The importance of ultrasound scans in pregnancy

Ultrasound scans in pregnancy create a black and white picture of the unborn baby while it’s in the uterus (womb). These scans, which are fundamental for modern maternal and fetal care, can help reassure the expectant mother that her baby is growing well.

Here, Dr Spyros Bakalis, renowned consultant in obstetrics and maternal and fetal medicine in London, provides a comprehensive overview of ultrasound scans in pregnancy.

What is the importance of scanning during pregnancy?

Ultrasound scanning in pregnancy is the cornerstone of modern maternal and fetal care, and it has vastly improved the quality of outcomes for women and their unborn children.

An ultrasound is used to reassure mothers by:

• Screening for potential pregnancy problems, such as a small or large baby, early (preterm) birth, and pre-eclampsia (a pregnancy disease involving high blood pressure and protein in the urine).
• Surveying the anatomy of the baby to identify fetal conditions that may benefit from treatment before or after birth.

What can be detected in the first and second trimester?

The first trimester scan (11 weeks and 2 days to 14 weeks) has multiple uses. It is often the first scan women have, and it allows:

• The number of babies to be counted and to date the pregnancy accurately.
• The full review of the baby or babies’ anatomy. This allows the baby’s head, brain and heart to be viewed, the arms and legs to be seen, and other parts of the body to be surveyed.

In combination with the measurement of fluid behind the baby’s neck (a nuchal translucency) and a blood test (a marker of how the placenta is working), the first trimester scan can also be used for the screening of Down’s syndrome.

The second trimester scan (18 to 21 weeks) has several facets. It allows:

• The measurement of the baby to assess its size.
• The complete anatomical survey of the baby.
• The assessment of the placental location to ensure it does not obstruct the delivery of the baby.
• The measurement of the blood flow from the mother to the baby (uterine artery Doppler studies) that helps calculate the chances of having a small baby or pre-eclampsia.

A measurement of the mother’s cervix, which is optional at the second trimester scan, can help predict the chances of having the baby early.

What is the purpose of third trimester scanning?

The aim of the third trimester scan is centred on measuring the size of the baby (the estimated fetal weight) by measuring the head, tummy (abdomen), and leg (femur) of the baby.

An ultrasound can also assess the amount of amniotic fluid around the baby, as well as the blood flow in the baby’s head (middle cerebral artery), the umbilical cord and, when necessary, the liver (ductus venosus). These all help assess the health of the baby.

Small or large babies or those with abnormal blood flows need further monitoring and may require an earlier delivery, hence the importance of these scans. In most cases, these scans reassure the mother that her baby is growing well.

The third trimester scan can also be used to detect previously unseen or newly developed fetal abnormalities. These are then managed similarly to those abnormalities that are detected in the second trimester.

Finally, the third trimester scan reviews whether the baby is lying head down (cephalic) or breech. This allows planning of the delivery.

How often are scans performed in the third trimester?

In the first instance, I recommend growth scans every 4 weeks from 28 weeks onwards. This allows the growth of the baby to be tracked, as well as any changes in the size or Doppler studies to be detected and acted upon.

In some cases, either for reassurance or when there are known medical issues, I recommend scans from 24 weeks onwards.

How accurate are scans performed in the third trimester?

Scans in the third trimester are accurate, but they can have up to a 10 per cent error in prediction of the estimated weight of the baby.

When is the last scan in pregnancy?

Usually, the last scan is at 36 weeks.

However, for women wanting to wait for natural labour beyond 40 weeks, I offer a scan at 39 - 40 weeks to detect any late growth or blood flow changes where delivery to protect the baby is indicated.

Dr Spyros Bakalis is one of the UK’s leading consultants in obstetrics and maternal and fetal medicine.

For expert pregnancy care, don’t hesitate to book an appointment with Dr Bakalis via his Top Doctors profile today.