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 position.

  • Placental position

  • 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