Doppler Ultrasound Scan during Pregnancy

Doppler Scan in Pregnancy- A Complete Guide

Medically Reviewed By
Dr. Rima Sonpal (Gynecologist/Obstetrician)
View more Gynecologist/Obstetrician Our Panel of Experts

Doppler sonography is a technique that uses reflected sound waves to measure movements such as blood flow and heartbeat. Doppler ultrasound scans can be used to determine the speed and direction of the blood flow. This information can be helpful in determining if the foetal growth is normal and whether the tissues are supplied with enough blood and nutrients. Doppler scans are performed with the same apparatus as a regular ultrasound scan and are normally used during the third trimester on women who have high-risk pregnancies.

What is a Doppler Scan?

A Doppler scan is similar to a regular ultrasound scan and works using high-frequency sound waves called ultrasound, which isn’t audible to our ears. The ultrasound generated by the equipment bounces off bones and tissues like an echo and is recorded with a microphone. All of this is done with a small hand-held probe called a transducer. A gel which helps in the process is applied over the belly, and the transducer is pressed gently against the skin to scan. Denser substances like bones give off a better echo than the softer tissue that the body is made up of, and by comparing the echo, an image of the baby is generated in a computer and displayed in real-time.

What sets a Doppler scan apart is that unlike a regular ultrasound scan, it can detect the flow of blood in blood vessels, estimate the speed of the blood flow, determine its direction, detect blood clots, etc. Most ultrasound equipment these days have an inbuilt Doppler feature and both the scans can be done together.

Is it Safe to Have a Doppler Scan?

Yes, like with all other ultrasound scans, Doppler is safe when done by trained professionals. The sonographer, who is the person carrying out the scans, follows a set of established guidelines that ensure that you and your baby are safe during the procedure. Since the scans work by using a focused beam of sound waves, the equipment can generate a small amount of heat as a consequence. Therefore, every ultrasound scan machine features a thermal index display that gives a rough estimation of how much heat is being generated.

The machines normally have a low thermal index and come with different output settings for different stages of pregnancy. Most of the ultrasound scans don’t exceed 30 minutes, and a typical Doppler scan lasts only a few minutes. This poses no risk to the baby or the mother. In decades of using ultrasound scans during pregnancies, there has been no evidence that suggests that these scans are harmful.

Why is a Doppler Scan Done During Pregnancy?

Generally, women need two basic ultrasound scans during their pregnancy. The first one is during the first trimester to look for the number of babies, check for the baby’s heartbeat, to determine the baby’s growth, and predict a due date. The second scan is done in the second trimester to check for physical abnormalities and confirm that the baby is developing normally.

If the doctor finds any anomaly during these scans, a Doppler scan is done for further investigation. Dopplers are often used to check the placental blood flow, the foetal umbilical blood flow, and blood flow in the heart and brain to ensure everything is normal. If any restriction to the flow is detected, the doctors can determine if it is caused by a restricted blood vessel, sickle cell anaemia or RH sensitisation.

Restricted blood flow in the foetus can cause lower birth weight, impaired development, reduced size, etc. A special type of Doppler ultrasound called a Transcranial Doppler is used to evaluate the risk of stroke in babies with sickle cell anaemia. Doppler scans are also recommended in conditions such as:

  • Carrying twins or more
  • The mother has a low or high body mass index (BMI)
  • Medical conditions such as diabetes and high blood pressure
  • The baby is affected by rhesus antibodies
  • The baby’s growth rate is low
  • Previous miscarriage or a small baby
  • The mother smokes

When Your Doctor Will Ask For a Doppler Test

Doctors ask for a Doppler test when complications or abnormalities are detected in earlier scans that demand extra care for the pregnant woman during the course of pregnancy. Some of the other common conditions when doctors ask for Doppler scans are:

  • Multiple Pregnancies

When the mother is carrying multiples, the pregnancy is considered a high risk one and is monitored regularly with Doppler scans. This is because such pregnancies have chances for several complications to arise. Some of them include – TTTS (Twin to Twin Transfusion Syndrome), IUGR (Intra Uterine Growth Retardation), umbilical cord entanglement, etc. These complications can be detected early with a Doppler scan.

  • Placental Problems

The placenta supplies the foetus with blood, nutrients and oxygen from the mother’s body. Healthy blood flow in the placenta is essential for normal development of the baby. During the second trimester’s anomaly detection scan, if problems such as slower foetal development are seen, a Doppler scan is used to detect any irregularities in the placental blood flow. A Doppler scan is also used if placenta previa (low lying placenta) is detected. The scan can show the placental position, which can change toward the end of the pregnancy.

  • Health Conditions of the Mother

The mother’s health has a profound effect on the growth of the foetus. Doctors use Doppler scans to determine the blood flow rate in the umbilical arteries and the placenta. There are conditions under which blood flow in the arteries can get restricted, such as the contraction of arteries due to smoking, certain medications, and other lifestyle-related causes. The contracted arteries offer high resistance to the flow of blood, resulting in improper oxygen and nutrient supply to the foetus. Conditions such as high blood pressure and diabetes can also have a significant effect on this.

  • Health Conditions of the Foetus

When the growth rate of the foetus in previous ultrasound scans is not satisfactory, doctors use a Doppler foetus scan for further analysis.

How a Doppler Scan is Different From Other Pregnancy Scans

A typical ultrasound scan bounces high-frequency sound off the tissues of the body to give a thin two-dimensional image that does not show any movement. A Doppler ultrasound scan, on the other hand, relies on the Doppler Effect. The phenomenon is a continuous change in the frequency of the bounced sound wave when it hits something that moves, like the blood flow in the arteries. It can measure the difference between blood that is moving away from the probe and blood that is moving towards the probe, as well as its speed. A Doppler can also detect the heartbeat of the foetus, which other scans cannot.

