Antenatal ultrasound scans

They're a safe way to check up on your baby - plus, they give parents an exciting glimpse of what's going on inside!

Ultrasound scans use sound waves to create an image of your baby in the womb. They are painless and non-invasive.

Most hospitals offer at least two ultrasound scans during pregnancy. The first is usually when you are 8-14 weeks pregnant, and is sometimes called the dating scan. The second scan is called the anomaly scan or mid-pregnancy scan, and it usually takes place between 18 and 21 weeks.

What does the first scan do?

As the name indicates, the main purpose of the dating scan is to work out accurately how far along you are in your pregnancy and estimate your due date. The sonographer will also be confirming that your baby has a heartbeat, and checking to see whether you might be expecting more than one!

At the end of this scan, you will probably have a more accurate idea of your due date, and you will have seen your baby for the very first time – though he or she won't look a whole lot like a baby just yet.

What happens during the dating scan?

The dating scan takes around 10 minutes. The sonographer will put some gel on your belly and will glide a small hand-held device (called a transducer) over your skin to get an image of your uterus (and your baby!).

If your uterus sits very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan instead, in order to get the transducer nearer to your baby.

Your partner should be able to come with you, if you like, and you may even be able to get a printout of the scan to take home with you.

Can I have a screening test at the same time?

Nuchal translucency (NT) is a way of estimating the risk of chromosomal abnormalities such as Down's Syndrome. It can be measured between 11 weeks and 13 weeks plus six days of pregnancy, by using ultrasound to measure the translucent (clear) space in the tissue at the back of your baby's neck. Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this fluid-filled space to be larger than average.

If you are having the NT screening test, the sonographer will do a dating scan first, to get an accurate estimate of how far along you are in your pregnancy. You will then have a blood sample taken. Your age and the information from the blood test and dating ultrasound are used to work out the chance of your baby having a chromosomal abnormality. A combined NT scan and blood test gives a more accurate diagnosis than an NT scan alone. This is available in most areas of the UK.

What is the anomaly scan?

This is a detailed ultrasound scan, usually carried out when you are between 18 weeks and 21 weeks pregnant. During this scan, the sonographer checks that the baby is growing normally, and looks at the position of the placenta. If your placenta is low in the uterus, you may need to have another scan late in your pregnancy to see whether it has moved; if it hasn't, you might be advised to have a caesarean section for safety the safety of yourself and your baby.

The other purpose of this scan is to detect potential physical abnormalities in your baby. As detailed as these scans are, they can't always pick up every abnormality. The anomaly scan is carried out in the same way as the dating scan. Most hospitals welcome partners into the room, and if your baby is in a cooperative mood and lying in the right position, you might well get some great images of that little face.

If you're keen to know the sex of your baby, ask the sonographer at the beginning of the scan. Depending on your baby's position in the womb, they might be able to tell you!

Are there any disadvantages to having a scan?

Ultrasound scans are painless and have no known physical effects on mum or baby - but in some cases, having a scan might create unnecessary worry for you and your partner. It might show a very minor problem, or something that may get better on its own.

As with all screening tests, scans aren't infallible: some scans yield false positive or false negative results. For example, about one in 20 women will appear to have babies at high risk of Down's syndrome. However, most of these babies turn out not to have Down's syndrome.

In the end, it's entirely up to you to decide if you want to have a scan. Your midwife, GP and obstetrician are all good people to talk to about any questions or concerns you might have.