The Anomaly Scan

Unless you have any complications with your pregnancy, or with a previous pregnancy, you will probably only be asked to attend two ultrasound scans. The first is at around 12 weeks and is known as the dating and nuchal translucency scan, the second is usually at between 18-20 weeks and is called the anomaly scan.

What is the anomaly scan for?

First of all, remember that this is a routine scan. Being asked to attend it is not an indication that the doctor or midwife believes that there is a problem with your pregnancy as all pregnant women are asked to attend a scan at this time. The purpose of the scan is to check that the baby is developing as it should be and to ensure that its limbs and organs are all growing properly. The sonographer (the person conducting the scan) will also be able to check that there is the right amount of amniotic fluid surrounding the baby and that the placenta is in the right location. You should view this scan as one of your baby's first routine health checks.

What problems can the anomaly scan detect?

Through taking the baby's measurements (head circumference, abdominal circumference and femur bone length) the sonographer will be able to confirm whether your baby is growing as expected for its gestation. Growth problems are relatively easy to detect.

As the sonographer will look at a cross section of the head (as well as all the other organs) they will be able to tell if there are any severe brain defects during the scan, but these are very rare.

The scan will also show whether the baby has a cleft lip, although it is possible that a cleft palate could go undetected as these are much harder to diagnose via an ultrasound scan.

They will assess the development of the skin and bones, particularly the spine. The condition known as spina bifida is likely to be picked up at this scan if your baby has it. The sonographer will check that all the major bones align correctly and that the skin has grown on the outside of all the bones. The scan will be able to show whether the abdominal wall is covering all the internal organs at the front of the baby's body; defects of this nature are nearly always picked up at this stage.

The major organs of the baby can be viewed in detail to see that they are working properly. For example, the chambers of the heart will be checked for size and to ensure that the valves are opening properly. A scan of the stomach should show that the baby is swallowing some of the amniotic fluid, you can usually see this as a black bubble. Other organs such as the kidneys and bladder will also be checked. Your baby fills and empties his or her bladder every half an hour at this stage, so during the scan it will visibly fill and/or empty!

The sonographer will check the location of the placenta. The main purpose of this is to ensure that it is not lying too low and covering or obstructing the opening of the cervix. If the placenta is low-lying then you will be called back for an additional scan at around 32 weeks to ensure that it has moved up and out of the way (which it usually does).

If no problems are found does it mean my baby is perfectly healthy?

Probably yes, since most defects in babies are rare. However, the detection rate for different defects varies considerably from around 16-85%, depending on the condition. The variance is accounted for by the fact that different conditions can be easier or harder to detect by nature, but also by the fact that the image quality during a scan can be variable. The key factors affecting image quality are the position of the baby at the time of the scan, and the mother's size. The latter reduces the image quality if the mother is overweight, as the sound waves used to produce the image have to pass through more body fat before bouncing off the baby.

What if a problem is detected?

Your sonographer will advise you if they have any concerns about either you or the baby as a result of the scan. You may be asked to attend further screening tests or another scan with a specialist present. The course of action will then depend on the condition that has been, or may be, diagnosed. Most defects can be treated soon after the birth. In very rare instances there are conditions which cannot be treated and which may leave parents with a very difficult decision to make - whether or not to terminate the pregnancy. Your antenatal and hospital team will provide you with support through this process if this does happen.

Remember that you may also be asked to attend another scan because the sonographer was unable to get all the information that they needed on this occasion, either because the baby is in an awkward position or because the scan was unclear if you are overweight. The sonographer is always likely to err on the side of caution so don't tail-spin into a panic if they ask to see you again. However, you are well within your rights to ask for more details if you are unclear as to why you are being asked to attend another scan so make sure that you have all your questions answered before you leave your appointment.

Do I have to attend the 20 week scan?

You cannot be forced to attend this scan, but there are no good reasons why you shouldn't have it. The scan is offered in order to ensure that your baby is developing properly and that they are healthy. It may help to give an early diagnosis to one of a number of conditions, making treatment easier to plan and increasing the chances of your baby having a healthy life.

How does an ultrasound scan work?

Usually the date of this scan is arranged at the time you attend your first ultrasound scan, but if you have not received confirmation of an appointment then contact your midwife and ask them who you need to contact in order to book it.

You will be asked to lie down on a bed and the person conducting the scan will cover your tummy in a clear gel. The sonographer then passes a small device known as a transducer backwards and forwards across your tummy. The transducer then emits sound waves which are bounced back from the baby and create an image on the screen.

What will I see at the anomaly scan?

There is usually a screen on the wall that is positioned so that you can see exactly what the sonographer can see during the scan. Ask the sonographer to explain any part of the image that you are unclear about. It can be a very interesting process and they are able to show you a great amount of detail if the baby is lying in a good position. They may prefer not to talk while they are conducting the scan but they will go through the images with you afterwards. Most hospitals offer a print out of one of the scan images for a small fee. Make sure you take some money with you to the hospital to cover this and let the sonographer know that you would like a print out before the scan takes place.

Watching your baby on the screen now that it is larger, with more distinguishable "human looking" features can be a remarkably special time. Make sure that your partner comes with you if possible, so that they can share the experience.

Can we find out the sex of the baby at the anomaly scan?

Yes, at this stage it is usually possible for the sonographer to see the sex of the baby and most hospitals will let you know if you ask. However, there are some hospitals who have a policy of not disclosing this information, so check your hospital's policy before you go. There may be somewhere else that you can go in order to find out the baby's sex, or you may have to pay for a private scan in order to get this information.

If you are keen to keep the sex of the baby a surprise until the big day then let your sonographer know before the scan starts so that they don't accidentally give it away!

Is the scan safe?

Yes, unlike an x-ray, an ultrasound works by emitting sound waves and there is no associated radiation. There is no evidence that they cause harm to either the mother or the unborn baby, but plenty of evidence that they do improve the health and quality of life of children where early diagnosis of conditions is made as a result of the scan.