However, the researchers - whose findings were published in the journal Fertility and Sterility - 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) - in which the mother's egg is fertilized outside of her body and then transferred to her womb - has been available to would-be mothers for more than three decades, and numerous studies have looked at the potential hazards of these techniques.

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 or using ICSI, in which a single sperm is injected directly into the egg, the risk of having a birth defect was 37 percent higher than that of the other children, they found.

"Children conceived by IVF and/or ICSI are at significantly increased risk for birth defects, and there is no risk difference between children conceived by IVF and/or ICSI," the team wrote.

According to the U.S. 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 United States.

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

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

The increase in birth defect risk was apparent across a range of functions and body systems, including the genitals, skeleton, digestive system and the nervous system, the authors reported.

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

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 said.

"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."

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." SOURCE: bit.ly/HW6WqZ

(Reporting from New York by Kerry Grens at Reuters Health; editing by Elaine Lies and Bob Tourtellotte)

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