A routine third trimester ultrasound has several advantages. A third trimester ultrasound can also be done on referral, for example if there is concern about the baby’s growth. If there is concern about the baby’s growth, you could be referred for a growth and Doppler ultrasound.  We measure the baby’s head and abdominal circumference and femur length and estimate the amount of amniotic fluid.  The measurements are plotted on charts of the expected size at different gestational ages. One of the common causes of a baby not growing well, is that the placenta does not function well.  This can also be associated with pre-eclampsia (high blood pressure caused by the pregnancy).  If the placenta does not function well, the baby’s blood circulation is distributed more to its heart and head, and less to its body and limbs.  Because of this, we commonly see that the head circumference is normal, while the femur length and especially the abdominal circumference are too small. With Doppler ultrasound the blood flow in the umbilical cord and an artery in the baby’s brain, the middle cerebral artery, are measured.  With decreasing function of the placenta, we see a decrease in blood flow in the umbilical cord during the portion of its heart beat when the heart relaxes. If it is severe enough, the flow in the umbilical cord momentarily stops or actually reverses direction during each heart beat.  At the same time, the flow in the arteries in the brain increases. Other blood vessels can be measured as well, yielding different information. We also measure the blood flow in the middle cerebral artery to determine whether the baby is anaemic.

Asymmetric growth restriction with normal head size and decreased femur length and abdominal circumference.

Increased resistance in umbilical artery and decreased resistance in middle cerebral artery.

Doppler ultrasound of blood flow in the middle cerebral artery.

Increased blood flow in the middle cerebral artery in an anaemic fetus.