Antenatal ultrasound scans

Most hospitals offer at least two ultrasound scans during pregnancy. The first is usually when you are around 10-14 weeks pregnant and is sometimes called the dating scan. The second scan is called the anomaly scan, which usually takes place between 18 and 20 weeks, plus 6 days.

dating scanWhen will I have my first scan?

The first scan is usually carried out when you are between 10 weeks and 13 weeks and 6 days pregnant. It is sometimes called the dating scan, because the scan's main purpose is to work out accurately how pregnant you are and estimate your due date.

However, the timing of your first scan will also depend on how your pregnancy is going, and where you live. You may be offered an earlier scan between six weeks and 10 weeks if you experience any bleeding.

What does the dating scan check?

  • The main purpose is to check your baby's measurements. This gives a better idea of how many weeks pregnant you are and your due date may be adjusted.
  • To check your baby has a heartbeat and is developing normally.
  • To check whether you're carrying more than one baby.
  • To detect some abnormalities, particularly in your baby's skull or abdominal wall.

Looking for abnormalities isn't the purpose of this scan, though. That's the job of the anomaly scan, when your baby is bigger and more detail can be seen.

What happens during the dating scan?

The dating scan will take around 10 minutes. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to come with a full bladder as this pushes your uterus up and tends to give a better picture. The sonographer will put some gel on your tummy and will move a small hand-held device (a transducer) over your skin to get an image of your uterus and baby.

If your uterus is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get nearer to your baby. It shouldn't be uncomfortable and you don't need a full bladder for this.

You'll be given a report of the scan which will tell you exactly how many weeks pregnant you are. It should be kept with your maternity notes so those involved in your care can see it.

It should be possible for your partner to come with you and you may be able to have a scan picture, although there might be a small charge for this.

Can I have a screening test at the same time?

Some units offer a dating scan and screening at the same time. If you are having a screening test for Down's syndrome, you'll be offered a dating scan first. This is because the screening test needs accurate dates to be reliable, otherwise it may lead to you being given a high risk factor when your baby is actually developing normally (false positive result).

Nuchal translucency (NT) can be measured from 11 weeks to 13 weeks plus six days. This estimates the risk of Down's syndrome and other chromosomal abnormalities. It uses 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.

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.

scan 2What is the anomaly scan?

This is a detailed ultrasound scan, usually carried out when you are between 18 weeks and 20 weeks plus 6 days pregnant. The scan checks for possible physical problems (abnormalities) in your baby, although it can't pick up every abnormality.

It also checks that the baby is growing and developing normally and checks to find the position of the placenta. It may detect a placenta that is low down. This isn’t an unusual finding at this stage, but you may be offered another scan later in pregnancy to check it has moved. If it stays low down in late pregnancy, special care may be needed at delivery, or a caesarean section may be advised.

The anomaly scan is carried out in the same way as the dating scan, with jelly on your tummy and the sonographer moving the ultrasound tranducer over your skin. 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. Most hospitals welcome partners into the scan room, and again you may be able to have a scan picture, although there might be a small charge for this.

Are there any disadvantages to having a scan?

Sometimes, having a scan can cause unnecessary worry. It may show a very minor problem, or something that may get better on its own.

As with all screening tests, there can be false positive and false negative results. For example, about one in 20 women will appear to be at high risk of Down's syndrome. However, most of these babies will turn out not to have Down's syndrome.

Ultrasound scans use sound waves to build a picture of the baby in the uterus (womb). Scans are painless and have no known side effects on mothers or babies. It is entirely up to you whether or not you have a scan. Talk to your midwife, GP or obstetrician about any questions or concerns you have.

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