An ultrasound gives your doctor -- and you -- a high-tech peek at your developing baby. During the procedure, high-frequency sound waves are directed at your baby. When the sound waves encounter your baby’s organs or tissues (or even his bloodstream, in one type of ultrasound), they bounce off, creating echoes that are converted into images. The images are displayed on a computer monitor, where your baby is suddenly the star of his own reality show.

The main purpose of an ultrasound, though, is not to catch the first glimpse of your baby or even to discover the sex of your child -- as thrilling as these disclosures might be. The ultrasound’s real job is to measure the growth and development of your baby and check on the health of your reproductive organs and placenta. Your doctor also uses the ultrasound images to detect or rule out certain birth defects.

While an increasingly common procedure since the 1970s -- around 70 percent of pregnant women in the United States today have at least one ultrasound -- this prenatal diagnostic procedure is not considered routine. So don’t worry if yours is a low-risk pregnancy and your practitioner doesn’t think an ultrasound is necessary. Studies have found that women with low-risk pregnancies who don’t have an ultrasound are no more likely to have babies with birth defects than women who do use the technology. Further reassurance comes from the American College of Obstetricians and Gynecologists: The group does not consider an ultrasound necessary for every pregnant woman.

A standard ultrasound is typically performed between the 18^th and 20^th weeks of pregnancy because by then your baby is developed enough to be examined in detail but not yet so big that it's hard to estimate his age. At this stage, many structural defects can be spotted in the brain, spine, stomach, kidneys, and bladder, along with all four chambers of the heart.

The procedure can be done at various times during a pregnancy to gather specific information, including:

First trimester

•8-12 weeks: to confirm a pregnancy and make sure it isn’t developing in the fallopian tube, and to estimate the age of the fetus

•10-13 weeks: to help guide the doctor during diagnostic procedures like chorionic villus sampling (CVS) and to check for possible defects of the brain and spinal cord

Second trimester

•16-20 weeks: as a guide during amniocentesis

•18-20 weeks: to assess the baby’s growth, development, and age, identify multiple pregnancies, and to check for birth defects

Third trimester

•24 weeks on: to measure the volume of amniotic fluid (too much or too little can indicate problems with fetal development or the pregnancy itself)

•24 weeks on: to evaluate the baby’s growth, heart rate, urine output, muscle tone, and activity if there is cause for concern

An ultrasound may be done during pregnancy for a variety of reasons, including to:

•Confirm a pregnancy and its location. Some embryos develop in the fallopian tube instead of the uterus. Known as an ectopic, or tubal pregnancy, early treatment is essential for the safety of the mother.

•Date the pregnancy. By measuring the size of your baby from crown to rump the technician can estimate his age.

•Rule out or determine miscarriage. An ultrasound can explain bleeding in early pregnancy or lack of movement later on.

•Detect possible defects.

•Evaluate the baby’s growth.

•Determine the position of the baby and the development of the placenta.

•Discover the sex of the baby. Some parents find out the gender of their baby as an exciting by-product of having an ultrasound for other reasons.

Doctors can do quick ultrasounds in the office, but for a standard more comprehensive test, a full bladder is necessary. This eliminates air pockets between the uterus and bladder that can distort the sound waves and produce unclear images.

It's best to wear a two-piece outfit so you won’t have to completely undress. You’ll lie on your side or back on a table and a gel will be spread on your abdomen. The technician will press down gently but firmly on the tranducer, while moving the device across your entire abdomen during the course of the exam. This may feel somewhat uncomfortable due to your full bladder, but it won’t be painful.

During the test, the technician will watch the images of your baby on the monitor and measure the baby’s head, abdomen, and thighbone, as well as the length from crown to rump. The technician will also look for evidence of any birth defects in your baby’s organs, limbs, or bones. During the procedure, the sonographer will capture some still images for review later on.

To the untrained eye, an ultrasound can look like lots of grainy black and white blobs. But the technician performing the procedure -- a radiologist, sonographer, or obstetrician -- can help you find a tiny face or fingers in those fuzzy pictures. She may even give you a snapshot to keep.

Yes, a variety of ultrasound tools are available to track the health and well being of your developing child. Although the techniques may vary, the process is essentially the same.

The most common, called a standard or level I ultrasound, offers basic information about the tissues, organs, and bones of your baby and takes about 20 minutes.

Advanced or targeted ultrasound (sometimes called level II or comprehensive ultrasound) is used to further investigate a suspected problem. It's more in-depth and takes longer -- from 30 minutes to several hours.

If your doctor suspects your baby may have birth defects, she may order a new technology called three-dimensional (3-D) ultrasound, which produces still images that are almost as detailed as photographs.

Taking the technology one step further, 4-D ultrasound produces a moving-picture version of 3-D ultrasound. The US Food and Drug Administration (FDA) notes that companies seeking to capitalize on this new technology are cropping up in shopping centers around the country, offering parents prenatal keepsake images for a fee. Because the long-term effects of repeated ultrasound aren’t known, the FDA says that a woman should have an ultrasound only when her doctor recommends it.

Another technology, Doppler ultrasound, targets the baby’s blood flow. A specialized Doppler transducer is used to measure the direction and speed of blood cells in the placenta. This information helps your practitioner determine if blood is flowing smoothly and delivering oxygen to all parts of the baby. This can be a particular concern for women who have high blood pressure or diabetes.

If your doctor suspects your baby might have a heart defect, he or she may suggest a fetal echocardiography -- a special ultrasound that zeroes in on the baby’s heart. Most ultrasounds examine the structure and function of the heart, but fetal echocardiography provides more detailed pictures that can help your doctor rule out a problem or define it more clearly.

Ultrasound is considered safe for both mother and child, since the procedure uses sound waves to gather information -- unlike an X-ray, which requires potentially harmful radiation. Some small studies have suggested, however, that ultrasound may have an impact on child development, possibly causing speech delays or a tendency for left-handedness, but further research is needed.

-- Sarah Henry is an award-winning health writer specializing in parenting and social issues. A former staff writer at the Center for Investigative Reporting, she has also reported on health issues for Hippocrates, Time Inc. Health, the Washington Post, the Los Angeles Times Magazine, and television programs such as "60 Minutes" and PBS's "Health Quarterly."

References

First published July 26, 2005
Last updated April 1, 2008
Copyright © 2005 Consumer Health Interactive

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