Morning sickness may be one of the worst things about being pregnant, but researchers say they've confirmed the common wisdom: It's a good sign.
Women who have nausea and vomiting earlier in pregnancy were much less likely to have a miscarriage, a team of government researchers found.
They watched a group of women who had already had at least one miscarriage, and found those who got morning sickness were 50 percent to 75 percent less likely to have another miscarriage.
"It's a common thought that nausea indicates a healthy pregnancy, but there wasn't a lot of high-quality evidence to support this belief," said Stefanie Hinkle, a staff scientist at the National Institute of Child Health and Human Development, who led the study.
The team thinks they've linked the nausea to levels of a hormone that makes sure an embryo gets firmly embedded in a woman's uterus.
They used data from women taking part in a different study of pregnancy between 2007 and 2011.
That study started collecting information before they got pregnant — so the researchers could see even very early miscarriages that women may not have noticed if they hadn't been in a study.
"Among women with one or two prior pregnancy losses, nausea and vomiting were common very early in pregnancy and were associated with a reduced risk for pregnancy loss," Hinkle's team wrote in their report, published in the Journal of the American Medical Association's JAMA Internal Medicine.
By the second month of pregnancy, more than 80 percent of the women in the study had nausea or vomiting or both, they found.
Other studies had shown just the opposite, but they did not have data on the very earliest of pregnancies, or used a woman's memory of events. "Thus, in the absence of prospective data, nausea and vomiting may be no more than a sign of still being pregnant as opposed to a sign of the health of the pregnancy," they wrote.
Overall, about a quarter of the pregnancies ended in a miscarriage — on average at seven weeks, the team found.
They didn't find any indicator of anything that might cause a miscarriage, or that might cause the nausea and vomiting, outside of pregnancy and hormone levels. The only consistent pattern: Women under 25 were more likely to have morning sickness than older women.
"Our modeling strategy accounted for smoking, alcohol and caffeine intake, and stress at each week, suggesting that the mechanism is likely not through avoidance of such substances," Hinkle's team wrote.