Guidance

20-week screening scan

Updated 8 October 2024

Applies to England

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

1. The 20-week screening scan

The NHS FASP recommends the offer of an ultrasound scan which is performed between 18⁺⁰ to 20⁺⁶ weeks to screen for 11 physical conditions (see section 4.4 of the handbook overview). The screening pathway must be completed by 23⁺⁰ weeks.

The purpose of the 20-week screening scan is to identify specified conditions that:

• benefit from treatment before or after birth
• need treatment in a specialist setting after birth to improve health outcomes
• could mean the baby may die shortly after birth
• lead to a discussion about the options of continuing or terminating the pregnancy

The scan should be performed in accordance with the NHS FASP base menu (see section 2) and fetal cardiac protocol.

The base menu shows the anatomical structures of the baby to be assessed. Unexpected development of these structures can indicate one of the 11 physical conditions screened for. Other conditions can also be detected but these are not part of the NHS FASP. This is because the data is insufficient to confidently predict the standard for detection that should be achieved.

All significant findings of the scan are reported. Any woman who does not wish to be informed of the 11 physical conditions or other unexpected findings should consider not having the scan.

Ultrasound scans contribute to both screening and the clinical management of pregnancy. The examination of placental position and amniotic fluid is not part of the NHS FASP but is good clinical practice.

The NHS FASP does not require fetal sex to be determined. There is no programme requirement to recall the woman if the sex of the baby is not identified.

Test standard FASP-S04 outlines test performance for specified conditions.

1.1 Conditions identified

Conditions identified that are part of the NHS FASP

If one of the 11 physical conditions screened for is suspected or identified, providers must make referrals to a local or tertiary-level centre as clinically appropriate. Referrals must be timely and in line with referral standard FASP-S08.

Conditions identified that are not part of the NHS FASP

If a condition that is not one of the 11 physical conditions screened for is suspected or identified, there should be a local guideline in place to make sure providers make timely referrals to a local or tertiary-level centre as clinically appropriate. This allows the woman to receive timely intervention and care. Ongoing audit is needed to monitor the outcome of these pregnancies in line with National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS) requirements.

1.2 Completing the 20-week screening scan

A single repeat ultrasound scan must be offered and completed by 23⁺⁰ weeks in cases where the image quality of the first scan is compromised by any of the following:

• maternal body mass index (BMI)
• uterine fibroids
• abdominal scarring
• baby or babies in a sub-optimal position

The woman can be rescanned on the same day and if the scan is still incomplete, a new appointment should be offered according to local guidelines

If image quality is sub-optimal and an unexpected structural development is suspected, a second opinion should be sought. Referral should be made for further investigation without delay. In these cases, there is no requirement to offer a repeat second appointment to complete screening. This should be recorded in the report.

Sometimes a woman attends for a repeat ultrasound scan and the image quality remains compromised. In these cases, there is no requirement to offer a further scan to complete screening. The woman should be informed that screening is incomplete and this must be recorded.

The 20-week screening scan can be performed and completed by 23⁺⁰ weeks when the provider has agreed a pathway with public health commissioners and screening quality assurance service (SQAS). This agreed pathway must facilitate referrals for further investigations and options for pregnancy choices in a timely manner. The screening pathway must still be completed by 23⁺⁰ weeks.

Ongoing audit of practice must be in place to monitor timeliness of the screening pathway. Coverage standard FASP-S02 outlines the completion timeframe for the 20-week screening scan.

The 20-week screening scan pathway requirements specification outlines failsafe systems providers must have in place. For example, follow-up of attendance at appointments.

Specific information is available on:

• ultrasound image archiving (see section 6.1 of Screening for Down’s syndrome, Edward’s syndrome and Patau’s syndrome)
• professional competence (see section 6.2 of ‘Screening for Down’s syndrome, Edward’s syndrome and Patau’s syndrome’)
• ultrasound machines (see section 6.3 of ‘Screening for Down’s syndrome, Edward’s syndrome and Patau’s syndrome’)

2. 20-week screening scan base menu

The 20-week screening scan base menu outlines the minimum anatomical structures to be assessed. Where appropriate these structures should be assessed in sagittal, coronal and transverse planes.