Which Areas Are Checked During a Doppler Ultrasound?

Dopplers are generally used to determine blood flow. When anomalies are detected in previous scans, the doctor generally scans the blood flow in the critical parts of the mother and the foetus’ body.

  1. Uterine Artery Doppler Scan

Uterine arteries are the blood vessels that carry blood to the mother’s uterus (womb). During pregnancy, these arteries become stretchy and expand in size to richly supply the uterus with blood. This physical change allows more amount of blood flow with low resistance through the arteries, so the foetus can be supplied with ample nourishment. If the mother suffers from conditions such as Pre-eclampsia, the blood flow to the artery is restricted. A Doppler scan can be used to detect this condition early on.

  1. Umbilical Artery Doppler Scan

In conditions where a pregnant woman is carrying twins, a baby appears to be growing slowly, or if the baby is affected by the rhesus antibodies, the doctor would recommend an umbilical artery scan. Umbilical arteries carry blood from the placenta to the baby through the umbilical cord. A Doppler scan of this artery reveals the amount of blood flow in it and how much nutrients and oxygen the baby is receiving. If any issues are discovered at this point, further Doppler tests might be done to reveal the cause and also check the blood flow in the baby’s brain and its aorta (a major artery in the body).

  1. Middle Cerebral Artery(MCA) Doppler Scan

This scan shows the amount of blood flow in the middle cerebral artery, which supplies blood to the baby’s brain. The scan is performed only if the baby is suspected to be anaemic, the baby is affected with slapped cheek syndrome, or if the baby is affected by rhesus antibodies.

  1. Ductus Venosus Scan

This is a rather uncommon Doppler scan. It is done in the first trimester, along with other tests to look for a chromosomal abnormality in the foetus. It is also done to scan the umbilical vein, which carries blood to the baby’s heart.

Types of Doppler Scan in the Pregnancy Phase

There are different types of Doppler scans that can determine various characteristics, such as the direction of the blood, the velocity of the blood, and the location. Depending on what needs to be assessed, three types of Doppler scans are used:

  1. Continuous Wave Doppler

This system utilises a continuous transmission and reception of ultrasound waves to accurately measure high velocities of blood flow. It does not show the direction or the location of the flow, but only the speed. It is compact and commonly used.

  1. Duplex Doppler

A duplex Doppler can generate an image of the blood vessel and the surrounding organs, while at the same time, measuring the speed and direction of the blood flow.

  1. Colour Doppler

A colour Doppler is similar to a duplex Doppler but gives a better visualisation of the scanned area. A computer overlaps colour images depicting the blood flow onto the image of the blood vessel and its surrounding tissues. The different colour schemes show the speed and direction of the blood flow. A variation of this instrument called a Power Doppler can be used to look at blood flow in solid organs.

How to Prepare For a Doppler Scan

There are no special instructions to be followed before undergoing a Doppler scan. Women who smoke must avoid smoking or using any nicotine-based products two or more hours before the procedure. As nicotine causes blood vessels to constrict, it could give a false diagnosis, which could be mistaken as a pathological constriction. Other general instructions include wearing loose-fitting clothes to the clinic and staying well hydrated.

How a Doppler Test in Pregnancy is Done

A Doppler pregnancy test is done in the same way that a normal ultrasound scan is done. The pregnant woman will be asked to lie down on the examination platform, and her skirt/pants will be lowered to expose the baby bump. The sonographer then applies a water-based gel on the tummy. The function of the gel is to facilitate a secure contact of the transducer with the skin so that no air bubbles get trapped in between and interfere with the sound waves as the transducer is moved about. The image of the scan is displayed on a computer screen in real-time and saved for further analysis. The scan should be over within a few minutes and is painless.

Other Risks

There are no risks from the ultrasound on the foetus or the mother in a Doppler scan. On the contrary, opting to not get an ultrasound scan throughout the pregnancy can be risky. Research has shown that Doppler scans lower the risk in high-risk pregnancies.

Please note that vaginal probe Doppler scans are usually not recommended during the early weeks of pregnancy.

Can You Use a Hand-Held Doppler Device at Home

Small hand-held Doppler ultrasound pregnancy heartbeat scanners can be purchased to listen to the baby’s heartbeat. The device won’t usually work until the woman is 13 weeks into the pregnancy, as the womb is still in the pelvis. It is possible to detect the heartbeat after 13 weeks.

However, doctors and midwives recommend against it, as the average untrained person won’t be able to tell the difference between a baby’s heartbeat and their own placental blood flow. Even though the technology is safe to use, it can be falsely reassuring and even misleading which can cause unnecessary stress and anxiety among pregnant women.

On the other hand, electronic fetal monitoring done by a doctor or a midwife using a Cardiotocograph has some real benefits to it. A Cardiotocograph or CTG is a small Doppler that monitors the baby’s heart rate for normal heartbeats and irregularities. A healthy baby’s heart rate varies all the time and changes as it moves around. Women with healthy pregnancies don’t need to have a CTG to monitor the baby. As long as it can be felt moving regularly during the day, the baby is fine.

CTG used during labour is a highly useful tool for when continuous monitoring of the baby is needed. The baby’s heart rate, along with the woman’s contractions, is measured to see a positive relationship between the two. If the CTG shows that the baby’s heart rate is decreasing with increasing contractions, the doctor administers a drug to lessen the contraction’s strength. If that doesn’t work, the woman would need an emergency caesarean.

Taking a Doppler Test can be extremely useful and help in detecting anomalies that cannot be detected with a normal ultrasound. If your doctor recommends you to take a Doppler test, rest assured that it is for the health and safety of you and your baby.

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