Recent 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.
Not all pregnant women who go on to have successful pregnancies experience nausea and vomiting early on or at all. 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 and previous studies have found that women who have these symptoms are less likely to suffer miscarriage.
To investigate the relationship in more detail, researchers 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. The study had several advantages over some of the earlier studies because pregnant women were recruited very early in their pregnancies or when they were trying to become pregnant, so they 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. 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 had a spontaneous abortion before 20 weeks.
It was found that the women who had no nausea or vomiting during their first trimester were 3.2 times more likely to miscarry as the women who did have morning sickness. 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 was her miscarriage risk. 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.
However, because of the nature of the study, the authors could not prove that there was a 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.