Your obstetrician may refer you for a scan in your third trimester for a variety of reasons:
• Growth and wellbeing of the baby
• Fluid loss
• Twins/multiple pregnancy
• Diabetic/Gestational Diabetes
• History of small or large baby
• Low lying placenta
• Pain or decreased fetal movement
• High blood pressure
An ultrasound in the third trimester has the ability to give important information to your obstetrician through the use of the Doppler assessment and by evaluating the fetal biophysical profile using the clinical history and examination. Fetal biophysical profiling is a test which attempts to gauge the amount of oxygen the fetus is getting via the placenta. This test will look at fetal movement, tone, baby’s heart rate and amniotic fluid volume. Placental insufficiency (reduced efficiency) can be detected by measuring the rate of blood flow through the main artery of the baby’s brain. This scan can also estimate fetal size, amount of amniotic fluid, placental position and blood flow patterns in the cord and ductus venosus (liver vein).
Because the baby is now occupying much more space in the uterus, images may not be as clear as previous scans. Quality of images in the third trimester rely on the fetal position and the mother's BMI. Images are greatly improved when there is a moderate amount of fluid in front of the fetal face.