A fetal ultrasound (sonogram) is an imaging technique that uses sound waves to produce images of a fetus in the uterus. Fetal ultrasound images can help your health care provider evaluate your baby's growth and development and monitor your pregnancy. In some cases, fetal ultrasound is used to evaluate possible problems or help confirm a diagnosis.

The first fetal ultrasound is usually done during the first trimester to confirm the pregnancy and estimate how long you've been pregnant. If your pregnancy remains uncomplicated, the next ultrasound is typically offered during the second trimester, when anatomic details are visible. If a problem is suspected, a follow-up ultrasound or additional imaging tests, such as an MRI, might be recommended.

There are two main types of fetal ultrasound exams:

• Transvaginal ultrasound. With this type of fetal ultrasound, a wandlike device called a transducer is placed in your vagina to send out sound waves and gather the reflections. Transvaginal ultrasounds are used most often during early pregnancy. This type of ultrasound also might be done if a transabdominal ultrasound didn't provide enough information.
• Transabdominal ultrasound. A transabdominal fetal ultrasound is done by moving a transducer over your abdomen.

Various other types of transabdominal ultrasounds are available, including:

• Specialized sonographic evaluation. This type of exam might be needed in specific circumstances, such as when a fetal abnormality is known or suspected. In this situation, a more detailed evaluation can provide additional information about the abnormality.
• 3D ultrasound. This exam provides a two-dimensional display of three-dimensional data. This type of ultrasound is sometimes used to help health care providers 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. It can provide details about a baby's blood flow.
• Fetal echocardiography. This exam provides a detailed picture of a baby's heart. It might be used to confirm or rule out a congenital heart defect.

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.

Diagnostic ultrasound has been used during pregnancy for many years and is generally considered safe when used appropriately. The lowest amount of ultrasound energy that provides an accurate assessment should be used.

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 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.

During the procedure

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.

Your health care provider or technician will move or scan the transducer back and forth over your abdomen. The sound waves reflected off your bones and other tissues will be converted into images on a monitor.

Your health care provider or technician will measure your baby's anatomy. 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. The transducer will be covered in a plastic sheath, like a condom, and be lubricated with gel. Your health care provider or technician will place the transducer in your vagina.

After the procedure

You can wipe off any residual gel or lubricant. If you had a full bladder during the ultrasound, you can 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.