Images of 6 specific anatomical sections should be archived. These are:

• head circumference (HC) measurement and the atrium of the lateral ventricle
• suboccipitobregmatic view demonstrating measurement of the transcerebellar diameter
• coronal view of lips with nasal tip
• abdominal circumference (AC) measurement
• femur length (FL) measurement
• sagittal (preferred) or coronal view of spine including sacrum

Examples of these images are included below, along with corresponding schematic diagrams.

The HC, AC and FL measurements should be taken to assess growth velocity. The British Medical Ultrasound Society (BMUS) Fetal size and dating: charts recommended for clinical obstetric practice (2009) guidelines provide guidance on how to date a pregnancy where the estimated due date (EDD) was not previously assigned.

2.1 Base menu ultrasound images with schematics to be archived

Image 1: head circumference (HC) measurement and the atrium of the lateral ventricle

Image 2: suboccipitobregmatic view demonstrating measurement of the transcerebellar diameter

Image 3: coronal view of lips with nasal tip

Image 4: abdominal circumference (AC) measurement

Image 5: femur length (FL) measurement

Image 6a: sagittal view of spine including sacrum

Image 6b: coronal view of upper spine

Image 6c: coronal view of lower spine including sacrum

2.2 Fetal cardiac protocol

The views to be assessed are:

• situs/laterality
• 4 chamber view (4CV): transverse section of the thorax including one complete rib and the crux of the heart
• aorta/left ventricular outflow tract (LVOT) – shows the outflow tract of the left ventricle
• pulmonary/right ventricular outflow tract (RVOT) – shows the outflow tract of the right ventricle only, or 3 vessel view (3VV): shows the outflow tract of the right ventricle including the pulmonary artery
• 3 vessel and trachea view (3VT): shows the main pulmonary artery in direct communication with the ductus arteriosus, the transverse aortic arch and the superior vena cava

Example images of these views are included below, along with corresponding schematic diagrams.

2.3 Fetal cardiac protocol ultrasound views with schematics

There is no NHS FASP requirement to archive images of the fetal cardiac protocol views.

Image 7: situs/laterality

Image 8: 4 chamber view (4CV)

Image 9: aorta/left ventricular outflow tract (LVOT)

Image 10: pulmonary artery/right ventricular outflow tract (RVOT) or 3 vessel view (3VV)

Image 11: 3 vessel and trachea view (3VT)

2.4 Normal variants

T21, T18 and T13 chance results must not be recalculated following the 20-week screening scan.

If one or more of the normal variants listed below are seen, the woman does not need referral for further assessment as part of the NHS FASP:

• choroid plexus cysts
• dilated cisterna magna
• echogenic foci in the heart
• 2 vessel cord

However, the scan findings listed below need to be reported and the woman referred for further assessment:

• nuchal fold (greater than or equal to 6.0mm)
• ventriculomegaly (atrium greater than or equal to 10.0mm)
• echogenic bowel (with density equivalent to bone)
• renal pelvic dilatation antero-posterior (AP) measurement (greater than 7.0mm)
• small measurements of HC, AC and/or FL (below the 5th centile on fetal biometry charts)

Although not part of NHS FASP, some scan findings can be associated with an increased risk of fetal growth restriction (FGR). The Royal College of Obstetricians and Gynaecologists (RCOG) guidance and the NHS England Saving Babies’ Lives Care Bundle (SBLCB) require additional assessment for FGR. This includes uterine artery Doppler assessment and an individualised plan of care. The scan findings that require referral for additional assessment for FGR are:

• echogenic bowel
• estimated fetal weight below the 10th centile
• single umbilical artery (2 vessel cord)

If these findings are seen during the scan, then locally agreed pathways for FGR should be followed.

3. 20-week screening scan results

All unexpected findings of the 20-week screening scan are discussed with the woman on the day.

The results must be reported and all actions recorded. Referrals should be made to appropriate services as per local guidelines.

Referral standard FASP-S08 outlines referral timeframes for the 20-week screening scan.