both the combined and quadruple tests.

Biochemical markers are analytes in the maternal blood that are measured by the laboratory and used to calculate the chance of a woman having a baby with T21, T18 or T13. The levels of these markers are affected by certain factors such as gestational age, ethnicity, smoking, maternal weight and maternal diabetes. To adjust these effects the levels are standardised and expressed as multiples of median (MoM) values. It is important that information about these factors is clearly written on the screening request form so the laboratories can make the correct adjustments.

The following analytes are measured by the laboratory to calculate the chance result.

Alpha fetoprotein (AFP)

A low level of AFP in the maternal blood in the second trimester is associated with T21.

Levels of AFP are low in maternal blood in the first trimester and increase until about 32 weeks.

Human chorionic gonadatropin (hCG) or free beta hCG

An increased level of hCG and/or the free beta subunit in the maternal blood in the first and second trimester is associated with T21. A decreased level is associated with T18 and T13.

Levels of hCG increase with time after conception, reaching a peak at about 9 to 12 weeks. It decreases and evens out at about 20 weeks.

Inhibin-A

An increased level of inhibin-A in the maternal blood in the second trimester is associated with T21.

Levels of inhibin-A increase during the first trimester. Levels decrease to even out in the second trimester, before increasing again in the third trimester.

Unconjugated oestriol (uE3)

A decreased level of uE3 in the maternal blood in the second trimester is associated with T21.

Levels of uE3 in the maternal blood increase during pregnancy, reaching a peak at term.

Pregnancy associated plasma protein-A (PAPP-A)

A decreased level of PAPP-A in the maternal blood in the first trimester is associated with T21, T18 and T13.

Levels of PAPP-A increase quickly during early pregnancy and continue to increase slowly until term.

Effect of vaginal bleeding on biochemical markers used in screening for T21, T18 and T13

Current evidence suggests that the biochemical marker levels are not substantially changed in a woman with a history of vaginal bleeding.

The NHS FASP recommends eligible women are offered the NHS combined or quadruple test as usual.

Effect of smoking on biochemical markers used in screening for T21, T18 and T13

Smoking affects biochemical marker levels, particularly PAPP-A and inhibin. Current evidence suggests there is a moderately large reduction in PAPP-A levels, and a large increase in inhibin levels, in women who are classed as smokers during pregnancy.

The NHS FASP recommends a minimum data field of ‘smoker (yes or no)’.

Women should be classified as smokers (smoker = yes) if they smoke tobacco (for example, cigarettes) in the 2 weeks before the blood sample is taken.

This includes women who smoke tobacco at the same time as using nicotine replacement therapy (NRT) and/or vaping products.

Women should be classified as non-smokers (smoker = no) if they:

• do not smoke tobacco
• stopped smoking tobacco 2 weeks or more before the blood sample is taken
• only use nicotine replacement therapy (NRT) and/or vaping products
• use NRT and stopped smoking tobacco 2 weeks or more before the blood sample is taken
• use vaping products and stopped smoking tobacco 2 weeks or more before the blood sample is taken

NRT includes nicotine patches, gum and inhalers. Vaping products include e-cigarettes and refill containers known as e-liquids.

Laboratories that can correct for NRT should do so.

4.3 Ultrasound measurements

Gestational age is calculated from ultrasound measurements of either the CRL or the HC.

The measurements for NT, CRL and HC recorded on the screening request form must be the same as the stored ultrasound images.

For purposes of consistency, all ultrasound measurements should be reported:

• in mm to one decimal place (for example, a CRL measurement of 61.0mm should not be reported as 61mm or 61.00mm; it must be reported as 61.0mm)
• as measured and not rounded to a whole number (for example, a CRL measurement of 45.6mm should not be rounded up to 46.0mm or rounded down to 45.0mm; it must be reported as 45.6mm)
• accurately even if it is less than 1.0mm (for example, an NT measurement of 0.8mm is valid and should be reported as this)

Nuchal translucency (NT)

The NT is the ultrasound appearance of a collection of fluid under the skin at the back of the baby’s neck. The thickness of the NT is measured and used as part of the combined test to calculate the chance of having a baby with T21, T18 or T13.

An NT measurement equal to or greater than 3.5mm may be associated with T21, T18 and T13 and serious cardiac conditions.

For a woman who accepts the offer of the combined test, an NT measurement equal to or greater than 3.5mm is usually associated with a higher chance result. In these cases, the blood sample must be taken, processed and the chance result calculated in the usual way. Referral, in line with local guidelines, must be made straight away and not delayed until the chance results are known. As soon as the chance results are available, they should be forwarded to the clinician to support discussion with the woman.

