Two new reports lay out the added difficulties that unvaccinated women with Covid have during pregnancy and childbirth, adding to research showing that they face elevated risks.

One study, published Thursday in the Archives of Pathology & Laboratory Medicine, suggests that the coronavirus can invade and destroy the placenta, through which the mother passes nutrients to the fetus.

The other, published this week in the Journal of the American Medical Association, found that pregnant women infected with the coronavirus are about 40 percent more likely to develop serious complications or die during pregnancy than those who aren’t infected.

The study of the effects of the virus on the placenta found that infection may deprive the fetus of oxygen in unvaccinated pregnant women, leading to a higher risk of delivering stillborn babies. While other infections can cause stillbirth by passing through the placenta and damaging the fetus, Covid-19 takes a different, dangerous tack.

“It causes extensive damage to the placenta, and stillbirth occurs from lack of oxygen,” said Dr. David Schwartz, a perinatal pathologist in Atlanta and the lead author of the study. “The placental destruction is so severe that whether or not the fetus becomes infected might be irrelevant.”

Dr. Schwartz’s research team analyzed 64 stillbirth cases and four neonatal deaths in 12 countries. All the pregnant mothers were unvaccinated, and all were thought to have been infected with the Delta variant. In the 68 cases, an average of 77 percent of the placenta had been destroyed.

Although stillbirths attributed to Covid-19 are uncommon — overall, about 24,000 babies are stillborn in the United States each year — a Centers for Disease Control and Prevention study released in November found that pregnant women who had Covid-19 when they delivered their babies were almost twice as likely to have a stillbirth as healthy women who did not have Covid.

The new study about complications for mothers found that the severity of their Covid symptoms was the key factor in their heightened risk. The most severely ill women were three times more likely to develop pregnancy complications than those who tested negative or had milder symptoms.

The researchers analyzed electronic medical records of about 14,000 pregnant women between March 1 and Dec. 31, 2020, before vaccines were widely available. Of those, about 2,350 tested positive during pregnancy or within six weeks of delivery.

The study also pointed to increased danger for newborns: Covid-19 was significantly associated with premature birth and admission to newborn intensive care units.

“We know from other studies that vaccination prevents the most severe symptoms of the disease,” said Dr. Torri D. Metz, an associate professor of obstetrics and gynecology at University of Utah Health, who led the study. “So, this is just another piece of the puzzle that should encourage pregnant people to get vaccinated.”

The new studies add to research showing the danger of Covid-19 to pregnant women and their babies. The C.D.C. has strongly encouraged vaccination for women who are pregnant, breastfeeding or planning to become pregnant. But vaccination rates are low among pregnant women, even though early research has found no evidence that the Pfizer-BioNTech or Moderna vaccines pose serious risks during pregnancy.