What the 20-Week Anomaly Scan Actually Checks

The appointment sits in your calendar for weeks before you actually get there. The 20-week scan, the anomaly scan, the anatomy scan: it goes by several names, and all of them carry a kind of gravity that your earlier scans didn't quite have.

When you finally walk in, the room is dark and quiet. The sonographer starts moving the probe, pausing often, typing things you can't see. Minutes go by. Nobody says much. Your baby is moving on the screen, which is reassuring, but you have no idea what exactly is being looked for, or whether the silence means something is hard to find.

The anomaly scan is the most detailed scan of pregnancy. Most people know it's happening around week 20, and most people know it's important, but surprisingly few are told in advance what the sonographer is actually checking. This is that explanation.

Why it's called the anomaly scan

The name comes from the purpose: it's screening for structural anomalies in your baby's development. "Anomaly" sounds alarming, but the scan is just a thorough look at your baby's anatomy to confirm everything has developed as expected.

It is not a test you pass or fail. It is a detailed look, performed by a trained sonographer, that picks up a range of conditions. Some of those conditions are serious. Many findings that come up in this scan turn out to be minor or resolve on their own. And most scans come back with nothing unexpected at all.

The scan usually happens between weeks 18 and 22. Week 20 is common, but your provider may schedule it a little earlier or later depending on your situation.

What the sonographer is checking

The list is long, and the sonographer works through it methodically. Most of the time, you won't be told what each measurement is in the moment. Here is what's on that list.

The brain. The sonographer checks the shape of the skull, the structures within the brain including the ventricles, and the cerebellum at the back. They're looking for normal development and the absence of features associated with neural tube conditions.

The spine. They check along the length of the spine for any gaps in the vertebrae, which can indicate spina bifida. The baby needs to be in the right position for this, which is one reason the scan can take a while.

The face. They check that the profile looks as expected and look specifically at the lip to screen for cleft lip. They also check the nose, jaw, and eye sockets.

The heart. The cardiac check is one of the most detailed parts of the scan. The sonographer looks at all four chambers, checks that they're the right size and in the right position, and looks at the major vessels coming in and out. Congenital heart conditions are the most common structural anomaly found at 20 weeks.

The stomach. A dark bubble on the screen shows that your baby has swallowed some amniotic fluid, which confirms the stomach is present and the swallowing reflex is working.

The kidneys and bladder. They check that both kidneys are present and look the right size, and that the bladder is filling and emptying. Small amounts of fluid collecting in the kidneys are common and usually resolve without intervention.

The limbs. All four limbs are checked: the long bones of the arms and legs, and the hands and feet. The sonographer is looking at bone length as well as structure.

The placenta. Position matters here. A placenta that's very low at 20 weeks may be covering the cervix, a condition called placenta praevia, which can affect how birth happens. Many placentas that look low at this stage move up as the uterus grows, which is why a follow-up scan is often recommended rather than immediate concern.

The cord insertion. They check where the umbilical cord attaches to the placenta.

Amniotic fluid. The level of fluid around the baby is measured. Too little or too much can be a sign worth investigating.

Measurements. Head circumference, abdominal circumference, and femur length are all measured. These help confirm how your baby is growing relative to the stage of pregnancy.

The sonographer is working through a long checklist. The silence is concentration, not concern.

What "soft markers" means

Sometimes the sonographer will note a finding called a soft marker. This is where the language gets confusing, and it's worth understanding what it actually means.

Soft markers are features that appear in some normal pregnancies but are also seen slightly more often in pregnancies with chromosomal conditions like Down's syndrome. Common examples include:

• A bright white spot on the heart, called an echogenic intracardiac focus
• Small fluid collections in the kidney
• A choroid plexus cyst in the brain
• A mildly short femur

The key word is "slightly." Any individual soft marker, in the absence of other findings, is most often found in a baby with completely normal chromosomes. The marker by itself does not mean something is wrong.

What a soft marker does is give your doctor more information for calculating risk. If you have already had first-trimester screening (the combined test, or NIPT), those results are part of the full picture. Your care team will look at everything together, not just the single finding.

If soft markers are mentioned, ask what the finding is specifically, whether it's isolated, and what the recommended next step is. In most cases it will be a review with your consultant and possibly a follow-up scan.

When they ask you to come back

Sometimes the sonographer doesn't manage to see everything they need to see. This is common, not alarming.

It usually means your baby was in the wrong position. The spine view requires a particular angle. The heart requires another. A baby who decides to curl up or turn away mid-scan can make some of the views impossible, no matter how patient everyone is.

