Pregnant women are unlikely to transmit COVID to babies, research shows

Pregnant women who test positive for COVID-19 at birth rarely transmit the virus to their newborns, according to a body of new research. The reason: COVID is not often found in a patient’s bloodstream.

While researchers have struggled to understand the impact of COVID on pregnancy and infants, these findings offer good news for expectant parents.

“Analysis shows that infection was uncommon in infants born to women with COVID-19,” said Kate Woodworth, a medical epidemiologist at the Centers for Disease Control and Prevention.

Despite this, a pregnant woman with COVID risks serious illness that can also have negative health consequences for her newborn, even if the baby is born COVID-free. Recent studies have linked COVID-19 infection during pregnancy to both preterm labor and stillbirth.

The CDC released a study in September that found the mother-to-baby transmission rate was less than 4%. Another study published in February, examining data from more than 4,000 women in COVID-19 newborn registries in the US and UK, estimates it’s even lower — around just 2%.

Research shows that this likely has to do with the lack of viruses in an expectant mother’s bloodstream. SARS-CoV-2 is not often present in blood samples, indicating that it does not usually enter an infected person’s bloodstream. For example, in a peer-reviewed study, only 6% of patients who presented to the emergency room with COVID-19 had the virus in their blood. Other recent data suggests that the presence of viruses in the blood could be linked to a more serious illness.

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“For [COVID-19] to reach a pregnant uterus, it must circulate in the bloodstream,” said David Schwartz, a medical epidemiologist and pathologist who until recently taught at Augusta University’s Medical College of Georgia. “And because [COVID-19] is not a pathogen that circulates very frequently in the bloodstream, there will not be many women whose uterus, placenta and baby are exposed to the virus.”

In the small number of newborns who test positive at birth, CDC studies have found that most infections are mild or asymptomatic. The World Health Organization has reported similar findings.

Schwartz’s recent work has focused on the negative effects that COVID can have on pregnancy. He emphasized that there are still many unknowns. For example, most of the studies on COVID transmission from pregnant women to their newborns took place before both the Omicron and Delta variants appeared.

Schwartz and health experts from 12 different countries recently teamed up to analyze the damage to the placenta caused by the virus, known as placental disease. Damage to the placenta may explain why adverse fetal outcomes sometimes occur, Schwartz and colleagues found in a study published last August. Placental damage can facilitate transmission of the virus, but perhaps more worryingly, it can also deprive a baby of the oxygen and nutrients it needs.

Denise Jamieson, director of OB/GYN at Emory Healthcare in Atlanta, Georgia, said that in most cases, the placenta does not result in transplacental transmission of SARS-CoV-2, which is not true of every virus. The Zika virus, for example, is extremely dangerous for pregnant women because it can cross the placenta more easily and cause serious problems for a fetus, including congenital defects such as microcephaly and brain damage. Such complications can occur even when maternal Zika symptoms are mild.

So far, it appears that COVID-19 behaves much more similarly than the flu during pregnancy, Jamieson said. The flu is another virus that is rarely transmitted to the fetus. In the case of influenza, pregnancy problems are more closely linked to illness in the pregnant woman than to transmission of the virus to her unborn child.

Flu vaccines are highly recommended for pregnant women, not only because they protect expectant mothers from serious illnesses, but also because antibodies from the vaccine can cross the placenta to boost immunity in babies before they are old enough to vaccinate themselves to become. There is evidence that the same beneficial effect can occur with COVID-19 antibodies, which offer some level of protection against the virus.

A peer-reviewed study by Weill Cornell Medicine that analyzed cord blood samples from more than 100 pregnant women in 2021 found that those who were vaccinated during pregnancy began producing antibodies a few days after their first dose. A little over two weeks later, they began transferring what they called “passive immunity” to their babies.

This means that even if an infant is unlikely to contract COVID-19 in the womb, they would be protected from the virus even after entering the world.

“Most babies who test positive are doing well,” Schwartz said. “But all of these things seem increasingly preventable by getting vaccinated and getting boosted.”

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