A fetal ultrasound can be done at any point during pregnancy. Your health care provider might use a fetal ultrasound to:
- Confirm the pregnancy and its location. Some embryos develop in the fallopian tube instead of in the uterus. A fetal ultrasound can help your health care provider detect a tubal (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 as well.
- 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.
- Identify birth defects. An ultrasound can help your health care provider detect various birth defects.
- Investigate signs or symptoms. 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. A C-section might be needed if the baby is in an abnormal position.
If your health care provider doesn't suggest a fetal ultrasound but you'd like the reassurance the exam can provide, share your wishes as you work together to determine what's best for you and your baby.
Routine fetal ultrasounds are considered safe for both mother and baby. Researchers haven't noted any adverse effects of fetal ultrasounds in children followed for several years after birth.
Still, caution remains important. The use of fetal ultrasound solely to create keepsakes isn't recommended.
Fetal ultrasound also has limitations. Fetal ultrasound might not detect all birth defects — or might incorrectly suggest a birth defect is present when it's not.
You might be asked to drink a certain amount of fluid or avoid urinating before a fetal ultrasound, depending on the type of ultrasound. When scheduling your ultrasound, ask your health care provider for specific instructions.
Also be aware that fetal ultrasound can be done through the vagina (transvaginal) or over the abdomen (transabdominal), depending on why it's being done or the stage of your pregnancy. If you're having a transabdominal ultrasound, consider wearing loosefitting clothing so that you can easily expose your abdomen.
There are two main types of fetal ultrasound exams:
- Transvaginal ultrasound. With this type of fetal ultrasound, a wand-like transducer is placed in your vagina to send out sound waves and gather the reflections. Transvaginal ultrasounds are used most often during early pregnancy, when the uterus and fallopian tubes are closer to the vagina than to the abdominal surface.
- Transabdominal ultrasound. A transabdominal fetal ultrasound is done by moving a transducer — a small plastic device that sends and receives sound waves — over your abdomen. This type of fetal ultrasound helps your health care provider determine your baby's gestational age and evaluate your baby's growth and development. The exam usually takes about 20 minutes.
Various other types of transabdominal ultrasounds are available, including:
- Specialized or targeted ultrasound. This type of fetal ultrasound targets a suspected problem and uses more sophisticated equipment than does a standard fetal ultrasound. The exam might take from 30 minutes to several hours.
- 3-D ultrasound. A 3-D fetal ultrasound can provide images of a baby with photo-quality details. This type of ultrasound is sometimes used to help health care providers evaluate a baby's growth and development, as well as detect facial abnormalities or neural tube defects.
- Doppler ultrasound. A Doppler ultrasound measures slight changes in the ultrasound waves as they bounce off moving objects, such as blood cells. A Doppler ultrasound can provide details about a baby's circulation.
- Fetal echocardiography. This type of fetal ultrasound provides a detailed picture of a baby's heart. It might be used to confirm or rule out a congenital heart defect.
During the exam
During a transabdominal fetal ultrasound, you'll recline on an exam table and expose your abdomen. Your health care provider or technician will apply a special gel to your abdomen. This will improve the conduction of sound waves and eliminate air between your skin and the transducer — the small plastic device that sends out sound waves and receives those that bounce back.
Your health care provider or technician will move the transducer back and forth over your abdomen. The sound waves reflected off your bones and other tissues will be converted into black-and-white or gray images on a monitor.
Your health care provider or technician will measure your baby's head, abdomen, thighbone and other structures. He or she might print or store certain images to document important structures. You'll likely be given copies of some of the images.
Depending on your baby's position and stage of development, you might be able to make out a face, hands and fingers, or arms and legs. Don't worry if you can't "see" your baby. Ultrasound images can be hard for an untrained observer to decipher. Ask your health care provider or technician to explain what's on the screen.
The procedure for other types of fetal ultrasound exams is similar. If you're having a transvaginal ultrasound, however, you'll be asked to change into a hospital gown or undress from the waist down. You'll recline on an exam table and place your feet in stirrups. Your health care provider or technician will place a lubricated transducer in your vagina rather than moving it across your abdomen.
After the exam
You can wipe off any residual gel or lubricant. If you had a full bladder during the ultrasound, you'll be able to urinate after the exam.
Typically, a fetal ultrasound offers reassurance that a baby is growing and developing normally. If your health care provider wants more details about your baby's health, he or she might recommend additional tests.