The coronavirus that causes COVID-19 rarely infects the placenta, but new research suggests that it is more likely to happen early in pregnancy when such an infection occurs. Analyzing 12 placentas from healthy women, ranging in gestational age from 5 weeks to 36 weeks, researchers found that the cells in the placenta that become infected with the coronavirus have the surface protein ACE2, which the virus uses as a gateway for entry. Late in pregnancy, the ACE2 proteins are positioned on cells in a way that does not expose them to the virus circulating in the mother's blood, possibly protecting the placenta from infection, said study co-author Dr. Drucilla Roberts of the Massachusetts General Hospital in Boston. This "protective" positioning pattern was less often seen in early gestation placentas when ACE2 was typically present over the entire cell circumference, the study found. That suggests "increased vulnerability of the early placenta to infection," Roberts said. "As more pregnant women recover from first trimester SARS-CoV-2 infection, it is important to remain vigilant to possible placental infection" and transmission from mother to fetus, the researchers said in a report published in Journal of Infectious Diseases. How often that happens, and effects on the fetus and newborn remain unknown, they added.