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Researchers are keeping an eye on a signal for preterm birth and excess maternal mortality in SARS-CoV-2-infected mothers, according to a discussion at the American Academy of Pediatrics (AAP) virtual meeting.

At the start of the pandemic, the main focus had been on whether newborns could get infected via vertical or horizontal transmission. The AAP's currently shows that the rate of newborn positivity among COVID-positive mothers is low, at .

However, newborn positivity rates can vary locally, with one center in Philadelphia reporting a 5.5% rate among newborns from April 2020 through September 2021. All but one of these neonates were asymptomatic, with one baby born with a cough, according to Karen Puopolo, MD, PhD, of Children's Hospital of Philadelphia and the University of Pennsylvania.

The evidence does not suggest greater likelihood of transmission when the mother rooms in with the baby and breastfeeds; instead, breast milk can harbor IgG and IgM antibodies that provide protection from SARS-CoV-2 infection, said Mark Hudak, MD, of the University of Florida College of Medicine in Jacksonville, during the AAP session.

He acknowledged that it remains to be seen what the natural history of perinatal COVID looks like with longer outpatient follow-up. For now, it appears that symptomatic neonatal SARS-CoV-2 is rare, and multisystem inflammatory syndrome in children (MIS-C) in neonates is even rarer, he said.

In contrast, a notable finding of immediate consequence has been the approximate 15% incidence of preterm birth (before 37 weeks of gestation) among infected mothers in the perinatal COVID registry compared with a 10% incidence in 2019.

Of preterm births to infected mothers during the pandemic, 42% represented delivery by induction or C-section due to concerns about the effects of SARS-CoV-2 on mother or fetus. The remainder may suggest secondary effects of the virus in stimulating preterm labor or causing fetal hypoxia requiring operative delivery, Hudak suggested.

"Continued data acquisition and trend analysis is needed to sort out possible causes of observed increase in preterm births," he said.

Hudak reported that maternal mortality is also in excess among pregnant women testing positive at or around the day of delivery, reaching about an order-of-magnitude higher than expected rates in registry data.

At his institution, there were six maternal deaths with COVID since June of this year -- corresponding with the spread of the Delta variant -- whereas there hadn't been any in the prior 12 months. However, the data are inconclusive regarding the Delta variant's contribution to the morbidity and mortality of perinatal COVID-19. The published literature likely has not included many infected mothers, Hudak said.

Puopolo noted that her institution saw in increase in the number and acuteness of COVID infections among women in the delivery rooms in August and September, when Delta was in circulation.

"At our center we see a lot of positive mothers, but I don't know if our rate of positive babies is any higher" with the variant, Hudak noted. He said other centers have shared mixed experiences anecdotally: some see more infected babies than before, while others have said they haven't seen any difference at all.

Delta was cited as one reason why the CDC started recommending COVID vaccination to pregnant people in August.

Puopolo presented data showing that maternal antibodies to SARS-CoV-2 appear to be lowest in asymptomatic infected individuals and those with mild-to-moderate illness, reaching at least 10 times higher in those receiving the Pfizer or Moderna vaccines.

Some vaccinated women had antibody levels so high that "our technician thought the assay had gone bad. But that was the response," she emphasized.

On the other hand, transplacental transfer ratios were roughly the same for illness as for vaccination, she said.

In Philadelphia, serology studies of pregnant women had shown an 11.5% seropositivity for IgG and/or IgM antibodies as of February. By now, an estimated 25% of women presenting for delivery are seropositive whether by infection or vaccination, according to Puopolo.