Doppler ultrasound scans in pregnancy (2024)

Doppler scans evaluate the blood flow through the placenta, umbilical cord and your baby's body. They help your doctor assess your baby's health and wellbeing. Doppler scans are most often done in the third trimester, along with a growth scan. They are also used to investigate possible pregnancy complications and monitor high-risk pregnancies. If your scan shows a problem, you'll need extra scans. If your baby needs to be born early, you'll need an induction of labour.

What is a Doppler ultrasound?

A Doppler scan is a non-invasive ultrasound that assesses your baby’s growth and wellbeing and identifies any health concerns.

It uses high-frequency sound waves that bounce off moving red blood cells and, so, is helpful to check blood flow. As blood cells move through vessels, they cause changes in sound waves. This is called the Doppler effect. A regular ultrasound also uses sound waves, but it doesn't show blood flow.

A Doppler scan will measure the blood flow through the umbilical cord and around different parts of your baby's body, such as his brain and heart. This helps to show how well the placenta is working and whether your baby is getting all the oxygen and nutrients he needs.

When the colour Doppler option is used, the computer changes sound waves into different colours. That's why the scan is also referred to as colour Doppler studies.

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When is a Doppler ultrasound done during pregnancy?

Generally, colour Doppler studies are done with a growth scan in the third trimester, between 36 and 40 weeks of pregnancy.

However, you may need it earlier if there are any pregnancy complications or health concerns.

Rarely, a Doppler scan is used in the early stages of pregnancy, for diagnosing genetic and cardiac disorders in a baby.

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Doppler scans are a kind of screening test, and not all hospitals or diagnostic centres routinely offer them. So, always follow your doctor's advice on when and where to get these scans.

Why might I need a Doppler ultrasound scan during pregnancy?

Doppler tests can help your doctor confirm your pregnancy is progressing as expected and diagnose and monitor any pregnancy complications. The scan can look at whether the placenta is working normally and delivering enough oxygen and nutrients to your baby.

The results can help your doctor decide whether your baby should be delivered early or if other medical measures are needed to protect you or your baby.

It's most often used during the third trimester in women with high-risk pregnancies and need extra care, for example, if:

  • you’re carrying twins or more, and there are signs of twin-to-twin transfusion syndrome (TTTS)
  • your baby is affected by rhesus antibodies (rhesus disease)
  • you catch slapped cheek syndrome (human parvovirus B19) in the first 20 weeks of your pregnancy
  • your baby is smaller than expected for his gestational age and may have intrauterine growth restriction (IUGR)
  • you are experiencing fewer baby movements
  • you have any bleeding during pregnancy
  • you’ve previously had a small baby
  • you’ve previously experienced a late miscarriage or suffered the loss of your baby at birth
  • you have an existing medical condition, such as diabetes or high blood pressure
  • you have a low or high BMI
  • you have low amniotic fluid (oligohydramnios)
  • you smoke

How is a Doppler ultrasound done?

You can have a Doppler scan at the same time as a normal scan, as it uses the same equipment. All modern ultrasound machines have a Doppler function.

For a transabdominal ultrasound, your doctor will spread gel on your belly and move a special wand, called a transducer, over your abdomen to find your baby.

The transducer sends out sound waves, which bounce off the blood flow to your baby’s body via the cord as well as his blood circulation system. Once your doctor locates a specific area or blood vessel that she wants to evaluate (one of the arteries in the umbilical cord, for instance), she presses a button to turn on the Doppler function.

This creates an image with blue or red highlights on a screen showing your doctor the speed and direction of the blood in real-time. It gives a good idea of how your baby’s doing.

The Doppler scan will take just a few minutes during your regular ultrasound.

Sometimes, Doppler ultrasound may be used during a transvaginal ultrasound (TVS) for gynaecological reasons. It may be needed to check on an ovarian cyst, for example, which is sometimes found during pregnancy.

Smaller, asymptomatic cysts usually don’t require treatment and will likely go away on their own, but larger ovarian cysts may require surgery.

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What is checked during a Doppler scan in pregnancy?

In general, Doppler scans look at the blood flow between you and your baby, to check whether he’s getting everything he needs to develop healthily.

The ultrasound doctor will check different areas depending on your personal situation. Generally, the blood flow in four main vessels is checked. These are:

  • uterine artery (mother’s vessels)
  • umbilical artery (from the cord)
  • middle cerebral artery MCA (that supplies the brain)
  • ductus venosus (that supplies the heart)

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Uterine artery Doppler scan

Uterine arteries are the vessels which carry blood to your womb (uterus). A uterine artery Doppler scan checks that enough blood is reaching your placenta.

Your baby needs plenty of nutrients and oxygen to grow at a healthy rate. Therefore, the walls of your uterine arteries should be stretchy, to allow as much blood through as possible.

In pregnancy, these normally small arteries increase in size to allow more blood to reach your womb easily. This is called low resistance.

If blood can’t get through to the placenta easily enough, your baby may not get the nutrients and oxygen he needs via the umbilical cord.

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You may have a uterine artery Doppler scan if you have a high chance of developing preeclampsia, which affects how well the placenta works.

Umbilical artery Doppler scan

You may be offered a different type of Doppler called an umbilical artery Doppler, if:

  • your baby seems to be growing slowly
  • your baby is affected by rhesus antibodies
  • you’re carrying twins

This looks at the blood flow from your baby, via the umbilical cord, to the placenta. This is a very safe and effective way of finding out whether your baby is getting everything he needs from the placenta.

