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Morning sickness linked to lower risk of pregnancy loss

When the researchers dug down further into the data, they found that, overall, the women who reported nausea, with or without vomiting, were between 50 percent and 75 percent less likely to miscarry than those who didn’t feel sick.

Here’s some consolation for the 74 percent of women who experience “morning sickness” for at least a month during pregnancy: Nausea and vomiting during early pregnancy are associated with a lower risk of miscarriage, according to a study published recently in JAMA Internal Medicine. 

The study’s findings “may provide reassurance to women experiencing these difficult symptoms in pregnancy,” the study’s authors write.

This isn’t the first research to show a protective association between morning sickness (which can happen at any time of day) and the risk for pregnancy loss. Earlier studies, however, came with many caveats. Most notably, they enrolled women after a doctor had confirmed their pregnancy, therefore excluding women whose pregnancies ended very early after conception. Previous studies also tended to rely on women’s recollections of their pregnancy symptoms, a method of collecting data that is prone to errors.

The current study set out to gather more definitive data by collecting information from the time a woman was trying to conceive to when her pregnancy ended, whether in miscarriage, stillbirth or the delivery of a healthy baby.                     

Study details

The study included 797 women, aged 18 to 40, who became pregnant while participating in a study that was investigating whether the daily use of low-dose aspirin prevented pregnancy loss. (Aspirin was not found to be protective.) All of the women had a previous history of one or two pregnancy losses, and all were actively trying to get pregnant again. 

The women provided daily urine samples, which made it possible to identify their pregnancies early. They were also asked to keep diaries in which they listed how many times they had experienced symptoms of nausea and/or vomiting during each day of the study.

Almost one quarter (188) of the women’s pregnancies ended in a miscarriage, with most — 93 percent — occurring during the first 12 weeks of pregnancy. One-third of those losses happened within the first six or seven weeks of pregnancy — before the pregnancy was detected by ultrasound.

About 20 percent of the women reported symptoms of nausea and/or vomiting by their second week of pregnancy (before they had positive pregnancy test results), 50 percent by their fifth week and 84 percent by their eight week. Interestingly, the symptoms weren’t continuously present, but would come and go from day to day. 

When the researchers dug down further into the data, they found that, overall, the women who reported nausea, with or without vomiting, were between 50 percent and 75 percent less likely to miscarry than those who didn’t feel sick.

Furthermore, the women who said they had very strong morning sickness — who were throwing up as well as feeling nauseated — tended to be the most likely to have a full and healthy pregnancy.

“These findings persist even after accounting for rarely measured lifestyle and fetal factors,” such as smoking, alcohol use, caffeine use, the mother’s stress levels during pregnancy and the gender of the baby, write the study’s authors.

Possible explanations

The underlying cause of morning sickness (which can occur at any time of the day) is unknown. This study appears to rule out the theory that morning sickness helps women avoid behaviors that might put their pregnancy at risk, for it controlled for many of those factors, such as alcohol use, caffeine use and smoking. 

The cause of morning sickness is more likely linked to rapid hormonal changes, especially the increase of a hormone known as human chorionic gonadotropin, or HCG (which is the hormone measured in home pregnancy tests), the study’s authors write.

“Another possibility,” they add, “is that nausea and vomiting are markers for viable placental tissue. Thus, less nausea and vomiting may identify failing pregnancies.”

Limitations and implications

Even though this study may have had a stronger methodology than past research on the topic, it still has several important limitations. In particular, the women in the study were highly homogenous — 96 percent white, 95 percent married and 72 percent employed. In addition, 63 percent of them were age 29 or younger. It’s therefore not clear if the findings would pertain to other groups of women.

The study was also unable to determine the impact of hyperemesis — a rare and severe form of morning sickness that can lead to hospitalization — on pregnancy outcomes.

Still, as the study’s authors emphasize, these findings should offer some comfort to women who spend some or all of their pregnancy enduring morning sickness.

Yet, pregnant women should not get overly complacent — or worried — about the study’s findings, the researchers add. Morning sickness is not a guarantee of a healthy pregnancy. Nor is its absence a sign that something will go wrong.

“Not all pregnancies are the same,” study author Stefanie N. Hinkle, a staff scientist at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, told CBS News. “Everyone’s individual experience is different so just because you don’t have symptoms does not mean that you’re going to go on to have a loss.”

FMI: The study, which was funded by a grant from the National Institutes of Health, can be accessed through the JAMA Internal Medicine website, but you will have to sign up for a (free) personal account first.

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