That's not enough to prove no risks exist, Chambers noted.
The researchers did account for other factors in a healthy pregnancy, such as the women's age and income and whether they smoked. But they couldn't factor in everything that affects pregnancy outcomes, Chambers said.
Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City, agreed. "It's an observational study, but that's what we have to go on," she said.
The best kinds of studies for showing a drug's benefits and risks are controlled clinical trials, where patients are randomly assigned to take a medication or not. It is generally considered unethical to include pregnant women in such studies, however, so doctors have to rely on studies like the current one -- records-based studies that look at whether pregnant women who used a given medication show any increased risks.
That's not ideal, but the new findings are reassuring, Wu said.
"This is a drug we've been using for a while," she said. "It can help keep women out of the hospital, and help keep them functioning on a day-to-day basis." For some women, Wu said, nausea and vomiting can be so severe that they can't get out of bed.
It's estimated that more than half of all pregnant women suffer from morning sickness. Usually, women can manage it without medication by, for example, eating smaller meals throughout the day, avoiding spicy foods and steering clear of strong smells that nauseate them.
But for some women, the problem is debilitating, and a small percentage develop hyperemesis, where a woman cannot keep any food down and she begins to lose weight.
Ondansetron fights nausea by lowering activity in the hormone serotonin. In the United States, it's the most commonly prescribed anti-nausea drug, and it is officially approved to treat nausea from cancer chemotherapy, radiation therapy and surgery. Its use against morning sickness has been based on only limited safe
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