After all these years, we still don't know what causes morning sickness. But new research indicates that at least it might be good for us.

Morning sickness, of course, is the wave of vomiting and nausea that affects most women during the early part of the day in the first four months or so of their pregnancies, and some women for their entire term.

The study, by the National Institutes of Health, offers evidence of a link between morning sickness and a lower risk of miscarriage. It was conducted by researchers at NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and other institutions. The results have been published in JAMA Internal Medicine.

“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 the study’s first author, Stefanie N. Hinkle, PhD, a staff scientist in NICHD’s Epidemiology Branch. “Our study evaluates symptoms from the earliest weeks of pregnancy, immediately after conception, and confirms that there is a protective association between nausea and vomiting and a lower risk of pregnancy loss.”

Hinkle and her team examined data from the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial. That study looked at whether women had experienced one or two pregnancy losses and who took a dose of aspirin each day were less likely to have a loss in the future.

The scientists only looked at data from the women in the study who had a positive pregnancy test. Daily, these women recorded their experiences of nausea and vomiting in the second through the eighth week of their pregnancies.  They also filled out a monthly questionnaire detailing their symptoms through the thirty-sixth week of their pregnancy. Hinkle and her colleagues noted that most previous studies on pregnancy loss and were not able to obtain such detailed information on symptoms in the early weeks of pregnancy. Usually the research relied on the women’s recollection of symptoms much later in pregnancy or after they had experienced a loss.

797 women had positive pregnancy tests in the EAGeR trial; 188 pregnancies ended in loss. By the end of the second month of pregnancy, 57.3 percent of the women reported experiencing nausea and 26.6 percent reported nausea with vomiting. compared to those who had not experienced nausea alone or nausea accompanied by vomiting, these women were 50 to 75 percent less likely to experience a pregnancy loss.