For a woman who declines the offer of the combined test, local guidelines should be in place for the clinical management of NT measurements equal to or greater than 3.5mm and any unexpected findings on the ultrasound scan.

It is important to note a baby with an increased NT may not have any of these conditions.

Crown rump length (CRL)

The CRL is the ultrasound measurement from the top of the head (crown) to the bottom of the buttocks (rump). The gestational age is calculated from this measurement, as outlined in the BMUS ‘Fetal size and dating: charts recommended for clinical obstetric practice’ (2009) guidelines available at BMUS: fetal measurements.

The combined test can only be performed when the CRL measurement is between 45.0mm and 84.0mm. The CRL measurement must be used rather than a gestational age of weeks and days.

The CRL measurement is essential for the analysis of the biochemical markers in the combined test. It may also be used in the quadruple test depending on gestational age.

Head circumference (HC)

The HC is the ultrasound measurement of the circumference of the baby’s head. The gestational age is calculated from this measurement when the CRL is greater than 84.0mm, as outlined in the BMUS ‘Fetal size and dating: charts recommended for clinical obstetric practice’ (2009) guidelines available at BMUS: fetal measurements.

The eligibility criteria for the quadruple test is when the HC measurement is between 101.0mm and 172.0mm.

5. Laboratory information for the combined and quadruple tests

All NHS FASP screening laboratories must:

• be accredited by the UK Accreditation Service (UKAS) according to International Organization for Standardization (ISO) Medical laboratories – requirements for quality and competence (ISO 15189)
• meet the screening programme quality assurance (QA) requirements mapped to ISO 15189

The laboratory must include an annual vertical audit of an antenatal screening sample in its laboratory audit schedule.

The audit should randomly select a higher chance T21, T18 or T13 sample. In laboratories providing both combined and quadruple testing it is acceptable for the annual audit to relate to either. Wherever possible this should alternate between the 2 tests.

The audit should include arrival and receipt of an antenatal screening sample at the laboratory and acknowledgment of higher chance results by clinical services.

UKAS looks at ISO 15189 and the screening requirements on behalf of NHS England Screening QA services and the NHS FASP. All NHS FASP screening laboratories must:

• participate in external quality assurance (EQA) schemes accredited to ISO’s Conformity assessment – General requirements for proficiency testing schemes (ISO 17043)
• use ISO 15189 accredited reference laboratories

The laboratory must have a viable contingency plan to continue the provision of screening in the event of any failures to the laboratory service.

5.1 Chance calculation software

Screening laboratories in England must use software that meet the requirements of the NHS FASP chance calculation software specification.

This sets out all aspects that need to be included in a software package to give consistent chance results.

Some variables that are entered into the software are defined by the local user to take account of the reagents used for screening and the characteristics of the local population. These are decided by the laboratory with support from DQASS.

The software used to calculate the chance result from the biochemical and ultrasound markers is complex. This is best provided and supported by commercial suppliers.

Laboratories must use chance calculation software that is (CE) marked. It must comply with EU directives and utilise software upgrades.

5.2 Data fields for the screening laboratory request form

Laboratory request forms must include the required data fields outlined in the Request form and data fields required when screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome. The data fields highlighted in bold text must be provided to DQASS. However, all data items specified are helpful to DQASS.

The NHS FASP recommends all the required data fields are included when current request forms are revised. Software and data entry methods may vary.

5.3 Chance result report contents

Only results and comments relating to the conditions screened for as part of the Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway must be included on the chance result report.

This report should include all relevant information used for calculating the chance result. This includes:

• dates and gestational age when:
  • maternal blood sample taken
  • ultrasound scan performed
  • chance result report issued
• all relevant measurements taken with units and multiple of the median (MoM) values; depending on the test, this will include:
  • biochemical markers
  • NT
  • CRL
  • HC
• maternal demographics:
  • date of birth
  • weight
  • smoking history
  • ethnicity
  • in-vitro fertilisation (IVF) status
  • diabetes status
  • previous pregnancy with T21, T18 or T13 (when appropriate)
  • singleton/twin pregnancy (chorionicity if known)
• T21 prior chance and final chance result
• T18 and T13 joint prior chance and final chance result
• final chance result reported using lower and upper limits (1 in 5,000 and 1 in 2 respectively)
• classification of the final chance result into the lower or higher chance category with the chance cut-off used, for example 1 in 150 at term

Test standard FASP-S05 outlines test turnaround times for T21, T18 and T13 screening.

