20-Week Anomaly Scan: Complete Guide
The 20-week anomaly scan (sometimes called the mid-pregnancy or "fetal anomaly" scan) is the most detailed ultrasound you'll have in a standard NHS pregnancy. It's offered to every pregnant woman in the UK between 18 and 21 weeks, and its job is to check your baby is developing as expected and to look closely at the structure of major organs. For most people it's a reassuring milestone, though it's worth knowing what the scan actually involves so you feel prepared.
What happens during the scan
The sonographer works through a detailed checklist, methodically measuring and photographing each part of your baby:
- The brain and spine — checking the shape of the skull, the structure of the brain, and that the spine is fully closed along its length
- The heart — looking at its four chambers, the major blood vessels, and how it's beating
- The abdomen, kidneys and bladder — checking these organs are present, correctly positioned, and functioning normally
- The face — including the lips, to screen for cleft lip
- Limbs — counting fingers and toes and checking the arms and legs are in proportion
- The placenta — its position is noted, particularly whether it's low-lying, which would prompt a follow-up scan later in pregnancy
- Amniotic fluid levels — too much or too little can sometimes indicate something worth monitoring
The scan usually takes 20 to 45 minutes, sometimes longer if your baby is in an awkward position or the sonographer needs extra views. You'll need a reasonably full bladder for a good picture, and most units allow one support person to come with you.
What the results mean
Most anomaly scans are completely reassuring, and the sonographer will let you know at the time whether everything looks as expected. If something needs a closer look, it doesn't automatically mean there's a serious problem — it's common to be asked back for a repeat scan a few weeks later simply because the baby was in a difficult position, or because a measurement needs rechecking. If a genuine concern is identified, you'll usually be referred to a fetal medicine specialist for further assessment and a clear explanation of what it means for your pregnancy.
Finding out the baby's sex
Many units will tell you the baby's sex at the anomaly scan if you ask, though this isn't guaranteed — some hospitals don't offer it, and accuracy depends on the baby's position at the time. If knowing (or not knowing) matters to you, it's worth telling the sonographer clearly at the start of the appointment.
Frequently Asked Questions
What is the anomaly scan actually checking for?
It's a detailed structural check of your baby's brain, spine, heart, abdomen, kidneys, limbs and face, along with the position of the placenta and the amount of amniotic fluid. The aim is to confirm normal development and to pick up any structural differences early enough to plan appropriate care. Most scans find nothing of concern.
Is the 20-week scan compulsory?
No, it's offered as a standard part of NHS antenatal care but you can decline it if you prefer. Most people choose to have it because of the reassurance it provides and because early detection of any issues allows time to plan care or, where relevant, further testing. Speak to your midwife if you're unsure and want to talk through the pros and cons.
What happens if they find something unexpected?
You'll be told what's been seen and, in most cases, offered a follow-up scan or a referral to a fetal medicine specialist for a more detailed assessment. Many "soft markers" picked up at this stage turn out to be normal variations rather than a genuine problem. Your care team will talk you through what any finding actually means and what happens next before you leave the appointment where possible.
This article is for informational purposes only and does not constitute medical advice.
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