Test-tube babies have double the usual risk of being underweight or having major birth defects, researchers say. But they say the findings should not necessarily discourage infertile couples from trying to conceive this way.

An Australian study found that almost one in 10 test-tube babies has birth defects, while a study by the U.S. Centers for Disease Control and Prevention found that newborns conceived this way tend to be smaller than usual. Being underweight can lead to physical and developmental disorders.

Neither study looked at why this was so. But the researchers said the problems might be caused by the women’s underlying fertility troubles or by the laboratory procedures themselves.

Test-tube, or in-vitro, fertilization is a common solution for many childless couples. Just under 1 percent of U.S. babies are conceived this way.

Some fertility experts questioned whether the latest studies exaggerate the risk. Even those who conducted the research said it should not necessarily dissuade couples from resorting to in-vitro fertilization.

“While the doubling in the risk is significant, you’ve got to remember that 90 percent of the babies didn’t have a major birth defect,” said Dr. Jennifer J. Kurinczuk, who led the Australian study. “We’re talking about things that happen anyway. They just seem to happen a little bit more commonly in this group than the other group.”

The two studies appear in today’s New England Journal of Medicine. Kurinczuk said some previous research pointed toward such birth defect risks from in-vitro fertilization, but this is the strongest evidence yet.

“These papers are the best of the lot so far to speak to these controversial issues,” said Dr. Allen A. Mitchell of the Boston University School of Public Health, who wrote an accompanying editorial. “Both of them suggest that there are greater risks associated with assisted reproductive technology than a lot of people thought.”