Information on reporting chance results in twin pregnancies (see section 7) is available.

Lower chance result letter templates are available.

5.4 Blood sample draw, transport and storage

Blood samples taken for T21, T18 and T13 screening must be collected in a plain or gel serum separating tube (SST).

A screening blood sample must always be taken first if a full blood count sample is being taken at the same time. It is important that the correct order of draw is followed.

Tubes containing ethylene-diamine-tetra-acetic acid (EDTA) must be avoided. This is due to significant interference of EDTA in the immunoassays used to measure the biochemical markers for screening. Blood samples must be processed as soon as possible.

Free beta hCG can increase in concentration over time due to the dissociation of the intact hCG molecule. This effect is temperature dependent. The rate of deterioration of the sample increases with a rise in temperature. Serum samples are stable up to 72 hours at room temperature.

All samples transported or stored at room temperature for longer than 72 hours must be rejected.

The stability of the samples is significantly improved by refrigeration at 4 degrees Celsius, with whole blood and serum samples stable for several days. High temperatures (over 30 degrees Celsius) should be avoided as deterioration of free beta hCG occurs within 4 hours of sample collection.

Most samples are stored and transported at room temperature. During periods of increased temperatures, providers should consider both storing and transporting samples in cool bags to reduce any effect on the markers. Whole blood samples must not be frozen or transported on dry ice as this makes the sample unsuitable for analysis.

Exceptions

If a gel separator tube is not used, it is good practice for the sample to be centrifuged and separated from the clot within 24 hours of collection.

If this is not possible, samples may remain as whole blood at room temperature for up to 48 hours. Samples received as whole blood after 48 hours from sample collection must be rejected.

Screening sample stability acceptance criteria

Whole blood (unseparated) samples can be stored at:

• room temperature (20 degrees Celsius) for 48 hours
• 4 degrees Celsius for 5 days

Serum (separated) samples can be stored at:

• room temperature (20 degrees Celsius) for 72 hours
• 4 degrees Celsius for 14 days

Test standard FASP-S06 measures the proportion of inadequate samples received for T21, T18 and T13 screening.

5.5 Analysers and kits

There are several suppliers in the UK providing assays and analysers that are CE marked for use in T21, T18 and T13 screening.

The NHS FASP does not have a preferred supplier and laboratories must decide which analyser best meets their purpose.

Some assays are available on random access analysers which overcome the need to batch samples. All assays should be verified and used according to the manufacturer’s instructions.

Any deviations must be fully validated in compliance with Medical laboratories – Requirements for quality and competence (ISO 15189).

Analysers must be maintained according to the manufacturer’s instructions and a comprehensive maintenance record kept, preferably in an electronic format.

Standard operating procedures that would pass a UKAS assessment must be in place in the laboratory. These should be for each assay and all procedures relating to the screening work of the laboratory.

5.6 Laboratory throughput

The NHS FASP requires a standalone screening laboratory to have a workload of at least 8,000 samples per screening test per year.

A high throughput, particularly where technology requires batching of samples, means that laboratories will be able to meet recommended turnaround times for results. Extended turnaround times can delay diagnosis and ongoing management options.

All laboratories with a workload of fewer than 8,000 samples per year for a specific screening test must be part of a network screening service for that test.

Laboratory network screening service

The total number of samples from all laboratories within a network screening service must be a minimum of 8,000 per screening test per year. Each laboratory in a network must have a minimum throughput of 2,000 samples per screening test per year.

Having a laboratory network screening service helps to improve the quality of the screening programme. The principle of a network allows laboratories with a smaller throughput to monitor performance while working in a consistent way to a common set of standards. It can develop capacity, technical expertise and proficiency in a safe and efficient way.

A laboratory network screening service must:

• be directed by a person with executive accountability and competence as assessed by UKAS
• have a clear governance structure in place that integrates with the organisation’s governance and quality structures
• have a service agreement in place between laboratories and commissioners

Each laboratory in a network screening service must have identical:

• screening policies
• analytical procedures, such as analysers and kit lots
• chance calculation algorithms and population parameters (this does not include factor adjustments) in their chance calculation software

The laboratory network screening service lead receives the combined 6 monthly DQASS detailed laboratory report (see section on ‘Report types’).

