Ultrasound in pregnancy

by | Tests and Investigations

Most pregnant women will have an ultrasound scan during their pregnancy. This simple test is quite safe for both mother and baby and causes only minor, if any, discomfort.

Ultrasound is a way of taking a look at the baby without using potentially dangerous X-rays. During an ultrasound scan, high-frequency soundwaves are used to create moving images of the developing baby, shown on a screen.

What can ultrasound scans show during pregnancy?

Ultrasound scans may be recommended at various stages of pregnancy for several reasons. Here are some examples.

  • To confirm the pregnancy.
  • To see if there is more than one baby, e.g twins.
  • To establish the date when the baby is due (baby’s due date).
  • To rule out an ectopic pregnancy – a pregnancy that implants outside the uterus (womb).
  • To assess the risk of the baby being affected by certain chromosomal abnormalities , e.g. Down syndrome. To check on the physical development of the baby and to check that it is growing adequately.
  • To check the amount of amniotic fluid surrounding the baby in the womb.
  • To determine the position of the placenta.
  • To check the baby’s position before delivery.

At what stage of pregnancy are ultrasound scans usually recommended?

For women in Australia with an uncomplicated pregnancy, the following ultrasound scans may be recommended.

First trimester ultrasound scans

A dating scan may be recommended if there is any uncertainty about when conception may have occurred (for example, women who have irregular periods and those who are uncertain of the date of their last menstrual period). Dating scans confirm the age of the pregnancy and provide an accurate due date. They can also show whether it is a single or multiple pregnancy (twins or more). Dating scans can be performed from 6 weeks of pregnancy.

The due date can also be confirmed during a nuchal translucency scan or 18-20 week pregnancy screening scan – see below.

A nuchal translucency scan (also called a first trimester pregnancy screening scan) is usually offered to pregnant women between 11 and 13 weeks. This scan is done in combination with blood tests to determine the baby’s risk of having certain chromosomal abnormalities, such as Down syndrome. The ultrasound looks specifically at the fluid space at the back of the baby’s neck. If the combined test indicates a higher than normal risk, you will be offered further tests.

Sometimes other problems can also be detected on ultrasound at this stage of pregnancy.

Second trimester ultrasound scan

The 18 to 20 week pregnancy screening scan, also called an anomaly scan, is recommended to check the developing baby.

Several measurements are taken to check the baby’s growth and development. The fetus is also assessed for any major anatomical abnormalities (such as problems with the head, limbs, heart and other internal organs).

At this stage of pregnancy, the sex of the baby can often be determined. However, if the genital area is difficult to visualise due to the baby’s position, it can be difficult to tell whether the baby is a boy or a girl.

In addition to checking the baby, the position of the placenta, the cervix and the amount of amniotic fluid are also usually assessed during this ultrasound scan.
ultrasound of baby

When might additional ultrasound scans be needed?

Your doctor may also recommend ultrasound scans at other times during the pregnancy. One common reason an ultrasound scan may be done is to check on the growth of the baby if they are measuring small at a routine antenatal visit.

Women pregnant with more than one baby (e.g. twins or triplets) may be advised to have more frequent ultrasound scans than usual.

Your doctor may also recommend having an ultrasound if you have an episode of vaginal bleeding.

Specialised ultrasound scans

3D and 4D ultrasound scans (4D scans are also called live, or moving, 3D ultrasounds) show images of the baby in three dimensions. They can provide clear images of the baby, including their face.

These detailed images are useful if further detail is needed to assess certain conditions, such as cleft lip or heart problems.

What happens during an ultrasound?

When you have a pregnancy ultrasound, a type of gel is spread on your abdomen and a device that produces and receives soundwaves (a transducer) is moved over your skin. The soundwaves ‘bounce’ off the baby and other internal structures, creating pictures on a TV screen.

Ultrasound scans that are done very early in pregnancy may need to be done transvaginally. This means that instead of moving the transducer device over the skin of your abdomen, a narrow device is gently inserted into the vagina to take images of the baby.

The sonographer (the health professional performing the ultrasound scan) may show you some of the images of your baby on the screen, and may also print some of the images for you. The images will also be reviewed by a radiologist (specialist in medical imaging) and your obstetrician or midwife.

Preparation for pregnancy ultrasound

Before having a pregnancy ultrasound, you are usually asked to drink several glasses of water and to hold off urinating for a period of time, so that you have a full bladder during the examination. This is especially important for early pregnancy scans, as a full bladder pushes your uterus out of the pelvis so that images of the baby can be more easily obtained.

Your doctor or the ultrasound clinic or hospital where you are having the scans will give you instructions on how long before the test you should drink the water, and how much to drink.

Ultrasound scans are not painful. You may feel a small amount of discomfort associated with having a full bladder, or when having a transvaginal ultrasound.

Do all pregnant women need to have pregnancy ultrasound scans?

Pregnancy ultrasounds offer an opportunity to check that your pregnancy is progressing as expected and that your baby is healthy. Many people feel reassured to see images of their developing baby.

If the ultrasound scan detects a problem, having the information early is often important. In some situations where a problem is detected during ultrasound, there may be treatments that can be undertaken or planned for while you are still pregnant.

But it should also be remembered that not all problems can be detected by ultrasound.

You can choose to not have ultrasound scans during pregnancy. However, if you have had problems with a previous pregnancy, you are older than 35 years or if there is a family history of certain conditions, ultrasound scanning is often strongly recommended.

Talk to your doctor or midwife before making your decision.