Ask the Doctors

Dear Doctor: I just entered my second trimester without any nausea at all. Considering a recent study linking morning sickness to a successful pregnancy, should I be worried?

Dear Reader: Nausea occurs in up to 80 percent of pregnancies. The cause appears to be related to a rapid increase of the pregnancy hormone beta hCG (BHCG) as well as the rise of estrogen. The nausea can be distressing for many women and can lead to worries about a future miscarriage.

Paradoxically, nausea may be an indication of a successful pregnancy. Preliminary studies have found this correlation, but they could not differentiate if the lack of nausea was related only to an early miscarriage or to an overall successful pregnancy.

A 2016 study in JAMA Internal Medicine looked at women between the ages of 18 and 40 who had a history of one or two miscarriages and who were trying to conceive. Of those women who were deemed to be pregnant based on a measurement of BHCG, 797 eventually became pregnant. These women kept daily diaries, recording instances of nausea and vomiting from the second week to the eighth week of their pregnancy. If a woman had nausea once, she was considered to have nausea in the pregnancy.

In week two of their pregnancy, only 20 percent of the women reported nausea, but by week eight of their pregnancy, nearly 85 percent of women had reported nausea. After the eighth week of the pregnancy, nausea was measured at monthly intervals. Also, to confirm a viable pregnancy, the women underwent an ultrasound at week six or week seven.

Investigators found that women who experienced nausea at some point during their pregnancy had a 50 percent reduction in the risk of a miscarriage. Those women who were afflicted with both nausea and vomiting at some point in their pregnancy had a 75 percent decrease in the chance of having a miscarriage. Moreover, based on the week-six and week-seven ultrasounds, the lack of nausea in the women who miscarried was not related to a lack of a viable pregnancy.

The authors proposed that nausea during pregnancy may have a protective effect by alerting a woman about her condition so as to alter her diet and avoid toxic substances, which could increase nausea or harm a pregnancy. I'm not so sure about this, but it is a possibility.

In regards to this study, I can understand the worry about your lack of nausea as you enter the second trimester of your pregnancy. But when you look further at the study, you find that nearly 94 percent of miscarriages happened in the first trimester of the pregnancy. So as you enter the second trimester, there is significantly less likelihood that you will have a miscarriage. Also, this study focused on women who had previously had a miscarriage, so its findings may be less applicable to you.

In summary, I wouldn't be concerned about your lack of nausea. But please do schedule follow-ups with your obstetrician for your regular prenatal care.

(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

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