DQASS assessment of laboratory performance

An annual throughput of 8,000 samples is needed to:

• achieve effectiveness and statistical precision in the DQASS assessment of laboratory performance
• enable DQASS to give additional focused support where required
• detect and respond to biases in MoM values in a timely manner
• enable SPR to be estimated reliably

If laboratory throughput is small, DQASS cannot measure performance reliably.

5.7 Internal quality control (IQC)

Laboratories must have internal quality control (IQC) procedures in addition to the external quality assurance service provided by DQASS. These should be performed frequently enough to enable timely changes.

IQC procedures must be rigorous. They must be able to identify:

• problems with the assays on a day-to-day basis
• problems such as a gradual drift of results with time
• changes that may occur when a new batch of reagents is used

Manufacturer recommendations for IQC must be strictly adhered to as a minimum requirement. Additional controls should be included, especially if an assay is particularly difficult.

It is good practice to include validated IQC material from another manufacturer. Laboratories should not rely solely on the controls provided by the manufacturer of the assay kit.

Long-term trends in IQC data should be analysed to detect assay drift.

It is important to compare sample and IQC results when a new batch of reagents is introduced. Significant shifts in population medians were observed on occasions with the introduction of a new reagent batch and after maintenance of an analyser. This can impact on the chance results reported if it is not anticipated and accounted for.

Vigilance is required at all times because chance calculation involves several biochemical markers. The measurement of each marker must be as accurate as possible. Small inaccuracies in these measurements can lead to a greater overall inaccuracy of the final chance result.

6. Ultrasound information for the combined and quadruple tests

6.1 Ultrasound image archiving

Every provider should save ultrasound scan reports and images onto an electronic reporting and archiving system. This system should be able to provide minimum audit data.

All imaging studies should also have an electronic report linked to the images.

6.2 Professional competence

All ultrasound practitioners must hold minimum certification as specified in the section on education and training. This information is also available in the:

• Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway requirements specification
• 20-week screening scan pathway requirements specification

6.3 Ultrasound machines

All ultrasound practitioners should be aware of the Royal College of Radiologists (RCR) and Society and College of Radiographers (SCoR) Standards for the provision of an ultrasound service.

All ultrasound practitioners should adhere to the BMUS Guidelines for the safe use of diagnostic ultrasound equipment which recommends time limits for obstetric scans.

Ultrasound machines should be capable of producing images of diagnostic quality and include the following features:

• adequate display/screen size for clear visualisation
• magnification facility
• cineloop function
• callipers set to measuring to one decimal point (for example, 0.4mm)
• pre and post processing facilities
• presets for individual clinical applications
• a range of suitable transducers
• Doppler and harmonic imaging facilities

All new ultrasound machines should be correctly set up with the manufacturer’s application specialist to make sure images can be optimised to allow accurate measurements.

6.4 Criteria for measurement of NT for the purposes of the combined test

Midline section

The criteria for midline section are:

• horizontal sagittal section extending from the crown to upper aspect of the heart which may be supine or prone
• section extending from crown to upper aspect of the heart which may be supine or prone
• head in line with the body with the NT visible along the length of the neck
• echogenic tip of the nose
• rectangular shape of the palate
• translucent diencephalon
• frontal process of the maxilla should not be visible

Position

The criteria for position are:

• pocket of fluid, at least equivalent in size to the width of the palate, visible between the chin and chest
• palate angle between 30 and 60 degrees to the horizontal
• nasal tip level with, or above, the anterior chest wall

Magnification

The criteria for magnification is the section fills over 60% of the screen.

Calliper placement

The criteria for calliper placement are:

• callipers placed on the upper and lower edges of the NT lines
• widest part of the NT measured with callipers perpendicular to the NT lines

Image archiving

The criteria for image archiving are:

• NT should be measured 3 times if possible
• maximum measurement that meets the criteria should be reported
• archive the image demonstrating the reported NT

If the NT line appears disrupted or distorted, check for nuchal cord. Guidance on how to measure the nuchal cord can be found in the “Measuring the Nuchal Translucency (NT) for the combined test” unit of the First trimester screening resource for ultrasound practitioners.

The NHS FASP does not recommend screening for nasal bone absence or hypoplasia, thus allowing measurement of the NT in the prone position.

6.5 Criteria for measurement of CRL for purposes of pregnancy dating and the combined and quadruple tests

Midline section

The criteria for midline section are:

• sagittal section with the head in line with the full length of the body
• echogenic tip of the nose
• rectangular shape of the palate
• translucent diencephalon
• CRL axis between 0 and 30 degrees to the horizontal
• clearly defined crown and rump

Position

The criteria for position are:

• pocket of fluid, at least equivalent in size to the width of the palate, visible between the chin and chest
• palate angle between 30 and 60 degrees to the horizontal
• nasal tip level or above the anterior abdominal wall

Magnification

The criteria for magnification is the CRL section fills over 60% of the screen.

