A new study suggests that only older antidepressants, particularly Prozac and Paxil, could increase a pregnant woman's risk of delivering a baby with birth defects. The findings dismissed an earlier belief that all Selective Serotonin Reuptake Inhibitors (SSRIs) make babies vulnerable to various birth defects.

About one in 33 babies in the United States is born with a birth defect, according to the U.S. Centers for Disease Control and Prevention. Earlier studies also found that 20 percent of those born with birth defects do not survive. Some of the common defects include Down syndrome, cleft lip or palate, anancephaly, and gastrointestinal defects.

Researchers at the CDC looked at the data of 27,859 women who gave birth between 1997 and 2009 at 10 centers. About 35 percent, or 9,857, of the women delivered babies with birth defects, and 13 percent of them were taking antidepressants during the first trimester of the pregnancies. The commonly used SSRIs by the participants included Zoloft (sertraline), Prozac (fluoxetine), Paxil (paroxetine), Celexa (citalopram) and Lexapro (escitalopram), according to the Washington Post.

The analysis showed that women who took Prozac and Paxil were up to 3.5 times more likely to deliver babies with birth defects compared to those who were taking other drugs. Prozac was linked to abnormally shaped skull while Paxil was associated with protruded intestines outside the body and anencephaly; both were found to elevate heart defect risk, Reuters reported.

While the risks are actually small, the study proves that some antidepressants are safe to use for pregnant women. However, further research is needed because it didn't prove that antidepressants caused the birth defects.

"Women definitely should not stop taking the antidepressants they are on without talking to their doctors," Jennita Reefhuis, study author and an epidemiologist with the U.S. National Center on Birth Defects and Developmental Disabilities at CDC, said to USA Today. "But this analysis can help guide doctors and patients."

The study was published in the July 8 issue of the British Medical Journal.