For those with a high-risk pregnancy, your doctor may order one or more ultrasounds during the third trimester to assess your baby's wellbeing. During an ultrasound late in pregnancy, the doctor may want to review the position and appearance of the placenta, particularly if you've had bleeding, or if a low-lying placenta was noted at your 20-week ultrasound. If you had too much or too little amniotic fluid at a previous ultrasound, your doctor will likely order several ultrasounds in the third trimester to keep an eye on your baby's condition. If you are pregnant with multiples, have high blood pressure or high blood sugar (gestational diabetes) or experience reduced fetal movements, a third trimester ultrasound will usually be ordered as well.
During an ultrasound late in the third trimester, your practitioner can determine the baby's position -- head down or breech. This becomes important as you get closer to your delivery date. The doctor can also determine if a caesarean section may be necessary.
Some practices will do an elective third trimester ultrasound at about 35 weeks to determine the baby's weight. This may also be ordered as medically necessary if the baby is particularly large or small due to a medical condition or unknown reasons. At about 35 weeks, an ultrasound may be used as a fairly accurate predictor of your baby's birth weight. If your baby is particularly large (especially if you have diabetes), a caesarean section may be necessary.
If an ultrasound determines there are abnormalities with the baby's size, position or another condition, don't panic. You need to stay as calm as possible for the health of your baby. Finding out about birth defects, problems with the placenta, health problems or other issues now can help your health care provider determine how to deliver your baby safely and also prepare the proper care for your baby at birth.