The latest NIH study, which was a secondary analysis of data from a separate clinical trial, involved tracking symptoms logged daily by nearly 800 pregnant women. All of the women in the study had at least one previous pregnancy loss, with about one-third of the participants having experienced two losses.
About 84 percent of the women reported nausea, with or without vomiting, by the time they were eight weeks pregnant. (Smaller percentages of women had morning sickness earlier in pregnancy—with about 20 percent of them reporting illness at two weeks pregnant, and more than 50 percent of them reporting nausea or vomiting by five weeks.) Nearly one-quarter of the pregnancies resulted in miscarriage, many of which occurred before the eight-week mark.
Overall, the women who reported nausea by itself or nausea with vomiting were between 50 percent and 75 percent less likely to miscarry than those who didn’t feel sick.(Earlier research, including a 2014 meta-analysis of 10 separate studies conducted between 1992 and 2012, has also found that women who had morning sickness experienced fewer miscarriages and gave birth to larger, healthier babies with fewer birth defects.)
But even though earlier studies have found similar associations, few researchers have taken into account the other potential indicators for miscarriage among study participants—like the number of previous pregnancy losses a woman has experienced, alcohol intake during pregnancy, and fetal characteristics such as chromosomal abnormalities. Such factors might increase the likelihood of miscarriage, even among women who experience morning sickness. The NIH researchers controlled for these and other circumstances in their assessment, giving them a clearer picture that the association between morning sickness and reduced pregnancy loss is strong—without confounding factors getting in the way.
The NIH study was unusual, too, in that it began with women who were still trying to conceive—rather than first enrolling participants who were already pregnant. “This is important because it allowed us to get detailed data from diaries that women were keeping about their symptoms in the earliest weeks of pregnancy—even before most women knew they were pregnant,” said Stefanie Hinkle, a staff scientist at the NIH, and the lead author of the study. “We found that in the week after conception, one in five women were already experiencing some nausea symptoms.”
The protective benefit of morning sickness was stronger among women who were throwing up compared with those who just felt crummy—up to a point, anyway. Researchers didn’t include findings from women who experienced hyperemesis, a severe form of nausea and vomiting during pregnancy that can lead to hospitalization—meaning the study’s findings do not apply to women with the very worst symptoms. The study had some other limitations, too. The vast majority of participants were married, highly educated, white women—so the extent to which the findings would apply to women in other demographics is unclear.