(Reuters Health) - Pregnant women on antipsychotic drugs can continue taking most of those medications without worrying the pills will increase the risk of their newborns having birth defects, a new study suggests.

“We did not see a meaningful increase in risk for any of the drugs with the exception of risperidone,” said lead author Krista Huybrechts, of Brigham and Women’s Hospital and Harvard Medical School in Boston.

Women taking the antipsychotic risperidone were about 26 percent more likely than women not taking antipsychotic drugs to give birth to children with birth defects, the researchers report in JAMA Psychiatry.

There is no biological reason risperidone would increase the risk of birth defects while others don’t, Huybrechts said, adding that the finding is a safety signal that should be investigated.

“It definitely needs more study,” she said. “We did a lot of analyses to see if we could explain that observation and we couldn’t.”

Because of concerns surrounding the unknown effects of experimental drugs on women and fetuses, researchers have traditionally excluded pregnant women from clinical trials. That exclusion has led to a lack of information about the safety of some drugs during pregnancy, Huybrechts’ team writes.

For the new study, the researchers used data on over 1.36 million U.S. pregnant women enrolled between 2000 and 2010 in Medicaid, the government-run insurance program for the poor.

The researchers attempted to match women who were similar to each other, except for taking antipsychotic medications. Then they looked to see if the newborns of women taking antipsychotics were more likely to be diagnosed with a birth defect in the first 90 days of life than the children of women not using the drugs.

The researchers also looked for differences between outcomes depending on which of the two main types of antipsychotics women used. So-called typical antipsychotics work by similar mechanisms to one another, but the second-generation atypical antipsychotics act differently from their predecessors.

About 33 of 1,000 children not exposed to antipychotics in the womb were diagnosed with birth defects. That compared to about 38 per 1,000 children exposed to typical antipsychotics and about 45 children per 1,000 who were exposed to atypical antipsychotics.

But after adjusting the numbers to account for other factors that may increase the risk of birth defects, the researchers found no increased risk for either type of antipsychotic drug - with the exception of risperidone.

Dr. Katherine Wisner, who co-wrote an editorial accompanying the new research, said the amount of data and the methods used make this a “landmark” study.

“For women taking other medications besides risperidone, it is really solid data to show there is no identifiable increased risk of birth defects,” said Wisner, of the Northwestern University Feinberg School of Medicine in Chicago.

Huybrechts cautioned that people should not draw the conclusion that antipsychotics are completely safe during pregnancy. The research team only looked at birth defects, which is one potential outcome during pregnancy.

The team has plans to evaluate antipsychotic use for the risk of gestational diabetes and other pregnancy related outcomes, she said.

SOURCE: bit.ly/2bdxBnk and bit.ly/2bdxCHU JAMA Psychiatry, online August 17, 2016.