The 20-Week Anomaly Scan: What to Expect

The 20-week scan is one of the most detailed check-ups your baby will have before birth. It's also the appointment where many parents find out their baby's sex. Here's everything you need to know, so you can walk in feeling prepared and leave feeling reassured.

What is the anomaly scan?

The anomaly scan (sometimes called the mid-pregnancy scan or 20-week scan) is a routine NHS ultrasound offered to all pregnant women in England between 18 and 21 weeks. It's a detailed check of your baby's physical development - their organs, bones, brain, heart, and overall growth.

It's called an "anomaly" scan because its primary purpose is to check for structural differences. But the vast majority of scans show a healthy, thriving baby. This is worth remembering if you're feeling nervous beforehand.

What does the sonographer check?

The scan is thorough. Your sonographer will systematically check:

Your baby's head and brain - They'll measure the head circumference and check the brain's structure, including the ventricles and cerebellum.

Face - They'll look at the lips to check for cleft lip (cleft palate is harder to detect on ultrasound).

Spine - Each vertebra is checked to ensure the spine has closed properly.

Heart - The four chambers of the heart are examined, along with the major blood vessels. Heart defects are among the most common structural differences, and while the scan picks up many of them, some are difficult to see at this stage.

Stomach and abdomen - They'll check that the stomach is in the right position and the abdominal wall is intact.

Kidneys - Both kidneys should be visible and a normal size.

Arms, legs, hands, and feet - Your baby's limbs are measured and checked.

Placenta position - If your placenta is lying low, it may be noted for a follow-up scan later (most low-lying placentas move up as the uterus grows).

Amniotic fluid - The amount of fluid around your baby is assessed.

The NHS screens for 11 specific conditions at this scan, with detection rates varying by condition. Some things are easier to spot than others, and no scan can guarantee that everything is perfect - but it provides an incredibly detailed picture of your baby's development.

How long does it take?

Typically 20-30 minutes, though it can take longer if your baby isn't in an ideal position. Some parents are asked to go for a short walk and come back so baby can shift around. This is completely normal and nothing to worry about.

Can you find out the sex?

At most NHS trusts, yes - but you need to ask. Some hospitals have a policy of not revealing sex, so check with your maternity unit beforehand. If you do want to know, the sonographer will look during the scan and tell you if baby is in a position where they can see clearly.

At 18-20 weeks, sex determination by ultrasound is around 99% accurate. But if baby's legs are crossed or they're facing away, it might not be possible to tell - and the sonographer won't guess.

If you'd prefer not to know, tell the sonographer at the start so they can avoid accidentally revealing it.

Practical tips for the day

Wear two pieces. You'll need to expose your belly, so a top and trousers/skirt is easier than a dress.

You may be asked to have a full bladder. Check your appointment letter - some trusts request this for a clearer image, especially if your placenta is at the front.

Bring your partner or a support person. Most trusts allow one person to join you. Check the current policy when you book.

Write down your questions. It's easy to forget what you wanted to ask once you're in the room. Common questions include: Is baby measuring on track? Where is the placenta? Is the amount of fluid normal?

Expect it to feel clinical. Unlike the dating scan, which often feels celebratory, the anomaly scan is more methodical. The sonographer needs to concentrate, so there may be long silences while they take measurements. This is normal - silence doesn't mean something is wrong.

What happens if something is found?

If the sonographer notices anything unusual, they may ask a colleague or consultant to take a second look. In some cases, you'll be referred for a more detailed scan at a fetal medicine unit. This can feel frightening, but a referral doesn't automatically mean there's a problem - it means they want a closer look to be thorough.

If a condition is confirmed, your care team will explain what it means, what the next steps are, and what support is available. You'll have time to ask questions and make decisions at your own pace.

After the scan

For most parents, the anomaly scan is a reassuring milestone. You'll likely leave with a clearer picture of your baby's development, an updated growth measurement, and possibly the knowledge of whether you're expecting a boy or a girl.

Some parents also leave with a printed scan photo or a short video clip - check with your trust whether this is offered and whether there's a small charge.

You're halfway through your pregnancy. That's worth celebrating.