Unlike the data from the C.D.C., pregnancy is the primary inclusion criteria, said Dr. Afshar, who is a co-principal investigator of the U.C.S.F. and U.C.L.A. study.

Women can register for the study themselves, regardless of where they are getting care, rather than relying on a clinician referral, she added. So far, more than 950 women have enrolled from across the country, and about 60 percent of them have Covid-19.

U.C.S.F. has also started another large, national study for pregnant women in their first trimester called ASPIRE that will follow about 10,000 women and their babies from the start of pregnancy through delivery and up to 18 months postpartum.

This data is vital given that there are gaping holes in the health information used by the C.D.C.

In the C.D.C. study, the researchers found that pregnant women with Covid-19 were more likely to be hospitalized than nonpregnant women who had the virus, but it did not say whether the pregnant women were hospitalized because of labor and delivery, or because of complications from Covid-19. Data sets that the researchers would have needed to make that distinction were not available, the study said.

The data on whether or not infected pregnant women were admitted to the I.C.U. or whether they required mechanical ventilation was missing for about 75 percent of the patients. Using the data that was available, the researchers determined that pregnant women with Covid-19 were more likely to be admitted to the I.C.U. than nonpregnant women (the numbers appeared to be slightly more than the percentage of pregnant women admitted in the past, when compared to data from a 2010 study). Similarly, the study found pregnant women with Covid-19 were more likely to end up on mechanical ventilators than infected nonpregnant women, though the differences were quite small.

Its really hard scientifically to know what that means unless you have an appropriate control group, Dr. Huddleston, one of the principal investigators of the ASPIRE study, said. In other words, researchers also need a control group of pregnant women who are not infected.

Despite the caveats of the C.D.C. study, it remains a signal that pregnant women could be more susceptible to severe Covid-19 symptoms, Dr. Bryant said, adding, its not super surprising given what we know about other respiratory illnesses like flu.

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Why We Still Dont Know Enough About Covid-19 and Pregnancy - The New York Times