Months before delivery, a fetal ultrasound may give you the chance to study your baby's profile and watch his or her tiny beating heart in action. But this first glimpse of your developing baby isn't meant primarily to provide parental thrills or souvenir snapshots. A fetal ultrasound helps your health care provider evaluate your baby's growth and development and determine how your pregnancy is progressing.

For most healthy women with normal pregnancies, routine ultrasounds don't affect the outcome of the pregnancy. So why are they done so often?

Your health care provider may use a fetal ultrasound to:

• Confirm the pregnancy and its location. Some embryos develop in the fallopian tube instead of in the uterus. An ultrasound exam can help your health care provider detect and treat a tubal (ectopic) pregnancy before it endangers your health.
• Determine your baby's gestational age. Knowing the baby's age can help your health care provider more accurately 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 may be done to make sure.
• Evaluate your baby's growth. Your health care provider can use ultrasound to make sure 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. The placenta provides your baby with vital nutrients and oxygen-rich blood. Any problems with the placenta need special attention.
• Identify possible fetal abnormalities. An ultrasound can help your health care provider detect many congenital abnormalities. An early diagnosis may lead to early interventions that help save or improve a baby's life.
• Investigate bleeding and other worrisome signs or symptoms. If you're bleeding or having other complications, an ultrasound may help your health care provider find out what's going on.
• Perform other prenatal tests. Your health care provider may use ultrasound to guide needle placement during certain prenatal tests, such as checking a sample of amniotic fluid for specific genetic problems (amniocentesis) or testing a sample of the placenta for genetic abnormalities (chorionic villus sampling).

Ultrasounds aren't recommended simply to determine a baby's sex — but it may be a bonus when an ultrasound is done for medical reasons.

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

During a fetal ultrasound, high-frequency sound waves are directed at the tissues in your abdominal area. These sound waves bounce off the curves and variations in your body, including your baby. The sound waves are visually translated into a pattern of light and dark areas — creating images of your baby on a monitor and on film.

Various types of ultrasound exams are available.

• Standard ultrasound. This type creates two-dimensional images that help your health care provider determine your baby's gestational age and evaluate your baby's growth and development. Most ultrasounds reveal a healthy, normal baby — or sometimes multiples. A standard prenatal ultrasound exam usually takes about 20 minutes.
• Advanced ultrasound. This type is similar to a standard ultrasound, but the exam targets a suspected problem and uses more sophisticated equipment. An advanced ultrasound may take from 30 minutes to several hours.
• Transvaginal ultrasound. With this type of ultrasound, the health care provider places a slender, wand-like device 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.
• Three-dimensional (3-D) ultrasound. This newer type of ultrasound offers 3-D images, sometimes with photo-quality details. It's currently used in selected medical centers to help health care providers further evaluate images from advanced ultrasounds.
• Doppler imaging. This type measures slight changes in the ultrasound waves as they bounce off moving objects, such as blood cells. This type of imaging can provide details about circulation — which is particularly helpful if you have high blood pressure or your baby is growing more slowly than expected.
• Fetal echocardiography. This type provides a detailed picture of your baby's heart. It may be used to confirm or rule out a congenital heart defect.

If you're having a standard ultrasound, you'll probably be asked to arrive with a full bladder. This eliminates pockets of air between your uterus and bladder, which helps create the clearest images.

Your health care provider or technician will begin by applying a special gel to your abdomen. The gel improves conduction of sound waves and eliminates air between your skin and the transducer — a small plastic device that sends out the sound waves and receives those that bounce back.

Then, the transducer will be moved back and forth over your abdomen. The sound waves reflected off your bones and other tissues will be digitally 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 may print certain images to document important structures.

Depending on your baby's position and stage of development, you may be able to make out a face, tiny hands and fingers, or arms and legs. Don't worry if you can't "see" your baby. Ultrasound images are hard for an untrained observer to decipher. Ask your health care provider or technician to explain what's on the screen.

You'll probably be given copies of some of the images. Some clinics may offer a videotape of the ultrasound as well.

Fetal ultrasound can be done at any point during pregnancy.

If your health care provider suspects an ectopic pregnancy or other problems, you may need a transvaginal ultrasound soon after you find out you're pregnant. A Doppler ultrasound may detect your baby's heart beat as early as six weeks into your pregnancy.

Routine fetal ultrasounds are typically done between 18 and 20 weeks, when many anatomic details are visible. If your baby's health needs to be monitored more closely, ultrasounds may be repeated throughout the pregnancy.

Typically, a fetal ultrasound offers reassurance that your baby is growing and developing normally. If your health care provider wants more details about your baby's health, he or she may recommend an advanced ultrasound or other tests.