The Anomaly Scan — What the 20 Week Scan Checks
What is examined, the conditions it screens for, what it honestly cannot detect, and where private scanning does and does not have a role.
The anomaly scan — usually called the 20-week scan — is the detailed examination of your baby’s anatomy offered to everyone on the NHS, normally performed between 18 and 21 weeks. Unlike earlier scans, which confirm viability and dates, this one works systematically through the developing structures: brain, face, spine, heart, abdomen, kidneys, limbs and the placenta. It takes around 30 minutes, sometimes longer if the baby is lying awkwardly.
The 20-week scan is part of the NHS Fetal Anomaly Screening Programme, performed by sonographers working to a defined national protocol with a referral pathway into fetal medicine when something is found. That pathway is the point of the scan, and your NHS appointment provides all of it free. We explain it here so you know what to expect; we do not replicate it, and we would not want you to substitute a private scan for it.
What is actually checked
The sonographer follows a set sequence, taking measurements and images of each region in turn:
- Head and brain — skull shape and bone integrity, brain structures and fluid spaces
- Face — principally the upper lip, to look for a cleft
- Spine — examined along its length and across, checking the skin covers it
- Heart — chambers, valves and the major vessels leaving it
- Abdomen — stomach, abdominal wall, bowel
- Kidneys and bladder — presence, appearance and drainage
- Limbs — arms, legs and the long bones (hands and feet are seen but fingers and toes are not counted)
- Placenta, cord and fluid — where the placenta lies, the cord vessels, and the volume of amniotic fluid
Growth measurements are taken at the same time — head circumference, abdominal circumference and thigh bone length — establishing a baseline against which later growth can be compared.
The eleven conditions
The NHS screening programme specifically looks for eleven rare conditions. Naming them is useful, because it clarifies what the scan is designed to do rather than what people assume it does:
| Condition | Condition |
|---|---|
| Anencephaly | Open spina bifida |
| Cleft lip | Diaphragmatic hernia |
| Gastroschisis | Exomphalos |
| Serious cardiac anomalies | Bilateral renal agenesis |
| Lethal skeletal dysplasia | Edwards’ syndrome (T18) |
| Patau’s syndrome (T13) |
Detection rates differ substantially between them. Some, such as anencephaly, are identified almost every time; others, particularly certain heart conditions, are considerably harder to see and are missed more often. The scan is a screening test, not a guarantee — and where something is suspected, you are referred to a fetal medicine specialist for a more detailed assessment rather than given a diagnosis on the spot.
What the anomaly scan cannot tell you
It cannot detect everything. Many conditions have no visible structural sign at 20 weeks; some develop or become apparent only later in pregnancy; and a number are simply beyond the resolution of ultrasound. A normal anomaly scan is genuinely reassuring, and it is not a guarantee of a baby without health conditions.
It does not assess chromosomes. Some chromosomal conditions produce structural signs the scan may pick up, but counting chromosomes is the job of NIPT and diagnostic testing, informed by the earlier combined screening. The two parts of the pathway ask different questions.
Image quality varies, and not because anyone did anything wrong. The baby’s position is the commonest obstacle — a baby lying face-down or curled tightly can hide the structures that need seeing. Scanning through more abdominal tissue also reduces detail. If views are incomplete, you will be asked to return for another attempt, which is routine and not a sign that something has been found.
It is optional. Like all antenatal screening, it is offered rather than required, and declining will not affect the rest of your care.
Where private scanning has a genuine role
Not in repeating the anomaly scan — but there is a real role afterwards. From around 24 weeks, growth scans track how your baby is growing against the baseline the 20-week scan established, measuring size, amniotic fluid and blood flow in the cord. These are the scans people book when growth has been queried, when a previous pregnancy had growth concerns, or when the gap between NHS appointments feels long.
What we will not do is offer a private “second opinion” anomaly scan to someone whose NHS scan raised a concern. If something has been flagged, the correct route is the fetal medicine referral your NHS team arranges — a specialist service with equipment, expertise and follow-through that a general imaging clinic cannot match. Sending you sideways at that moment would waste time you may need.
Your questions answered
When is the anomaly scan done?
What does the 20 week scan check?
Can the anomaly scan detect everything?
Will they tell me the sex of my baby?
Why do I have to come back for another scan?
Can I have a private anomaly scan instead?
What happens if something is found?
Across the pregnancy pathway
First-trimester combined screening, and why screening is not diagnosis.
Read the guide →
£149Growth ScansFrom 24 weeks — tracking growth against the 20-week baseline.
About the scan →
Blood TestUnderstanding NIPTWhat chromosomal screening does that no scan can.
Read the guide →
Growth scanning after your 20-week scan
From 24 weeks we measure growth, fluid and cord blood flow against your baseline — findings explained on screen, with a written report usually within two hours for your midwife or GP.
Kensington, London W8 4ED
3 mins from Notting Hill Gate
Screening scope, timing and the conditions listed follow the NHS Fetal Anomaly Screening Programme; general scanning pathways per NHS guidance on pregnancy ultrasound scans. This article is general information, not individual medical advice — your screening results should be discussed with your NHS midwife or screening team.
Written and clinically reviewed by the HCPC-registered sonographers who perform this scan at IUS London — a CQC-registered diagnostic ultrasound clinic (Provider ID 1-2775844974). Your own findings are explained to you at the scan and set out in your report.
Author: Yianni Kiromitis, Senior Sonographer, HCPC RA38415 — over 20 years’ experience in NHS and private ultrasound
Medically reviewed: 1 August 2026
