Third Trimester Ultrasound
Not all pregnant women need a third trimester ultrasound. Some of the reasons why a third trimester ultrasound may be requested are listed below:
If clinically the fetus feels larger or smaller than expected for the gestation
If the mother has a medical condition that may influence fetal growth and well-being
If the placenta was low-lying on the midtrimester ultrasound.
If the clinician is uncertain about the position of the fetal head
If there is pain of bleeding
If the fetus is not moving well
If a fetal abnormality was noted on an earlier ultrasound
On a third trimester ultrasound we always check:
Fetal anatomy, although late in pregnancy the views of certain parts of the fetus may be difficult.
Fetal size. An estimated fetal weight is calculated from measuring the fetal head, abdomen and femur. There can be a 15% error on the weight estimate.
Fetal well-being. We check fetal movement, tone, breathing and liquor volume and give the fetus a biophysical score which is indicative of its well-being. The maximum score on ultrasound is 8/8.
The blood flow in the umbilical cord, which gives an idea of placental function