This is usually done at 30 to 37 weeks. The following details are assessed at this scan:
a) Measurements of head circumference, abdominal circumference and femur length to check if the biometric measurements of the fetus are appropriate for the gestational age of the fetus. Estimated fetal weight is obtained from abdominal circumference and femur length measurements, and estimates the actual weight with up to about 15% accuracy.
b) Detection of some late onset structural abnormalities (1 out of 200 structural abnormalities are detected after a normal first and second trimester scan - examples include congenital diaphragmatic hernia, hydrocephalus, renal pelvic dilatation or hydronephrosis, sacrococcygeal teratoma, some heart abnormalities like coarctation of aorta).
c) Doppler studies of the uterine, middle cerebral and umbilical arteries. Calculation of the cerebroplacental ratio (this is an assessment of the placental reserve) from 36 weeks onwards may be useful in timing the delivery of the fetus, especially when the cerebroplacental ratio is performed weekly. Recently, there is good data now to show that planning the timing of delivery based on the estimated fetal weight and the cerebroplacental ratio reduces the risk of severe morbidity to the baby (see the study here https://pubmed.ncbi.nlm.nih.gov/38219773/). Measurement of the uterine artery Doppler on the Astraia program that the clinic uses allows the risk assessment for pre-eclampsia before 36 weeks and at term.
d) 3D and 4D scans - These are only possible when the fetus is facing the front of the mother's abdomen and there is adequate amniotic fluid in front without obstruction from the limbs or umbilical cord. This may promote bonding with the parents.