Dr. Marie McCormick, a pediatrician and the chairwoman of a new report on cannabis from the National Academies of Sciences, Engineering and Medicine, said smoking cannabis “does confer, in terms of birth weight, the same risk as cigarettes.”

Some of the gathering evidence is reassuring. So far, prenatal cannabis exposure does not appear to be linked to obvious birth defects. “That’s why some providers and lay people alike think there’s no effect,” said Dr. Erica Wymore, a neonatologist at Children’s Hospital Colorado. But she warned, “Just because they don’t have a major birth defect or overt withdrawal symptoms doesn’t mean the baby’s neurological development is not impacted.”

Most research in this area was done when the drug was far less potent. Marijuana had 12 percent THC in 2014, while in 1995 it was just 4 percent, according to the National Institute on Drug Abuse.

“All those really good earlier studies on marijuana effects aren’t telling us what we need to know now about higher concentration levels,” said Therese Grant, an epidemiologist and director of the University of Washington’s fetal alcohol and drug unit. “We need to do a whole lot more research now.”

There are two additional problems with studies of maternal cannabis use. Research is often based on reports by pregnant women — instead of, say, tests of urine or the umbilical cord — and they consistently underreport their use. (Researchers know of underreporting because samples reveal discrepancies.) And pregnant women who roll joints also tend to smoke tobacco or drink alcohol; it can be hard to tease out the risks of cannabis itself.

Few realize that THC is stored in fat and therefore can linger in a mother’s body for weeks, if not months. It’s not known whether the fetus’s exposure is limited to the hours a woman feels high.

The American College of Obstetricians and Gynecologists advises clinicians to ask pregnant women about marijuana use and to urge them to quit.