Why it's done

First trimester ultrasound examination is done to evaluate the presence, size and location of the pregnancy, determine the number of fetuses, and estimate how long you've been pregnant (gestational age). Ultrasound can also be used for first trimester genetic screening, as well as screening for abnormalities of your uterus or cervix.

In the second or third trimester a standard ultrasound is done to evaluate several features of the pregnancy, including fetal anatomy. This exam is typically done between weeks 18 and 20 of pregnancy. However, the timing of this ultrasound might be altered for reasons such as obesity, which could limit visualization of the fetus.

During the second and third trimesters, limited ultrasound evaluation might be needed when a specific question requires investigation. Examples include the evaluation of fetal growth and the estimation of amniotic fluid volume. A specialized or detailed exam is done when an anomaly is suspected based on your history or other prenatal exam results.

Your health care provider might use fetal ultrasound to:

• Confirm the pregnancy and its location. Some fetuses develop outside of the uterus, in the fallopian tube. A fetal ultrasound can help your health care provider detect a pregnancy outside of the uterus (ectopic pregnancy).
• Determine your baby's gestational age. Knowing the baby's age can help your health care provider determine your due date and track various milestones throughout your pregnancy.
• Confirm the number of babies. If your health care provider suspects a multiple pregnancy, an ultrasound might be done to confirm the number of babies.
• Evaluate your baby's growth. Your health care provider can use ultrasound to determine whether your baby is growing at a normal rate. Ultrasound can be used to monitor your baby's movement, breathing and heart rate.
• Study the placenta and amniotic fluid levels. The placenta provides your baby with vital nutrients and oxygen-rich blood. Too much or too little amniotic fluid — the fluid that surrounds the baby in the uterus during pregnancy — or complications with the placenta need special attention. An ultrasound can help evaluate the placenta and amniotic fluid around the baby.
• Identify birth defects. An ultrasound can help your health care provider screen for some birth defects.
• Investigate complications. If you're bleeding or having other complications, an ultrasound might help your health care provider determine the cause.
• Perform other prenatal tests. Your health care provider might use ultrasound to guide needle placement during certain prenatal tests, such as amniocentesis or chorionic villus sampling.
• Determine fetal position before delivery. Most babies are positioned headfirst by the end of the third trimester. That doesn't always happen, though. Ultrasound imaging can confirm the baby's presentation so that your health care provider can discuss options for delivery.

Fetal ultrasound should be done only for valid medical reasons. Fetal ultrasound isn't recommended only to determine a baby's sex. Similarly, fetal ultrasound isn't recommended solely for the purpose of producing keepsake videos or pictures.

If your health care provider doesn't suggest a fetal ultrasound but you'd like the reassurance an ultrasound can provide, share your wishes with your care provider so that you can work together to determine what's best for you and your baby.