Doppler scan

Women are offered an average of two primary scans throughout their pregnancy: one in the first trimester for dating the baby, and another in the second trimester to check the developing baby for anomalies (see our relevant articles for more information). The Doppler ultrasound is one of the extra scans required for certain mothers, usually taken at about 28 weeks into pregnancy.

Why might extra scans be necessary?

Extra scans are mostly necessary when complications or irregularities are identified during earlier scans. Uterine fibroids are a typical cause of extra scans; these are benign tumours arising on the uterine wall, and can affect the baby’s position or lead to bleeding, premature labour or even miscarriage. Placental problems are another common reason for extra scans, and the Doppler scan concentrates on these. Outside of problems, multiple pregnancies (e.g. twins) also necessitate extra scans to regularly check on development.

Placental problems

At the anomaly scan during the second trimester, one of the sonographer’s priorities is to examine the placenta. They check that it is positioned correctly, of a suitable size, and functioning as it should. The placenta is crucial for the baby’s healthy development because it feeds the foetus blood, oxygen and nutrients from the mother’s body. If problems or irregularities are noted at this stage, such as slower than average foetal growth, a further scan will be scheduled in order to check that the placenta is working properly. This is the Doppler scan.

Doppler and low-lying placenta

One of the most common reasons for doctors to suggest a Doppler scan is that the placenta is low-lying (placenta praevia), meaning that it rests at or close to the neck of the womb. If the placenta stays in this position it can cause problems with natural birth, which is why it is vital to check on placental changes before the due date. In a majority of cases the Doppler scan shows movement of the placenta to a better position. When there is not enough change to a low-lying placenta, the doctor and hospital arranges for the mother to have a caesarean section when the baby is due. This is not anything to panic about, and your doctor will talk you through the process; what’s most important is to know about these issues before the due date.

What does the scan involve?

Doppler scanning is a form of ultrasound and therefore uses the same procedure as far as the mother-to-be is concerned. The scan is painless, non-invasive, and should cause a minimum amount of discomfort. The difference is the type of sound waves projected into the womb and echoed back to the transducer, which are able to bounce off red blood cells as they move, giving an accurate picture of their speed. From this the sonographer can calculate the rate of blood flow moving through the umbilical cord between baby and placenta. Functionality and health of the placenta are thereby checked, and also that of other major blood vessels. Blood clots, blocked arteries or reduced blood flow can all be identified, as well as the risk of pre-eclampsia.

Depending on the results, appropriate preventative care can be administered or advice given.