A recall for a second look means the scan was incomplete, not that something was found. You'll be asked to come back, walk around, have something to eat and drink, and try again. The vast majority of incomplete scans are resolved at the repeat appointment.

If something is found

If the sonographer does see something that needs further discussion, you will likely be asked to wait to speak with a doctor or consultant rather than going straight home. This can feel frightening, but it is the standard process.

What happens next depends entirely on what was found. Some findings are monitored with follow-up scans and turn out to be insignificant. Others are referred to a fetal medicine specialist for a more detailed assessment.

What you are entitled to in any of those conversations: a clear explanation of what was seen, what it might mean, what the options are, and time to ask questions. You do not have to take everything in at once. You can ask for a nurse or midwife to stay with you. You can ask for a repeat conversation once you've had time to think.

What the anomaly scan doesn't check

The scan is thorough but not complete. A normal anomaly scan cannot rule out every possible condition.

It screens for many but not all chromosomal conditions. It does not replace the first-trimester combined screening or NIPT. It cannot detect some genetic conditions, some subtle heart conditions, or changes that develop after the scan is done.

A normal result is genuinely reassuring. It is also not a guarantee, and understanding that distinction matters. This is not a reason to fear the scan. It is a reason to understand what it actually provides.

Before you go in

A few things that make the experience easier:

• Drink water beforehand. A comfortably full bladder helps with some of the early views.
• Plan for up to an hour. The scan itself takes around 30 to 45 minutes on average, sometimes longer.
• Ask in advance whether your partner or support person can attend.
• It is completely reasonable to ask the sonographer to explain what they're looking at as they go. Some are naturally more talkative than others, but you are allowed to ask.

After the scan

For most people, the scan ends with a straightforward conversation, a set of measurements, and a printout of a blurry profile photo. The information can take a little time to land. You spent 40 minutes watching something carefully without always knowing what you were looking at.

By around 20 weeks, you're also starting to feel your baby move more consistently. If you want to understand those patterns and what's normal week by week, How Much Should Your Baby Move? goes through that in detail.

Having somewhere to put the questions

The questions that don't get asked in the room often surface on the way home, or at 11pm. Something the sonographer said, a term you didn't recognise, a result you're not sure how to read.

Seri Bloom is in your WhatsApp through pregnancy and into postpartum. You can share what you heard, describe a finding you're trying to understand, or just work out what questions to bring to your next appointment. She won't replace your fetal medicine consultant or your midwife, and she'll say clearly when you need one. But for the hours when the appointment is over and the thinking is still happening, it helps to have somewhere steady to put it.

Questions that come up a lot

What does the anomaly scan check?

The 20-week anomaly scan checks your baby's brain, spine, face, heart, stomach, kidneys, bladder, limbs, and umbilical cord insertion. It also checks the position of the placenta and the level of amniotic fluid around the baby, and takes measurements of your baby's head circumference, abdominal circumference, and femur. It is the most detailed scan of pregnancy.

How long does the anomaly scan take?

The scan usually takes 30 to 45 minutes, though it can run longer if the baby is in an awkward position. Some parents finish closer to 20 minutes; others take longer. Plan to be at the clinic for at least an hour in total to allow time for waiting and any follow-up conversation.

What happens if a soft marker is found at the 20-week scan?

A soft marker is a feature that appears in some normal pregnancies but is also associated slightly more often with certain chromosomal conditions. Finding a single soft marker is common and, on its own, does not mean something is wrong. Your doctor will look at the full picture, including any earlier screening results, and may recommend a follow-up scan or a review with a fetal medicine consultant.

What does it mean if they can't see everything at the anomaly scan?

If the sonographer can't complete all the views, it usually means your baby was in a position that blocked the necessary angles. You'll be asked to come back for a repeat appointment. A recall for an incomplete scan is not a finding. It is a practical problem caused by a baby who moved at the wrong moment.

Can the anomaly scan detect Down's syndrome?

The anomaly scan is not the primary screening test for Down's syndrome. That screening happens earlier, through the combined first-trimester test or through NIPT. The anomaly scan may note soft markers that are more common in pregnancies with chromosomal conditions, and those findings will be discussed alongside your earlier screening results. If you are uncertain what tests you've had or what your results mean, ask your care team to walk through the full picture with you.

Will I find out the sex at the 20-week scan?

In many hospitals and clinics, the sonographer will offer to tell you the sex during the anomaly scan, if you'd like to know. In some settings, gender disclosure is not standard practice. Ask in advance so you know what to expect, and let your sonographer know at the start of the appointment if this matters to you either way.

Seri Bloom shares general guidance and is not a substitute for your doctor, midwife, or care team. If you have specific concerns about your scan results, please speak with your care provider or a fetal medicine specialist.