Your pregnancy and baby guide

Anomaly scan at 18-21 weeks pregnant

This detailed ultrasound scan, sometimes called the mid-pregnancy or 20-week scan, is usually carried out when you are between 18 and 21 weeks pregnant. In some areas it may be carried out later than 21 weeks. The scan checks for major physical abnormalities in your baby, although it can't pick up every problem.

The anomaly scan is carried out in the same way as the dating scan, with gel on your tummy and the sonographer passing the ultrasound device backwards and forwards over your skin.

Picture of an anomaly scan Credit:

Chad Ehlers / Alamy Stock Photo

You will be offered a scan that produces a two-dimensional (2-D) black and white image that gives only a side view of the baby. The NHS screening programme does not use 3-D or colour images.

The anomaly scan is offered to everybody, but you do not have to have it if you do not want to.

The scan is a medical examination. You will be asked to give your permission for it to be carried out. Make sure you understand what is going to happen, and feel free to ask any questions.

What is the scan for?

The mid-pregnancy anomaly scan looks for some physical abnormalities in the baby. The scan only looks for these problems and cannot find everything that might be wrong.

What kind of problems does the scan look for?

The scan looks in detail at the baby's bones, heart, brain, spinal cord, face, kidneys and abdomen. It allows the sonographer to look specifically for 11 conditions, some of which are very rare.

For an information leaflet on each of these conditions, including treatment options, click on the links below:

In most cases, the scan will show that the baby appears to be developing normally, but sometimes the sonographer will find or suspect a problem.

Some problems can be seen more clearly than others. For example, some babies have a problem called open spina bifida, which affects the spinal cord.

Spina bifida can usually be seen clearly on a scan, and will be detected in around 9 out of 10 babies who have this problem. Some other problems, such as heart defects, are more difficult to see. The scan will find about half of those babies who have heart defects.

Some of the problems that can be seen on the scan, such as cleft lip, will mean the baby may need treatment or extra surgery after they are born.

In a small number of cases, some very serious problems are found – for example, the baby's brain, kidneys, internal organs or bones may not have developed properly. In some very serious, rare cases where no treatment is possible, the baby will die soon after it is born or may die during pregnancy.

What happens at the scan?

Most scans are carried out by specially trained staff called sonographers. The scan is carried out in a dimly lit room so the sonographer can get good images of the baby.

You will be asked to lie on a couch and to lower your skirt or trousers to your hips and lift your top to your chest so your abdomen is uncovered. The sonographer or their assistant will tuck tissue paper around your clothing to protect it from the gel, which will be put on your tummy.

The sonographer then passes a handheld probe over your skin to examine the baby's body. The gel makes sure there is good contact between the probe and your skin. A black and white image of the baby will appear on the ultrasound screen.

Having the scan does not hurt, but the sonographer may need to apply slight pressure to get the best views of the baby. This might be uncomfortable. The sonographer needs to keep the screen in a position that gives them a good view of the baby. The screen may be directly facing them, or at an angle.

Sometimes the sonographer doing the scan will need to be quiet while they concentrate on checking your baby. However, they will be able to talk to you about the pictures once they've completed the check.

The appointment for the mid-pregnancy scan usually takes around 30 minutes. Sometimes it is difficult to get a good picture if the baby is lying in an awkward position, if they are moving around a lot, or if you are above average weight. This does not mean there is anything to worry about.

You may need to have a full bladder when you come for the appointment. The doctor or midwife looking after you will let you know before you come. If you are not sure, you can contact them and ask.

Can my partner or a friend come to the scan with me?

Yes. The mid-pregnancy scan can sometimes find problems with the baby. You may like someone to come with you to the scan appointment.

Most hospitals do not allow children to attend scans as childcare is not usually available. Please ask your hospital about this before your appointment.

Can the scan harm me or my baby?

There are no known risks to the baby or you from having an ultrasound scan, but it is important to think carefully about whether to have the scan or not.

The scan may provide information that may mean you have to make some important decisions. For example, you may be offered further tests that have a risk of miscarriage, and you will need to decide whether or not to have these tests.

Do I have to have this scan?

No, you do not have to have the scan – it is your choice whether to have it or not. Some people want to find out if their baby has problems and some do not. If you choose not to have the scan, your antenatal care will continue as normal.

When will I get the results of the scan?

The sonographer will be able to tell you the results of the scan at the time.

What if the scan shows a possible problem?

Most scans show that the baby seems to be developing normally and no problems are found.

If any problem is found or suspected, the sonographer may ask for another member of staff to look at the scan and give a second opinion.

Scans cannot find all problems, and there is always a chance that a baby may be born with a health problem that scans could not have seen.

Will I need any further tests?

If the scan shows that there might be a problem, you may be offered another test to find out for certain. If you are offered further tests, you will be given more information about the tests so you can decide whether or not you want to have them.

You will be able to discuss this with your midwife or consultant. If necessary, you will be referred to a specialist, possibly in another hospital.

Find out more about what may happen if a screening test shows a possible problem.

Page last reviewed: 25/02/2015
Next review due: 30/11/2017