COVID’s severe risk to pregnant women is real, a large Kaiser study in California shows

Unvaccinated pregnant women infected with the coronavirus have more than twice the risk of having dangerous blood clots or other severe medical problems, including preterm birth, than those who don’t have the virus, according to a study of thousands of Northern California women published Monday.

The analysis of 43,886 women who gave birth at Kaiser Permanente Northern California between March 1, 2020, and March 16, 2021 — before coronavirus vaccines were widely available — revealed that babies born to mothers who contracted COVID were also more likely to be born prematurely, placing them at greater risk for brain and heart problems.

“These findings add to the growing evidence that having COVID-19 during pregnancy raises risks of serious complications,” said Dr. Assiamira Ferrara, the study’s lead author and a Kaiser Permanente expert in diabetes and obesity during pregnancy.

The study, published in the journal Jama Internal Medicine, supports evidence that getting vaccinated against the coronavirus during pregnancy is essential for avoiding serious medical problems, Ferrara said.

Despite broad evidence that it’s safe to get the shot during pregnancy — and the danger is worse without it — many pregnant women are reluctant to get the protective shot. Less than a third of pregnant women had been vaccinated by last fall, the Centers for Disease Control and Prevention reported.

Nationwide, 188,581 pregnant women have had COVID-19 since the pandemic began, which reflects a 10% increase from just last month, according to the CDC. In all, 292 pregnant woman have died from COVID, including at least 16 this year.

The Kaiser study looked at medical outcomes for 43,886 pregnant women who had not been vaccinated. They included 1,332 who tested positive for the coronavirus, and they were three times more likely to have a blood clot than uninfected women.

Those with the virus were also 2.5 times more likely to experience a severe medical condition during pregnancy, from kidney failure to an aneurysm, the study found.

In all, 5.7% of those with COVID had to be hospitalized during pregnancy because of the virus. Those most likely to need hospitalization were Black or Asian/Pacific Islander, and women who had diabetes before getting pregnant.

Roughly two-thirds of the study’s participants were non-White — including 28% Latina, 26% Asian or Pacific Islander and 6.5% Black.

Dr. Stephanie Gaw, an expert in maternal-fetal medicine at UCSF who was not affiliated with the study, praised it for “building evidence that COVID-19 in pregnancy is harmful to the mother and pregnancy,” and for supporting the need for protective public health measures, from vaccines to masking in high-risk situations.

Because all participants were members of Kaiser, a managed care system, the results didn’t include more vulnerable women without insurance, Gaw noted.

“We would expect the outcomes to be even worse for those patients,” she said. Other drawbacks were that the study ended before the more severe delta variant of the virus showed up last summer, and that no vaccinated women were included — although vaccines became available on a limited basis three months before the yearlong study ended.

Meanwhile, the new study found no increase in stillbirths among infected mothers. A November study of 1.2 million women by the Centers for Disease Control and Prevention found a heightened risk of stillbirths, but the rate was generally less than 1%.

Nanette Asimov is a San Francisco Chronicle staff writer. Email: nasimov@sfchronicle.com Twitter: @NanetteAsimov