"This study came from the long-standing idea that nausea and vomiting in pregnancy indicated that a woman was still pregnant," said lead study author Stefanie Hinkle, a scientist at the National Institutes of Health in Bethesda, Maryland.

As many as 80 percent of pregnant women experience nausea, vomiting, or both, Hinkle and colleagues note in JAMA Internal Medicine.

The current study included about 800 pregnant women with at least one or two prior miscarriages.

All of the women had pregnancies confirmed by lab tests and they were around 29 years old on average at the start of the study.

A typical pregnancy lasts 40 weeks, and women have the highest risk of miscarriage in the first trimester, roughly the first 12 weeks. Odds of a miscarriage are higher for women that are older or have certain medical problems such as diabetes, lupus or thyroid disorders.

For the study, women recorded nausea and vomiting symptoms in daily diaries from weeks 2 through 8 of their pregnancies. Then, starting with week 12, they reported symptoms in monthly questionnaires.

After two weeks of pregnancy, 18 percent of the women reported nausea without vomiting, while 4 percent said they experienced both symptoms.

By eight weeks of pregnancy, 57 percent experienced nausea alone and 27 percent had a combination of nausea and vomiting.

As women approached the 12-week mark, 86 percent reported nausea and 35 percent reported nausea combined with vomiting.

In general, women younger than 25 were more likely to experience nausea and vomiting than the older participants in the study.

Overall, 188 pregnancies (24 percent) ended in another miscarriage.

Nausea and vomiting were associated with a 50 percent to 75 percent lower risk of pregnancy loss, the study found.

"Our findings should be reassuring to women experiencing these symptoms, as the risk for a pregnancy loss is greatly reduced in women with these symptoms," Hinkle said by email.

The study doesn`t explain why women who have these symptoms may be more likely to have successful pregnancies, the authors caution.

It`s possible that nausea and vomiting may be the body`s way of getting women to alter their diets during pregnancy, or that a surge in pregnancy hormones triggers these symptoms, the authors suggest.

Limitations of the study include the reliance on women to accurately recall and report symptoms in their diaries, the researchers point out. Researchers also lacked data on the severity of nausea and vomiting.

Still, the findings add to a large body of evidence linking nausea and vomiting to a lower risk of miscarriage, Dr. Siripanth Nippita, a reproductive health researcher at Harvard Medical School in Boston, noted in an accompanying editorial.

Women need to keep in mind that these symptoms don`t protect against miscarriage, and that severe morning sickness can require treatment to minimize the risk of complications during pregnancy, Nippita said by email.

"Nausea and vomiting are common during pregnancy," Nippita said. "Many women don`t experience it and still go on to have normal, healthy pregnancies. On the other hand, women who do experience it may still have a loss."

"The association between nausea and vomiting and a continuing pregnancy is true for the population in this study, but an individual`s experience may be different," Nippita added.

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