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If any issues are found during an umbilical artery Doppler scan, your doctor will give you further advice. She may request additional Doppler studies to look at the blood flow in your baby’s brain and his aorta, which is the main artery in his body.

Alternatively, she may request that you visit her a couple of times a week for more umbilical artery Doppler scans.

These will check that your baby is healthy and help you and your doctor to decide the best time and place for you to give birth. If your doctor doesn't have a Doppler machine, she might ask you to get these additional tests done at a diagnostic imaging centre.

It's perfectly safe for your baby to have frequent Doppler scans. The benefits of having more scans to monitor your baby’s health outweigh any potential risks.

If any of the scans show that your baby isn’t getting enough nutrients or oxygen, you may be advised to give birth early.

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It’s a different picture if you just happen to be carrying a small baby. A small baby with normal blood flow via the umbilical cord is probably doing well, and getting all the nutrients he needs.

The middle cerebral artery (MCA) scan

This is done to see the amount of blood flow in the middle cerebral artery, which supplies the brain. It is done when the baby is suspected to be anaemic, in cases your baby is affected by rhesus antibodies or if your baby is affected by slapped cheek disease (parvovirus). It is also done sometimes when the umbilical artery flow is abnormal.

This test gives an idea of whether the baby's blood has enough oxygen-carrying capacity or not and whether the baby requires any interventions like a blood transfusion.

Ductus venosus scan

This is rarely done. If needed in the first trimester, along with the other tests, it gives an indication about a chromosomal abnormality in a baby.

In the third trimester, it helps to determine whether the baby is getting enough nutrition and oxygen.

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What if my Doppler scan shows a problem?

Once you get your reports, your doctor will evaluate the results of your Doppler test.

If your Doppler ultrasound results are abnormal, it may mean that your baby isn’t getting adequate blood flow. Your doctor may want to run other tests to determine the cause.

If your baby isn’t getting enough oxygen or nutrients, he may need to be delivered preterm. Therefore, your ob-gyn may recommend an early induction of labour.

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Your doctor should explain what the abnormal result means for your pregnancy and your baby. If you have questions or concerns, don’t hesitate to ask.

Are Doppler scans safe in pregnancy?

As with all ultrasound scans, Doppler scans are safe in trained hands. When carried out by a trained ultrasound doctor, a Doppler scan helps to give a clear picture of your baby’s health and wellbeing.

In the early stages of pregnancy, a Doppler scan should only be used for diagnosing genetic and cardiac disorders. Even if the actual ultrasound takes a bit longer, your doctor will ensure that the Doppler function on the scan is used for the shortest time possible (less than the recommended 5-10 minutes).

Doppler and colour scans have a slightly higher thermal index. While this is still safe in small doses, scans should be limited to the minimum length of time it takes to get a good reading. In most cases, a Doppler scan should only take a few minutes.

It's helpful to know that the high-end scan machines automatically reduce the power of the ultrasound beam when a Doppler is used. This helps to reduce its intensity.

What is the difference between a Doppler scan and a handheld Doppler?

Doppler scan

During a Doppler ultrasound, sound waves are projected onto a screen, enabling the ultrasound doctor to see the blood flow from the placenta to your baby and in your baby's body. Doppler studies are usually done in the third trimester along with your growth scan, or when needed to investigate possible pregnancy complications.

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Handheld Doppler

A handheld Doppler is a machine used to listen to your baby's heartbeat throughout pregnancy. It also uses these sound waves. It's often done in your doctor's clinic when you go for each prenatal visit.

To check your baby's heart rate with a handheld Doppler, your doctor will cover the device with ultrasound gel and move it around on your belly until she finds a spot where the heartbeat can be detected.

The handheld Doppler sends and receives sound waves that safely bounce off your insides, including your baby's heart. The returning sound waves are processed and amplified by the device so you and your doctor can hear the heartbeat.

Is it safe to use a portable Doppler machine at home?

It's possible to hire or buy a handheld Doppler machine, also known as a fetal Doppler, to listen to your baby's heartbeat at home. However, doctors strongly advise parents-to-be against using one.

The technology that handheld Dopplers use is safe, but how some interpret the results could be a problem.

Hand-held Dopplers can be falsely reassuring. It's very easy to pick up the sound of blood flowing through the placenta, or your own heartbeat, and mistake this for your baby's heartbeat. Doctors receive specialist training to interpret heart rate patterns and sounds, so it’s best to leave Dopplers to them.

It’s also very difficult to pick up your baby’s heartbeat before your second trimester, so it may cause a lot of needless anxiety and panic.

Going to all your antenatal appointments and being aware of your baby's pattern of movements is a far more reliable way of monitoring your baby’s health.

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If you're at all concerned about a change in your baby’s activity, contact your doctor straight away.

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What is a cardiotocograph (CTG)?

A cardiotocograph (CTG), also known as a non-stress test (NST), is a form of Doppler which uses only sound, and doesn’t produce an image. It monitors your baby’s heartbeat and checks that it’s beating at a normal rate. A healthy baby’s heart rate varies from beat to beat and increases when he moves around.

Your doctor is likely to ask you to have a CTG as part of a routine check-up in your third trimester. She'll especially want to check if your baby’s movements have slowed down or if you have any pregnancy complications.

The use of a CTG during labour is known as electronic fetal monitoring (EFM). Find out more about when and why a cardiotocography (CTG) test is done.

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