Ultrasound scans: How do ultrasound scans work and how many will you have during pregnancy?

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  • Ultrasound scans are offered to all pregnant women and if you are expecting a child you will be offered at least two ultrasound scans on the NHS.

    The NHS defines an ultrasound scan (also sometimes called a sonogram) as: “a procedure that uses high-frequency sound waves to create an image of part of the inside of the body.”

    Pregnant women will normally have an ‘early pregnancy scan’ (also known as a dating or booking scan) between 11 and 14 weeks and a ‘mid pregnancy scan’ (also known as a fetal anomaly scan). This second scan, which happens between 18 and 21 weeks pregnant, is usually when parents can find out a baby’s gender if they wish.

    Midwife and childcare expert Anne Richley explains what happens during these ultrasound scans, the difference between them and what additional tests you might be offered:

    How do ultrasound scans work?

    Gel is squeezed onto your abdomen and a transducer (or probe) is gently glided over it during an ultrasound scan. The transducer emits and receives high-frequency sound waves, which are transmitted through the uterus and bounce off the baby. The ultrasound scan images and sounds are then translated into a picture on a screen.

    If you’re having an ultrasound scan in very early pregnancy, the sonographer may need to do a ‘trans-vaginal’ rather than an abdominal scan, to get an image.

    The evidence so far is that ultrasounds are safe but it’s recommended that its use is ‘justified and limited to the minimum necessary’.

    The various ultrasound scans offered will depend on your local health authority. Some might not be offered in your area but will be available privately if you ask.

    The different ultrasound scans available in pregnancy

    Dating ultrasound scans

    To accurately date your pregnancy, the sonographer will measure your baby from the top of its head to the bottom of its spine. Many women know exactly when their baby was conceived or the date of their last period, so the midwife can work out when the baby is due.

    Despite this, most women still choose to have this early scan if it’s offered, as they find it so exciting to see the image of their baby, with their beating heart. Also, for many, this is the moment when the pregnancy becomes real. This early scan may also be when the mum-to-be finds out that she’s having twins – or more!

    Nuchal ultrasound scans

    This is done between 11 and 13 weeks of pregnancy, and a collection of fluid under the skin at the back of a baby’s neck can be measured. All babies have some fluid, but in many little ones with Down’s syndrome and some other conditions, the nuchal translucency is increased.

    This scan is a screening test, meaning that it can estimate the chance of your baby having Down’s, but it can’t diagnose. If the result comes back and shows that it’s likely, you might then decide to have a diagnostic test such as amniocentesis or chorionic villus sampling to confirm.

    A blood test combined with scan can help you to find out how likely it is that your baby has Downs syndrome, Edwards Syndrome or Pateaus Syndrome. Ultrasound scans can also help to detect  some conditions such as spina bifida.

    Anomaly ultrasound scans

    This is offered at around 18-20 weeks and the purpose is to check that your baby is developing normally. The sonographer will look at the structure of his bones and major organs, measure him and check the position of the placenta and the amount of fluid around him.

    If a problem is found or suspected, you’ll be told at the time of the scan and an appointment to have another scan with a specialist should be made within 72 hours. Around 15 per cent of scans will need to be repeated for one reason or another; most problems that need repeat scanning are not serious.

    Growth ultrasound scans

    Some women are advised to have extra growth-monitoring scans from around 28 weeks of pregnancy, if they have previously had a very small baby, are carrying twins or have high blood pressure – which can affect growth.

    Group B Strep test

    This is probably the least common test, but just as important. Group B Strep (GBS) is a bacterial infection that can affect a baby around labour and birth and potentially cause problems.

    The test works using something called an enriched culture method (ECM). If you want it done, get your doctor or midwife to order the test. There are some NHS hospitals that do the test, but you’ll probably have to pay to get it done privately.

    The GBS Screening Pack is sent out by post and involves two swabs, one vaginal and one rectal. These should be taken between 35-37 weeks of pregnancy – you can do them yourself or ask your doctor or midwife to do it. The swabs are then sent straight to the testing lab in packaging that comes as part of the GBS Screening Pack. If you don’t get it on the NHS, the test is usually free but you will have to pay to get the results analysed – usually around £30.