A new study concludes that marijuana use during pregnancy increases the risk of premature birth.

That study, of Canadian women and their babies, also found a hint of benefit: Pregnant pot users were slightly less likely than nonusers to develop gestational diabetes and the high blood pressure disorder called preeclampsia.

A second study found that more pregnant women are using cannabis, a trend that fits with its growing social acceptability and legalization in many states. Using data from a federal survey, researchers from the National Institute on Drug Abuse (NIDA) found the prevalence of pregnant women who reported using marijuana grew from 3.4 percent in 2002 to 7 percent in 2017. During the first trimester of pregnancy, use increased even more, doubling from about 6 percent to 12 percent over the 15-year period.

The two analyses, published Tuesday in the Journal of the American Medical Association, add to decades of inconclusive and conflicting research on the risks of pot use during pregnancy. Amid the uncertainty, a growing minority of pregnant women tout the plant to treat nausea, pain, depression, and other complaints, while medical, addiction, and legal authorities contend the dangers are clear.

“These two studies send a straightforward message: Cannabis use in pregnancy is likely unsafe. Its potential for harm may represent a public health problem,” says an editorial accompanying the new analyses. The lead author of the opinion piece, Michael Silverstein, is a pediatrician and addiction researcher at Boston University School of Medicine.

Tetrahydrocannabinol (THC), the psychoactive compound in cannabis that causes a high, readily crosses the placenta to the fetus. In theory, THC and related compounds could interfere with fetal growth and brain development.

The Canadian study, led by scientists at the Ottawa Hospital Research Institute, compared 652,190 pregnant women who reported no use of cannabis with 9,427 self-reported users. The rate of preterm birth — defined as less than 37 weeks out of a full-term 40-week pregnancy — was 12 percent among users, compared with 6 percent among nonusers. Even after researchers made adjustments to account for potential confounding factors such as income and smoking, cannabis users were more likely to have premature babies. Users were also more likely to have babies who were small and needed intensive care.

This contrasts with a comprehensive review of previous research that did not find a link between cannabis use and premature birth or low birth weight, the Ottawa researchers noted.

As for the apparent small reduction in preeclampsia and gestational diabetes — 0.5 percent less for cannabis users — the researchers said it “may not be clinically important.” Previous studies and animal models suggest cannabis has antioxidant properties that could be beneficial.

On websites, women describe how marijuana relieved their morning sickness, anxiety, and other medical problems. And medical marijuana is now legal in most states.

But medical marijuana does not appear to be driving the surge in pot use by pregnant women, NIDA researchers concluded. They classified women as having “medical-only cannabis” if all use was recommended by health-care providers. Between 2013 and 2017, only 0.5 percent of pregnant woman reported such strict medical usage — the same rate as nonpregnant women.

“Although many states have approved cannabis for nausea/vomiting, the results suggest that clinicians might not recommend it during pregnancy, perhaps reflecting the American College of Obstetricians and Gynecologists’ recommendation that pregnant women discontinue cannabis consumption,” the NIDA researchers wrote.