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Personal Health Jane Brody on health and aging.

More than 30 years ago, a 37-year-old friend of mine with an unplanned fourth pregnancy was told by her obstetrician that an amniocentesis was “too dangerous” and could cause a miscarriage. She ultimately bore a child severely affected by Down syndrome, which could have been detected with the test.

Today, my friend’s story would have a different trajectory. She would have a series of screening tests, and if the results suggested a high risk of Down syndrome, then an amniocentesis or chorionic villus sampling (C.V.S.) to make the diagnosis. She’d be given the option to abort the pregnancy.

In the future, a woman who decides to continue a Down syndrome pregnancy may also be offered prenatal treatment to temper the developmental harm to the fetus.

Prenatal diagnosis, today a routine part of obstetric care, has made great strides since the mid-1970s and is now on the cusp of further revolutionary developments.

In the nearly four decades since amniocentesis became widely accepted, new techniques have gradually improved the safety and accuracy of prenatal diagnosis. Prenatal tests for more than 800 genetic disorders have been developed. And the number of women who must undergo amniocentesis or C.V.S. has been greatly reduced.

The newest screening test, highly accurate and noninvasive, relies on fetal genetic fragments found in the mother’s blood. Available commercially from four companies, this test is so accurate in detecting Down syndrome that few, if any, affected fetuses are missed, and far fewer women need an invasive procedure to confirm or refute the presence of Down, according to studies in several countries.

The new test, done late in the first trimester of pregnancy, can also detect other genetic diseases, like extra copies of chromosomes 13 and 18, and a missing sex chromosome. It is not yet approved by the Food and Drug Administration, however, and the American College of Obstetrics and Gynecology currently recommends it only for women at high risk for having a baby with a chromosomal abnormality.

But most women can get the new screening test if a doctor orders it and they are willing to pay for it themselves, according to Dr. Diana W. Bianchi, a neonatologist and geneticist at Tufts University School of Medicine. Dr. Bianchi, who serves on the advisory board of a company selling fetal DNA tests, said she expects these tests will soon become routine for all pregnant women because, in addition to their “extraordinary accuracy” in detecting a Down syndrome pregnancy, they can be done earlier than other tests, and reduce costs and the risk of complications.

Down syndrome, which occurs in about one in every 700 births in the United States, is by far the most common chromosomal abnormality. It causes physical and intellectual disabilities that range from mild to severe.

In the past, the decision to undergo an amniocentesis or C.V.S. was based on a woman’s age or genetic history. The older the woman, the greater her risk of bearing a child with genetic defects due to an abnormal number of chromosomes.

It is now standard practice to offer all pregnant women a series of noninvasive screening tests in the first and second trimesters of pregnancy to assess the chances that a fetus has the extra copy of chromosome 21, which causes Down syndrome. It is then up to the woman to decide whether to undergo C.V.S. or an amniocentesis.

Taken together, these older screening tests pick up about 92 percent of cases of Down syndrome but miss 8 percent. And they yield false-positive results incorrectly indicating the presence of Down in about 5 percent of fetuses.

“This means a lot of women are needlessly worried and a lot have amnios that are not medically necessary,” Dr. Bianchi explained at a meeting last December of the March of Dimes.

Amniocentesis is performed between 15 and 20 weeks of gestation and involves inserting a long needle into a woman’s uterus to extract some of the fluid and cells surrounding the fetus. In C.V.S., usually done at 11 weeks gestation, the fetal cell sample is taken from the placenta. The cells are then analyzed for possible genetic defects. There is a small risk of miscarriage associated with both procedures.

But the new tests of fetal DNA from the mother’s blood detect all or nearly all cases of Down syndrome, and they return false-positive results in fewer than 1 percent of cases. Only those with a positive result need a C.V.S. or amniocentesis to confirm a Down syndrome pregnancy, and only about one woman in 1,000 who are tested requires such an invasive procedure to learn that her fetus does not have Down syndrome.

In a recent article in The New England Journal of Medicine, however, Stephanie Morain, a doctoral candidate at Harvard who studies medical ethics, and her co-authors said the fetal DNA tests have some disadvantages. They miss some chromosomal abnormalities detected by standard screening techniques, and they are “not widely covered by insurance.” Prices for the tests range from about $800 to more than $2,000, although some companies offer “introductory pricing” specials at about $200.

The new tests are also not required to meet the strict standards of safety and effectiveness established by the F.D.A., and they are valid only in singleton pregnancies. Nonetheless, tens of thousands of the fetal DNA tests have been performed to date.

The new test requires just a small sample of a pregnant woman’s blood. During pregnancy, a woman’s blood contains cell-free DNA from herself and her unborn baby. On average, Dr. Bianchi said in an interview, at around 10 weeks of gestation, about 10 to 12 percent of the DNA in a woman’s blood will be fetal DNA from the placenta.

Using modern genetic sequencing techniques, the fetal DNA can be rapidly analyzed at a relatively low cost, in part because multiple samples from different women can be examined simultaneously.

As with other screening tests, the new DNA tests are “not diagnostic,” Dr. Lee P. Shulman, a geneticist at Northwestern University Feinberg School of Medicine, emphasized. A positive result on the test must always be confirmed by a C.V.S. or amniocentesis. Dr. Bianchi is now most excited by the prospect of treating a Down syndrome fetus before birth to minimize the disorder’s neurological effects. Studies have shown that giving pregnant mice medication that counters fetal oxidative stress results in more normal brain development of pups with Down syndrome.

When prenatal testing reveals that a fetus has a serious birth defect, some women may consider ending their pregnancies. The March of Dimes recommends that those who continue:

* Consult a specialist in maternal-fetal medicine who is specially trained to treat high-risk pregnancies.

* Arrange to give birth in a hospital with a Level III newborn intensive care unit (NICU) staffed and equipped to provide the expert care needed by infants with special needs.

* Discuss the best way to deliver the baby with the doctor; a cesarean section is safer than a vaginal birth for babies with certain birth defects.

* Consult a pediatrician who specializes in the kind of care (for example, cardiac or neurological) that the baby will need after birth.

* Join an organization or volunteer group that can provide support and education about how best to care for the baby’s specific needs.

This is the first of two columns on healthy babies. The second one can be found here.