COVID doesn’t pass from mom to fetus during pregnancy

(KYMA, KECY/NBC News) - A new study may prove reassuring for expectant moms: Pregnant women who are infected with COVID-19 during their third trimester appear unlikely to pass the infection to their fetuses.

This study was conducted between April and June 2020 among women who came to one of three Boston area hospitals either for treatment of COVID-19 or for delivery.

None of the newborns of the 127 pregnant women, including 64 who had varying levels of illness from the virus, tested positive for the coronavirus.

"I think that's probably one of the more reassuring pieces to a patient, just that if you get COVID-19 in pregnancy, still there seems to be a relatively low chance that your fetus is going to be born with active COVID-19 infection," said study author Dr. Andrea Edlow, a maternal-fetal medicine specialist at Massachusetts General Hospital.

The study participants who had tested positive for COVID-19 included 36% who were asymptomatic, 34% who had mild disease, 11% who had moderate disease, 16% with severe disease and 3% with critical disease.

The study also included 63 pregnant women who tested negative for the virus and 11 reproductive-age women who were not pregnant, to provide comparison.

Researchers evaluated the levels of virus in respiratory, blood and placental tissue samples. They also looked for the development of maternal antibodies, how well those antibodies passed through the placenta to the fetus and examined placental tissue.

They found detectable levels of the virus in the women's saliva, nasal and throat secretions. They found no virus in the bloodstream or the placenta.

Though in this study none of the babies were born positive for COVID-19, the risk isn't zero, Edlow cautioned. Other studies have shown the range is quite low, however, much lower than in other viruses, including Zika or cytomegalovirus, she noted.

Another significant but less encouraging finding from the study is that the moms infected with COVID-19 did make antibodies to the virus but did not transfer them across the placenta as much as would be expected.

While finding lower-than-expected levels of protective antibodies in umbilical cord blood, researchers found high levels of influenza antibodies, possibly from maternal flu vaccination, according to the study.