Doctors know that viruses can cause complications in pregnancy and now they are racing to understand the effect of Covid-19. So far there is evidence of risks, but they may not be as severe as the complications from influenza and some other viruses.

Pregnant women are more vulnerable to infections due to changes in the immune system during pregnancy as well as factors such as shifts in the respiratory system. “The big risk in pregnancy is not as dramatic as some other respiratory diseases like influenza,” says Denise Jamieson, chair of gynecology and obstetrics at Emory University School of Medicine in Atlanta. But experts say more research and long-term studies are needed to understand the virus’s effect on this population given how differently it affects people.

New studies on the virus’s effect on pregnant women and their babies also have turned up both reassuring and alarming findings about the effects on mother and child. Mothers with Covid-19 seem to have prolonged bouts of the illness but largely give birth to healthy babies, according to two recent published studies from a national registry run out of the University of California, San Francisco and the University of California, Los Angeles.

Centers for Disease Control and Prevention studies have found hospitalized women with Covid-19 may develop serious illness, deliver a baby prematurely or have a miscarriage or stillbirth. (The study didn’t have a control group of pregnant women without Covid-19, so didn’t measure whether the virus raises a mother’s risk of those developments.) Other studies have found rates of preterm birth in mothers with Covid-19 aren’t higher than those without the virus.

The virus and its still-unknown impact has many pregnant women worried. “There are just so many unknowns that are accompanied with a positive Covid test when you’re pregnant,” says Melinda Castillo, a 39-year-old financial analyst in Texas who tested positive for Covid-19 in June and is due to have a baby boy in December.

Questions remain, particularly about how coronavirus may affect women in the first trimester. Another UCSF study is enrolling 10,000 women to look at whether contracting the virus in the first trimester can influence neurological development in the first 18 months of life. A UCSF/UCLA study published last week in Obstetrics & Gynecology found that 25% of 594 pregnant women with Covid-19 had symptoms for two months or longer. The median time for symptom resolution was 37 days. Mrs. Castillo—who is participating in the study—says she felt better after about five weeks.

“If you have Covid, the disease course can be quite prolonged for pregnant people,” says Vanessa Jacoby, vice chair of research in the department of obstetrics, gynecology and reproductive sciences at UCSF, and co-principal investigator of the study.

The findings are part of the Pregnancy Coronavirus Outcomes Registry, which goes by the acronym Priority. It is a UCSF/UCLA study of 1,300 pregnant or postpartum women with a confirmed or suspected case of Covid-19.

“In prior flu outbreaks, pregnant women had much worse disease courses,” said Dr. Jacoby, referring to the H1N1 swine flu and SARS. “So we felt this really intense urgency to provide information on Covid-19 and pregnancy.”

An earlier Priority study found that among about 179 infants born to women with Covid-19, there were few bad health outcomes compared with 84 mothers who didn’t have the virus—except for a higher rate of NICU admissions for women who contracted Covid-19 up to two weeks before delivery. The study found that only 1% of infants born to Covid-19 mothers tested positive for the virus and it didn’t appear to affect them.

But two recent studies from the Centers for Disease Control and Prevention had more sobering news. They found that hospitalized pregnant women infected with the coronavirus may develop serious health complications, deliver their babies prematurely or have a miscarriage or stillbirth.

Experts caution the study may not be capturing the problem fully since researchers looked only at women who had Covid-19 and didn’t compare them to pregnant women without Covid-19.

The study included nearly 600 pregnant women with Covid-19 but more than half of the women were asymptomatic when admitted to the hospital for different reasons. Of the 272 who had symptoms, 8% needed a ventilator for breathing assistance and 16% were admitted to intensive care. About 2% of women had a pregnancy loss. Two mothers died during hospitalization and two newborns died in the hospital after delivery.

About 23% of symptomatic women had a preterm birth while 12.6% of all women in the study—including the asymptomatic ones—had a preterm birth. The U.S preterm birth rate is 10%, according to the most recent CDC data.

Most recent studies have focused on women in the later stages of pregnancy. Marcelle Cedars, professor and director of the division of reproductive endocrinology and infertility at UCSF is heading up a study called Aspire, for Assessing the Safety of Pregnancy in the Coronavirus Pandemic. The effort aims to enroll women early in their pregnancy and follow them through delivery and until their infants are 18 months old.

“This is the time period of greatest risk for the pregnancy,” says Dr. Cedars. “This is a particularly critical time for brain development.” The researchers have enrolled more than 1,000 of an anticipated 10,000 women. They will compare outcomes of mothers who get Covid-19 with those who don’t and examine differences between asymptomatic and symptomatic infections.

Even though studies have shown that the virus that causes Covid-19 rarely crosses the placenta, it can still cause inflammation of the placenta which poses risks for the pregnancy and child, Dr. Cedars says. Studies from other viruses have shown that inflammation in the first trimester can affect a baby’s neurological development.

Mounting an immune response can result in inflammation, which also could affect asymptomatic women. “If mounting an immune response, a mom may not be physically sick but still there may be downstream effects on the baby,” says Dr. Cedars.

The impact may not show up at birth but could manifest in subtle neurological and developmental problems, such as autism, in early years, says Dr. Cedars. “The earliest you can start to see those changes is about 18 months,” she says.

Laura Johnson, of Hickory, N.C., is taking part in the Aspire study and is due in March. Ms. Johnson, 33, tested positive for Covid-19 in August. For about six days she had congestion and body aches and lost her sense of smell.

“I had a relatively mild case,” she says. “But in terms of being pregnant, I was terrified. It wasn’t like the level of sickness that scared me. Nobody wants to call their doctor and have them say, ‘We don’t know.’ They were as reassuring as they could be but information is constantly changing.”

Write to Sumathi Reddy at sumathi.reddy@wsj.com

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