Assisted reproductive technology is one of the great medical success stories of the late 20th century. Thanks to fertility drugs, in-vitro fertilization (IVF) and a growing list of even more sophisticated techniques, tens of thousands of healthy babies are born each year that otherwise might never have been conceived. But the process is neither foolproof nor risk free. There are limits to what science can do for infertile couples, and the more doctors have to intervene with drugs, needles and surgery to get sperm to meet egg, the greater the chance that something will go wrong. Among the pitfalls:

Ovarian Hyperstimulation
The first step in most assisted-fertilization techniques is to trick the ovaries into producing a lot of eggs at once. But the hormones doctors use to do this are powerful drugs and in rare cases can cause serious complications, including blood clots and kidney damage.

Multiple Gestation
Not being able to have a baby can be heartbreaking. But having too many at once can be even worse. About 20% to 35% of IVF pregnancies produce multiple fetuses, usually twins. Having more than two or three babies at once is often a medical disaster. Babies that develop in a crowded uterus or are born too early are at risk for a lifetime of developmental problems, including mental retardation, paralysis and blindness. Trying to reduce the number of fetuses through selective abortion has its own problems, not the least of which is an increased chance of miscarriage.

Low Birth Weight
Twins and triplets (not to mention septuplets) often weigh less than normal at birth. But a recent study from the U.S. Centers for Disease Control suggests that even single babies conceived through IVF are more likely to be born underweight. Whether that also puts them at greater risk of developmental problems is uncertain.

Birth Defects
An Australian study published in March reported that IVF children are twice as likely to suffer birth defects — such as cleft palate, a hole in the heart or kidney problems — as children conceived the usual way. Several earlier studies have shown no differences between the two kinds of babies, so further research is needed. Even if the apparent increase is real, it might not be clear whether the birth defects are caused by the artificial reproductive technology or by whatever underlying problem caused the infertility in the first place.

Even the most powerful techniques can turn back a woman's biological clock only so far. Women in their early 30s who want to use their own eggs have a better than 30% chance of delivering a live baby by artificial means. After age 43, the success rate drops to a forbidding 3%.

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