An ultrasound can be performed in the third trimester (after 28 weeks) for many reasons, primarily in order to assess the health and wellbeing of the fetus.

What can a third-trimester ultrasound show about my baby?

A third trimester ultrasound can show a lot of features about the health of the fetus at this late stage of your pregnancy.

• A third trimester ultrasound has the ability to measure the blood flow through the umbilical cord. This is a very useful assessment of fetal well-being as it directly provides information as to how the placenta is functioning.
• A third trimester ultrasound measures the size of the fetus to ensure the fetus is not too big (macrosomia) or too small (growth restriction).
• It measures the amount of amniotic fluid. There can be too much amniotic fluid (polyhydramnios) or too little amniotic fluid (oligohydramnios).
• It confirms presentation: head first (cephalic), bottom first (breech) or transverse (lying sideways).
• It confirms the position of the placenta to ensure that the placenta is not sitting too close to the cervix (placenta praevia).
• If there has been vaginal bleeding then an ultrasound can also be useful to look for blood clots in the uterus, particularly around the placenta.
• Occasionally, a third trimester ultrasound is performed to monitor for potential fetal abnormalities (although usually these are better assessed at the second trimester ultrasound).

As the fetus becomes larger it tends to adopt a more curled up position. This means that views of the limbs and face are often more difficult to obtain and three-dimensional pictures in particular are often not as pleasing as at the second trimester ultrasound.