Calliper placement

The criteria for calliper placement are:

• correct calliper placement on outer borders of crown and rump
• longest CRL section measured

Image archiving

The criteria for image archiving are:

• CRL should be measured 3 times if possible
• maximum measurement that meets the criteria should be reported
• archive the image demonstrating the reported CRL

7. Screening in twin pregnancies

NHS FASP screening for T21, T18 and T13 is only available for singleton and twin pregnancies.

Guidance is produced by National Institute for Health and Care Excellence (NICE) on higher multiple pregnancies (triplets and more).

More than one baby in a pregnancy complicates screening.

Each feto-placental unit contributes to the levels of the biochemical markers used in the chance calculation. Each baby has an individual NT measurement.

7.1 Combined test in twin pregnancies

The test of choice for twin pregnancies is the combined test. Every effort must be made to offer and complete this test. The larger of the 2 CRL measurements should be used in the chance result calculation. Depending on a woman’s personal choice, chance results (see section 9) are reported as:

• a term chance of T21 and a joint term chance of T18 and T13
• a term chance of T21 only
• a joint term chance of T18 and T13 only

In a monochorionic twin pregnancy, the chance result is the same for each baby and one ‘pregnancy’ chance result is reported.

In a dichorionic twin pregnancy, the chance result is reported for each baby.

7.2 Quadruple test in twin pregnancies

The quadruple test is offered to a woman with a twin pregnancy to screen for T21 only when one or both of the:

• NT measurements cannot be obtained
• CRL measurements are greater than 84.0mm

The larger of the 2 HC measurements should be used in the chance result calculation.

The quadruple test in twin pregnancies is not as sensitive as the combined test and the decision-making process can be more difficult for a number of reasons. Women considering the quadruple test should have a discussion with a healthcare professional with a special interest, experience and knowledge of managing multiple pregnancies. This is to help support personal informed choice (see section 3 ‘Personal informed choice’ in the handbook overview.

Monochorionic twins

The performance of the quadruple test in monochorionic twins is comparable to that in singleton pregnancies.

Dichorionic twins

In dichorionic twins where one baby has the condition and the other does not, the performance of the quadruple test is not as sensitive as it is in monochorionic twins.

Quadruple test chance results relate to the pregnancy not to individual babies. They are not interpreted in the usual way but used, with a cut-off of 1 in 150 at term, to define a higher chance group.

An appropriately trained healthcare professional should interpret and explain these results and pregnancy options due to the complexities involved.

7.3 Prenatal diagnosis in twin pregnancies

Chorionic villus sampling (CVS) and amniocentesis sampling options are available.

The risk of miscarriage is twice as high in twin pregnancies compared to singleton pregnancies.

Prenatal diagnosis (PND) is performed at a tertiary-level fetal medicine unit. This is due to the specialised nature of the procedure and the increased risk of miscarriage. This is in line with the Royal College of Obstetrics and Gynaecology (RCOG) and NICE guidelines.

7.4 Vanished twin

For the purposes of screening, the definition of a vanished twin is when one fetus in a twin pregnancy is non-viable at the dating scan.

The ultrasound scan may show either:

• an empty second pregnancy sac
• a second pregnancy sac containing a non-viable fetus

In both scenarios, the biochemical markers can be affected and therefore the NHS FASP combined and quadruple screening tests cannot be offered.

When a vanished twin pregnancy is identified, local guidelines for the measuring of the CRL and NT should be followed and the woman referred to a fetal medicine service to discuss local testing options as part of a clinical management pathway.

Other testing options are being introduced but time frames for implementing these will vary depending on the software used in the screening laboratory. The fetal medicine service should discuss available options with the screening laboratory.

The testing options available as part of a clinical management pathway when fully implemented are:

• maternal age and NT
• a modified combined test (2 options available):
  1. Maternal age, NT and free beta hCG (excluding PAPP-A)
  2. Maternal age, NT, free beta hCG and adjusted PAPP-A
• a modified quadruple test (2 options available):
  1. Maternal age, hCG (beta), inhibin-A and uE3 (excluding AFP)
  2. Maternal age, hCG (beta), inhibin-A, uE3 and truncated AFP

The laboratory request form must clearly indicate when there is a vanished twin. If a modified combined test with adjusted PAPP-A is offered, the request form must include the CRL of the non-viable fetus if present.

