Ultrasound scans in pregnancy

You usually have one or more ultrasound scans during pregnancy. The two scans usually offered are the nuchal scan and the anatomy scan.

It's not usual to have a dating scan early in your pregnancy to work out how many weeks pregnant you are and your due date. However, you may need a scan in early pregnancy if you have complications such as bleeding.

Sometimes other scans are carried out to check the growth of your baby or the position of your baby or the placenta. You will need extra scans if you’re carrying twins or if you have had complications in this pregnancy or a previous pregnancy.

Nuchal translucency scan

You will be offered this scan at 12–14 weeks' gestation. This scan confirms your pregnancy due date, finds out if you are having twins and looks for abnormalities in your baby. However, your baby is still quite small at this stage (about 5–8cm long) so most abnormalities are better checked for at 20 weeks when your baby is bigger.

The nuchal translucency is the measurement of fluid behind your baby’s neck. This result can be used (along with blood tests) to calculate the chance of your baby being born with some genetic conditions such as Down syndrome. If the scan and blood tests show that your baby has an increased chance of having Down syndrome, you have the option of having further testing such as amniocentesis or NIPT (non-invasive prenatal testing).

Sometimes the sonographer will need to do an internal (transvaginal) ultrasound to get good images of your baby.

Anatomy scan

You will be offered a scan at around 18–20 weeks’ gestation, which is also called the second trimester scan or morphology scan.

Many important structural problems can be seen with a scan at this stage. This scan is usually the most detailed examination and includes assessment of the development of your baby and the position of the placenta.

Placenta previa (where the placenta is covering your cervix) may be diagnosed during the anatomy scan. However, as your baby develops and your uterus gets bigger, the placenta usually moves away from your cervix. It is usually not possible to know if the placenta has moved far enough for a normal birth until 32 weeks and sometimes even later. You may be offered additional scans later in your pregnancy to check for this.

At this scan, you can usually find out your baby’s sex. However, if your baby is lying in an awkward position their sex can’t be worked out.

A report of the ultrasound findings will be sent to your doctor or midwife. They will discuss the results with you.

There are several important things to remember regarding ultrasound scans.

• Not all problems cause a change in the anatomy of a baby, so not all abnormalities will be detected by an ultrasound scan.
• Sometimes the changes are very difficult to see, especially if there are twins or your baby is in a difficult position, or if you are overweight.
• Sometimes, something seen on the scan might indicate there is a problem or further monitoring or scanning of your pregnancy is needed. However, not all abnormal ultrasound findings mean there is something wrong with your baby. This situation can be very difficult because it obviously causes parents and whānau to worry about their baby.
• If any problem is found on the ultrasound scan, you might have follow-up scans arranged or you may be referred to an obstetrician to further tell you about the scan, arrange investigations and explain what your options are.
• Routine third trimester scanning of people whose pregnancies are progressing normally does not lead to healthier babies or fewer problems during labour and birth.
• An ultrasound scan will not be able to tell whether you can have a vaginal birth as this depends on your baby’s position and the shape of your pelvis as well as your baby’s weight. Scans in late pregnancy have a range of error of +/- 15% in estimating your baby’s weight. This means that if a scan estimates your baby weighs 4kg, your baby could actually weigh between 3.4 and 4.6kg.

A pregnancy ultrasound is normally carried out by specially trained clinicians called sonographers. You will be asked to lie on the examination table and to lift your top to your chest and lower your skirt or trousers to the top of your hips, to expose your puku (stomach).

The sonographer will put ultrasound gel on your stomach to ensure good contact between your skin and the machine. They will then pass a probe over your skin. This sends out ultrasound waves and picks them up again when they bounce back. A black and white picture of your baby will be shown on a small screen.

The scan usually takes about 20–30 minutes. Having the scan does not hurt, but the sonographer may have to push quite firmly at times in order to see the deeper structures. 

Sometimes, a transvaginal (internal) ultrasound needs to be done. This is more likely in early pregnancy (when your womb is still quite small) or if a better view of the edge of the placenta is needed.

There are no known risks to you or your baby from having an ultrasound scan. Ultrasound scanning has benefits when performed for medically necessary reasons. However, ultrasound scans can have unexpected findings that cause anxiety, so an ultrasound scan should not be done unless there is a medical reason for it.

The ultrasound scans you will be offered in your pregnancy are optional. It is your choice whether you have them.