Obstetrics

Ultrasound investigations in pregnancy

Most women will have one or more ultrasound scans in their pregnancy. There are several benefits of having scans in pregnancy and these vary depending upon when you have the scan done.

The following are the most common scans:

  • A dating scan is often arranged to confirm your pregnancy dates and is particularly important if you did not know when your last period was or you have an irregular cycle. The best time to have this scan is around 7 weeks as before this it is not always possible to see a baby with a heart beat in the uterus. Prior to 7 weeks if there is uncertainty about your pregnancy, blood tests looking at the rise in pregnancy hormone level are more helpful. A transvaginal scan is often done at this stage of pregnancy as it gives more information than looking through your abdominal wall.
  • A combined first trimester screen (CFTS) is a combined blood test and ultrasound scan which is used as a screening test for abnormal chromosomes- the most well known one is Downs syndrome. The nuchal translucency test can be done between 11 weeks 3 days and 13 weeks and 6 days of pregnancy (ideally at 12 weeks). The blood test is most informative if collected during the tenth week of pregnancy, however it can be collected between 9 weeks and 12 weeks 6 days of pregnancy.Two hormones in your blood are measured. The result is sent to the Radiology service where you are having your scan. The blood test increases the accuracy of the test to about 90% meaning for 100 babies affected 90 will be detected by this test. The test has a false positive rate of 5% meaning for every 100 scans done the test will be abnormal 5% of the time when in fact the baby is normal. If the test suggests your baby is high risk, a diagnostic test is offered which is able to see if your baby really is affected or not. This test is optional and not all women wish to know this information and that is fine, however the other advantage of a scan around 12 weeks is it gives an opportunity to pick up major abnormalities with the baby which would otherwise not be identified until the next scan at 18-20 weeks.
  • Morphology scan at 18-20 weeks. This is probably the most important scan as it identifies where the placenta is. This is important to know about for your baby’s birth and if the baby has abnormalities which might be very important to know about prior to baby being born. It is often possible to see the sex of the baby at this scan however that is not the only reason for the scan and is not 100% reliable.
  • Placental position will be rechecked around 34 weeks if it was low in the uterus at the time of the morphology scan. If it stays in front of the cervix that might mean a caesarean section is required however, often as the uterus grows later in pregnancy the placenta moves away from the cervix and a vaginal birth is still possible.
  • Growth scans are not performed routinely on all women but may be indicated based on your history, the findings of the earlier scans or complications which develop along the way in your pregnancy.

It is up to you what scans you may choose to have in your pregnancy. I would recommend at the very least a morphology scan and I am happy to talk about the details at your appointment.