Tough to believe, but doctors still don't know exactly what causes morning sickness, that nausea and vomiting that plagues pregnant women in the first few months of pregnancy. But what they do know, is that morning sickness is a Good Thing.

In a study done at the US National Institute of Child Health and Human Development in Bethesda, Maryland, those who endured the nausea and vomiting of morning sickness were 50 to 75 percent less likely to experience a pregnancy loss.

“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. “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.”

The researchers used data available from the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial. The study focused on 797 women who had already lost one or two pregnancies. All were newly pregnant, as confirmed by a urine test. Over the course of the investigation, 188 of the pregnancies ended in loss.

The researchers examined data from all the women in the study who had a positive pregnancy test. The women kept daily diaries, tracking whether they experienced morning sickness from the 2nd through the 8th week of their pregnancies. They then responded to a monthly questionnaire on their symptoms through the 36th week of pregnancy. The scientists cited this as one of their advantages over most previous studies on nausea and pregnancy loss, whose authors were not able to obtain such detailed information on symptoms in these early weeks of pregnancy. Instead, most of studies had relied on the women’s recollection of symptoms much later in pregnancy or after they had experienced a pregnancy loss.

By the 8th week of pregnancy, 57.3 percent of the women reported experiencing nausea and 26.6 percent reported nausea with vomiting. The researchers found that these women were 50 to 75 percent less likely to experience a pregnancy loss, compared to those who had not experienced nausea alone or nausea accompanied by vomiting.

The study was published in JAMA Internal Medicine.