Time of blood sampling when using a modified combined test with adjusted PAPP-A

When using a modified combined test with adjusted PAPP-A, it is not possible to use a maternal blood sample taken before the ultrasound scan.

In these cases, the maternal blood sample must be performed on the same day as the ultrasound scan that identifies the vanished twin pregnancy.

Reporting requirements

Chance result reports for vanished twin pregnancies must include one of the following statements:

• ‘This is a vanished twin pregnancy, and the chance result was calculated using a modified combined screening test as part of a clinical management pathway.’
• ‘This is a vanished twin pregnancy, and the chance result was calculated using a modified quadruple screening test as part of a clinical management pathway.’

Detection rates (DR)

The tables below outline modelled DR per testing strategy for vanished twin pregnancies.

The modelled performance for the combined test and quadruple test in singleton pregnancies are included for comparison.

Combined test

Testing strategy	Expected DR (%) for T21	Expected DR (%) for T18	Expected DR (%) for T13	False positive rate (%)
Maternal age and NT	77	74	69	2.7
Modified combined test excluding PAPP-A	82	75	61	2.7
Modified combined test adjusting PAPP-A	87	81	72	2.3
Current NHS FASP combined test in singleton pregnancies	87	81	72	2.3

Quadruple test

Testing strategy	Expected DR (%) for T21	False positive rate (%)
Modified quadruple test excluding AFP	74	4.2
Modified quadruple test truncating AFP	82	5.3
Current NHS FASP quadruple test in singleton pregnancies	80	3.8

Reference: First-trimester screening for trisomies in pregnancies with vanishing twin - PubMed (nih.gov)

8. Screening in in-vitro fertilisation (IVF) pregnancies

The eligibility criteria for the combined test in IVF pregnancies is when the CRL is between 45.0mm and 84.0mm. The CRL measurement must be used for the purposes of screening.

The quadruple test is also performed in IVF pregnancies.

Maternal age in IVF pregnancies using donor eggs is calculated from the date of birth or age of the donor at the time of egg collection. This is required for calculation of prior chance.

If the date of birth or age of the donor at the time of egg collection is unknown, every effort must be made to find this information for the chance result calculation. This includes IVF treatment received outside England.

If this information cannot be determined, laboratories can use the upper age limit of egg donation in England which is under 36 years (that is; 35 years and 364 days). When this approach is used, it must be clearly written on the chance result report. We recommend using this cut-off due to limited information on egg donation outside England. In these cases, the healthcare professional should discuss the limitations of this approach and the impact on the chance result.

Calculating the estimated due date (EDD) of an IVF pregnancy is not part of the NHS FASP. Local guidelines should be in place to date these pregnancies.

9. Screening results

Chance results from combined or quadruple tests must only be recalculated in exceptional circumstances. For example, incorrect data on the screening request form.

All higher chance results and/or significant findings on ultrasound scan must be reported and followed up in line with the screening standards.

Referral standard FASP-S07 outlines communication timeframes for higher chance results. Following a higher chance result from the combined or quadruple test, women must have a discussion with a healthcare professional about their results. Women can choose to have:

• no further testing
• NIPT screening
• prenatal diagnosis (PND)

If the woman chooses to have no further testing, routine antenatal care will continue.

If the woman chooses to have PND, the options are either chorionic villus sampling (CVS) or amniocentesis.

The PND procedure should be made available to the woman before or by 3 working days from receipt of higher chance combined or quadruple test results.

Lower chance result letter templates are available.

9.1 Chance cut-off

Chance cut-off determines whether a woman has a lower chance or higher chance result.

The chance cut-off is 1 in 150 at term for both the combined and quadruple tests, as defined by the NHS FASP.

A lower chance result is less than or equal to 1 in 151.

A higher chance result is greater than or equal to 1 in 150 (between 1 in 2 and 1 in 150).

A woman with a higher chance result is offered a PND test, such as CVS or amniocentesis.

The cut-off is based on a chance at term, rather than a chance at the time of the screening test. The chance at the time of screening is not known because there is significant pregnancy loss between the time of screening and birth. This loss rate is not known.

9.2 Combined test chance results

Depending on the woman’s screening choice, the following chance results are reported:

• a term chance for T21 and a joint term chance for T18 and T13
• a term chance for T21 only
• a joint term chance for T18 and T13 only

9.3 Quadruple test chance results

For a woman screened using the quadruple test a single term chance of T21 is reported.