This applies to ultrasound after 28 weeks, commonly much later. Some examples of reasons a third trimester ultrasound is:
The fetus, or the mother’s abdomen, feels too big or small for the stage of pregnancy.
The mother has a medical condition that can affect fetal growth and well-being, such as high blood pressure or gestational diabetes.
The placenta was low-lying on the midtrimester ultrasound.
The position of the fetus could be unusual eg breech.
Abnormal symptoms such as pain or bleeding.
A fetal abnormality noted at an earlier ultrasound requires follow-up.
Third trimester ultrasound (late ultrasound) includes:
Fetal size. An estimated fetal weight is calculated from measuring the fetal head, abdomen and femur. The weight ‘estimate’ is truly that, with one standard deviation being 15% either side of the true weight.
Some assessment of fetal anatomy, bearing in mind that late pregnancy limits the views and the check will not generally be a universal check like the 19-week ultrasound.
Placental position and maturity.
Amniotic fluid volume.
The Doppler blood flow in the umbilical cord, which gives an idea of placental function and reserve.
Other factors as requested by the referring doctor.
Late Pregnancy Ultrasound Images
Further late pregnancy images are seen under 3D-4D Ultrasound