New research confirms that women plagued by morning sickness in early pregnancy are less likely to miscarry.
But women who don't experience nausea and vomiting during their first trimester shouldn't be alarmed, Dr. Ronna L. Chan of The University of North Carolina at Chapel Hill, one of the study's authors, told Reuters Health.
"Not all pregnant women who go on to have successful pregnancies experience nausea and vomiting early on or at all," she said by e-mail. "In addition, pregnancy symptoms can vary from one pregnancy to the next, even for the same woman."
From 50 percent to 90 percent of women have morning sickness in early pregnancy, Chan and her team note in the journal Human Reproduction, and previous studies have found that women who have these symptoms are less likely to miscarry.
To investigate the relationship in more detail, Chan and her colleagues looked not only at the presence or absence of these symptoms, but how long the symptoms lasted, in more than 2,400 women living in three US cities.
"Our study had several advantages over some of the earlier studies because we recruited pregnant women very early in their pregnancies or when they were trying to become pregnant, so we were able to follow them over the course of their pregnancies and collect data regarding the timing and occurrence of nausea and vomiting early on," the researcher explained.
Eighty-nine percent of the women had some degree of morning sickness, while 53 percent had vomiting as well as nausea. Eleven percent of the women miscarried before 20 weeks.
The women who had no nausea or vomiting during their first trimester were 3.2 times as likely to miscarry as the women who did have morning sickness, Chan and her team found.
This relationship was particularly strong for older women; women younger than 25 who had no morning sickness were four times as likely to miscarry compared to their peers who had nausea and vomiting, while miscarriage risk was increased nearly 12-fold for women 35 and older with no morning sickness.
And the longer a woman had these symptoms, the lower her miscarriage risk, the researchers found; this association was especially strong among older women. Women 35 and older who had morning sickness for at least half of their pregnancy were 80 percent less likely to miscarry than women in this age group who didn't have these symptoms.
Still, because of the nature of the study, the authors could not prove that there was any cause-effect relationship between morning sickness and a healthier pregnancy, just that the two were linked.
A number of theories have been put forth to explain why morning sickness might signal a healthier pregnancy, Chan said. "Some postulate nausea and vomiting during pregnancy is a mechanism to help improve the quality of a pregnant woman's diet or a way to reduce or eliminate potentially harmful substances from the mother in order to protect the fetus," she explained.
While these ideas are "plausible," the researcher said, she thinks the symptoms reflect a pregnant woman's sensitivity to the sharp rise in certain hormones key for sustaining pregnancy that occurs during the first trimester.