The 20-Week Anomaly Scan: What It Checks and What Happens if Something Is Found

The mid-pregnancy scan is a detailed check of how your baby is developing, from the brain and heart to the spine and limbs. Most scans are reassuring, but it helps to know beforehand what the scan is looking for and what happens if something is found.

Imagine looking forward to seeing your baby again and maybe finding out the sex. The sonographer goes quiet as they look at the heart for longer than you expected. Knowing in advance how the scan works can make moments like this less frightening.

When it happens

The anomaly scan, also called the mid-pregnancy or fetal anomaly scan, is usually offered between 18 and 21 weeks of pregnancy. At this stage, the baby is big enough for many structures to be seen clearly, but early enough for further tests and planning if needed.

What the scan checks

• The brain and skull
• The face, including checking for a cleft lip
• The spine, including checking for spina bifida
• The heart, including its chambers, valves and main blood vessels
• The abdominal wall, stomach and kidneys
• The arms, legs, hands and feet
• The baby’s growth, the placenta position and the amount of fluid around the baby

What the scan cannot do

The anomaly scan does not find every condition. Some heart problems, for example, are hard to see, and some conditions only develop later in pregnancy. A normal scan is reassuring but cannot guarantee that the baby has no health problems.

How to prepare

• Check whether you need a full bladder; usually you do not at this stage
• Wear loose, comfortable clothing
• Ask whether you can bring a support person, and whether children are allowed
• Think in advance about whether you want to know the sex, if your hospital offers this

What happens during the scan

The scan is done through your tummy using gel and a hand-held probe, like the dating scan. It usually takes 20 to 40 minutes. Sometimes the baby is lying in a position that makes it hard to see everything, and you may be asked to walk around, empty your bladder or come back another day.,The sonographer may be quiet while concentrating. This is normal and does not necessarily mean there is a problem. You can ask them to explain what they are looking at.

The placenta

The scan checks where the placenta is lying. If it is low, close to or covering the cervix, you will usually be offered another scan later in pregnancy, at around 32 weeks, to see if it has moved up. Most low-lying placentas move away from the cervix as the womb grows.

If something is found

If the sonographer sees a possible problem, they will usually ask a senior colleague to look. You may be referred to a fetal medicine specialist, usually within a few days, for a more detailed scan.,Some findings are minor and resolve on their own. Others may need monitoring, planning for treatment after birth, or further tests such as amniocentesis. You will be given information and support to make decisions that are right for you.

Finding out the sex

In many countries, you can ask to find out the sex at this scan, if the baby is in a position where it can be seen. It is not always possible, and it is not 100% accurate. Some hospitals do not share the sex as a matter of policy. Tell the sonographer at the start whether you want to know.

Common findings that are usually harmless

Some findings on the anomaly scan sound worrying but are usually harmless. These include a “soft marker” such as a bright spot in the heart (echogenic focus), a slightly enlarged kidney collecting system or a choroid plexus cyst in the brain. On their own, these are common and usually have no effect on the baby.

The sonographer will explain whether any finding needs follow-up. If you are given a term you do not understand, ask them to write it down so you can discuss it with your midwife or healthcare provider.

Conditions the scan aims to find

Screening programmes list specific conditions the scan looks for, such as spina bifida, anencephaly, cleft lip, serious heart conditions, abdominal wall defects, kidney problems and some limb abnormalities. Detection rates differ between conditions; some, like anencephaly, are almost always detected, while others, like some heart conditions, are found in around half of cases.

Planning ahead if a condition is found

Finding a condition before birth can help your team plan the safest delivery, arrange for specialists to be present, and prepare treatment for after your baby is born. For example, some heart conditions need surgery soon after birth, and knowing in advance means you can deliver in a hospital with the right specialist team.

Questions to ask at the scan

• Were you able to see everything you needed to?
• Is the placenta in a normal position?
• Is the baby growing as expected?
• Do I need any further scans?

Frequently asked questions

Will I get pictures?

Many hospitals offer printed pictures, sometimes for a small fee.

Is the scan safe?

Yes. Ultrasound has been used safely in pregnancy for decades.

Can I refuse the scan?

Yes. All screening in pregnancy is your choice.

But what if I am anxious the whole time?

That is very common. Tell the sonographer; they can talk you through what they are checking. Bringing someone with you can also help.

Related: The 12-week dating scan

Sources: NHS Fetal Anomaly Screening Programme, World Health Organization, Royal College of Obstetricians and Gynaecologists, American College of Obstetricians and Gynecologists.