The report "confirms what most people accepted anyway, that, yes, there is an increased risk in congenital abnormality associated with assisted reproductive technology," said Dr. William Buckett, a professor at McGill University, who was not involved in the study.

The researchers did not determine why fertility treatments are tied to a higher risk of birth defects, or whether the technology is even responsible.

In vitro fertilization (IVF) has been available to would-be mothers for more than three decades, and numerous studies have looked at the potential hazards of these techniques.

IVF involves fertilizing the mother's egg outside of her body and implanting it in the womb.

Dr. Zhibin Hu at Nanjing Medical University and colleagues collected the results of 46 studies that compared the number of birth defects among children conceived using an IVF technique to children conceived normally.

For more than 124,000 children born through IVF, the risk of having a birth defect was 37 percent higher than that of the other children.

According to the Centers for Disease Control and Prevention, major birth defects, such as malformation of a limb or organ, occur in about three out of every 100 babies born in the U.S.

A 37 percent increase would bump up that rate to four out of every 100 babies.

The increase in birth defect risk was apparent across a variety of body parts and functions, including the genitals, skeleton, digestive system and the nervous system, the authors report in the journal Fertility and Sterility.

The question of why most studies find birth defects more common among IVF-conceived babies remains to be answered.

"I think that it is very complex reasons that we don't fully understand yet," said Dr. Charlotte Hobbs, director of the Arkansas Center for Birth Defects Research and Prevention and a professor at the University of Arkansas for Medical Sciences, who did not participate in the review.

It's possible that the same reasons people have trouble conceiving and seek out fertility treatment could influence their increased risk of having a baby with a birth defect.

It's also possible that the IVF techniques themselves - the jostling and handling of the embryos, or the drugs that go along with fertility treatment - could be involved.

A third theory is that birth defects only appear to be more common in babies conceived through fertility treatments because they're monitored more closely than other babies, Buckett suggested.

"Couples who have had babies born as a result of IVF are followed up more closely, and therefore subtle abnormalities may be detected that otherwise might not have been detected," he told Reuters Health.

As far as trying to reduce the risk of birth defects for parents using IVF, Hu said in an email that "it is really too early to find out ways to reduce the risk, because the reasons accounting for the risk are largely unknown."

Hobbs said that working to understand why the risk of birth defects is elevated will help in developing ways to prevent them.

Hu added that strict screening of parents to see if IVF is appropriate for them and genetic testing are recommended.

Buckett said that it's important for physicians to explain the potential risks related to IVF to parents who opt for the procedure.

SOURCE: bit.ly/HW6WqZ Fertility and Sterility, online April 3, 2012.

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