Third trimester ultrasound

Most commonly, your practitioner would order a third-trimester ultrasound to:

Determine the cause of vaginal bleeding

If you're bleeding during the second half of your pregnancy, it may be due to problems with the placenta. An ultrasound can help your practitioner figure out what's going on.

Check on the baby's growth

Later in pregnancy (around 34 weeks), if there's any concern that your baby is not growing properly, your practitioner may schedule an ultrasound (or a series of them) to measure certain parts of his body.
In particular, the practitioner will be measuring the size of the baby's head, the length of his femur (thigh bone), and the distance around his midsection (the abdominal circumference).

Check on your amniotic fluid level

If you've been diagnosed with too much or too little amniotic fluid, your practitioner will likely order regular ultrasounds throughout your third trimester to monitor your baby's condition.

Check on the baby's well-being late in the pregnancy

If you have high blood pressure or diabetes or you're overdue, your practitioner may order a fetal biophysical profile to check on the baby's movements and breathing and to measure the amount of amniotic fluid.

Determine whether you should plan for a cesarean section

This may be the case if your baby is especially large (particularly if you have diabetes) or in an abnormal (breech) position or if the placenta is blocking its exit from the uterus.