Scans in later pregnancy

When will I have my last ultrasound scan?

If your pregnancy is straightforward, your last scan will probably be your anatomy scan, which is usually offered between 18 and 22 weeks (SOGC nd, HealthLink BC 2019a).

Your prenatal care provider will continue to keep an eye on how your baby is growing by measuring your bump (Mayo Clinic 2017).

She will also listen to your baby’s heartbeat, and check the baby’s position by feeling your belly (Ontario Midwives nd).

To make sure you're keeping healthy, your prenatal care provider will also:

• take your blood pressure
• test your urine
• take blood samples
  (SOGC nd)

Learn more about what happens at your prenatal appointments.

If any problems are picked up, or if you had complications in a previous pregnancy, you may be offered extra scans (HealthLink BC 2019a).

Why are scans done in later pregnancy?

A common reason for a scan later in the second trimester or in the third trimester is to check that your baby is growing normally.

If a bump measurement at your prenatal visit suggests that your baby is not growing at the expected rate, either less or more than anticipated, your prenatal care provider will refer you for a growth scan (Mayo Clinic 2017, ACOG 2016).

Your prenatal care provider will also recommend a scan if she suspects there’s a problem with the amount of amniotic fluid around your baby – either too low (oligohydramnios) or too high (polyhydramnios) (Mayo Clinic 2017, HealthLink BC 2019a, HealthLink BC 2019b).

If you had complications in previous pregnancies, or have a medical condition such as diabetes (Berger 2016) or high blood pressure (Magee 2014), your prenatal care provider will probably recommend regular scans.

A scan later in the second trimester or in the third trimester may also be offered to:

• Check the position of the placenta if it was found to be low lying on the anatomy scan (HealthLink BC 2019a, ACOG 2016).
• Check that the blood is circulating properly in your baby, uterus and placenta with a Doppler scan if there was a previous concern about it ( Alfirevic 2017).
• Check the length of your cervix. This is done if there’s a chance you might go into preterm labour (when your body is getting ready for birth too early in your pregnancy) (HealthLink BC 2019a, Kagan, Sonek 2015).
• Monitor twins or triplets (HealthLink BC 2019a, ACOG 2016).
• Check your baby's position (ACOG 2016) - for instance, if he’s thought to be breech.

Can a scan in later pregnancy tell if my baby is healthy?

This image shows a baby's face in the third trimester, and gives you an idea of the kind of detail you may be able to see.

If you're offered a scan later in the second trimester or in the third trimester, it can check your baby's wellbeing by measuring him, looking at the amniotic fluid and blood flow, and by simply watching him move around. This is usually called a biophysical profile (TOP 2017).

A healthy baby stretches and flexes, moves his arms and legs, opens and closes his hands, wiggles his fingers and toes, and moves his lips and tongue. He practices breathing the amniotic fluid into his lungs, which helps them to mature. All these things can be checked on a later pregnancy scan (Sunnybrook nd).

If your baby doesn’t appear to be moving, there may be a problem; although it's possible that your baby may simply be asleep. Before your baby is born, he will have sleep periods during the day and night that last 20 to 40 minutes, sometimes up to 90 minutes in later pregnancy. If you have any concerns about your baby's movement, be sure to speak to your prenatal care provider.

Any concerns can be checked with a Doppler scan, which checks the levels and speed of blood flow from the placenta to your baby (Maulik 2019).

How is a pregnancy scan later in the second trimester or in the third trimester less useful?

Estimating your correct due date

An estimate of your due date (based on how far along your pregnancy is) becomes less accurate as your pregnancy advances. Ideally, your stage of pregnancy and due date should be established earlier, between 11 weeks and 14 weeks (Butt 2014).

Assessing your baby’s weight

Using ultrasound, the sonographer takes measurements of your baby’s head circumference, abdominal circumference, and thigh bone (femur) length, and combines them to assess his weight.

However, the bigger your baby and the nearer he gets to full term, the harder it is to measure him, and the less accurate the estimated weight (Milner, Arezina 2018).

Is it safe to have several scans?

Ultrasound scans, when used for the right purpose, are considered safe for moms-to-be and their babies, with no side-effects. Your prenatal care provider will only recommend a scan if there are clear medical reasons for it. The use of non-medical scans (such keepsake videos and photos) is strongly discouraged (CAR 2014).

Watch a video on how to monitor your baby's movements in pregnancy.