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Examining Babies before Birth

Ultrasound is under fire, but there are other important new tests

For years, ultrasound testing has been as much a part of a pregnant woman's experience as stepping on the scale at the ob-gyn's. The test to diagnose birth defects is routinely given to over two-thirds of all mothers-to-be. But a recent study shows that the benefits of ultrasound may have been oversold.

Along with detecting anatomical abnormalities such as heart defects in the fetus, ultrasound has been used to determine a fetus's age, rate of growth, heart and breathing rate, and position of the fetus and placenta within the womb. It is also a way of establishing the sex of the fetus, gauging the amount of amniotic fluid in the uterus, and determining whether there is more than one child on the way.

The new study by researchers at the University of Missouri at Columbia divided 15,500 women with low-risk pregnancies into two groups - those who had two ultrasound tests during their pregnancies, at four months and at seven months, and those who either didn't have the test or had it only when there was a specific medical need.

The six-year study, the largest ever to examine prenatal testing, found that there was no difference between the two groups in the incidence of premature babies, lung failure, spinal chord injury, infection, probllem deliveries, or prolonged hospital stays. What's more, both groups had the same rate of birth defects - 5 percent. The study's conclusion: Although ultrasound testing, or a sonogram as it is also known, detects more fetal abnormalities, getting the test doesn't change the odds of delivering a baby with birth defects because most birth defects aren't detected within 24 weeks of the onset of pregnancy, which is the cutoff time for an abortion in most states. "Our study shows that most normal pregnant women will have a healthy baby regardless of ultrasound screening," says the study's lead investigator, Dr. Bernard G. Ewigman.

While the routine use of ultrasound is being called into question, other prenatal tests for birth defects and genetic disorders have never shown more potential. They include amniocentesis, alpha-fetoprotein testing, and the latest procedure - percutaneous umbilical blood sampling, or PUBS.

The tests are regularly recommended for: women 35 and over; women with a family history of genetic disorders; women who have had a previous child with a birth defect; women who have been exposed to a virus known to cause birth defects; women who ingest substances such as alcohol or certain medicines during pregnancy or at the time of conception; and women who were insulin-dependent prior to pregnancy. A breakdown of the procedures:

Amniocentesis: The procedure can be done as early as the 12th and as late as the 18th week of pregnancy. The process requires that a needle be inserted into the woman's abdomen and amniotic fluid withdrawn so that it can be tested. Since the genetic makeup of the cells in the amniotic fluid are the same as those of the fetus, the cells can be tested for several chromosonal abnormalities, including those indicating the presence of Down syndrome, a form of severe mental retardation, and spina bifida, a central nervous system disorder that prevents the spinal column from developing fully.

With amniocentesis, there is a 1 in 200 chance of miscarriage. Some women have cramps after the test, and, in rare cases, there is a chance that the fetus will be injured, according to the American College of Obstetricians and Gynecologists. And test results, which take two to four weeks, may show an innacurate "abnormal" reading if the fetus is older or younger than had been estimated.

Alpha-fetoprotein: This involves a simple blood test given to women who are 15 to 18 weeks pregnant. Also known as maternal serum screening, the simple test uses blood taken from a woman's arm to measure the amount of alpha-fetoprotein - a protein produced by the fetus - in the mother's blood. Unusually low levels of the chemical are linked to chromosome disorders. When levels of the chemical are higher than normal, it can mean that the fetus has a neural tube defect such as spina bifida. But alpha-fetoprotein levels can also be elevated if the fetus is merely older than originally thought, or if there are twins. When alpha-fetoprotein levels are suspicious, the test is usually followed by a second test such as amniocentesis.

PUBS: The percutaneous umbilical blood sampling procedure, introduced in 1983, uses blood drawn from the fetus to determine if it has chicken pox, anemia, a form of pneumonia known as cytomegalovirus, or toxoplasma gondii, a potentially fatal parasite. The test can also be used to identify several genetic defects, including Down syndrome and spina bifida. The process takes about five minutes, unless a blood problem is found in the fetus and a transfusion is required. Test results can be processed quickly - 2 days as opposed to at least 10 for amniocentesis, although the risk of fetal death from the test is slightly higher for PUBS - 1 to 1.5 percent as opposed to a half of 1 percent for amnio. Since amnio can be done as early as the twelfth week of pregnancy, and PUBS can't be done until the 17th or 18th week, amnio remains the primary test for identifying potential genetic abnormalities.


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Updated on October 8, 1995 editor@obs-us.com