Cannabis use is associated with a greater risk of an unhealthy pregnancy outcomes, especially low birth weight, according to a study funded by the National Institutes of Health. While the study did not identity why cannabis use might have these effects, it underscores the potentially damaging impact of the substance on fetal health, the authors say.

Many pregnant people use cannabis to help manage symptoms, including nausea and pain. The prevalence of the drug has surged in the past decade as more states have legalized its use for medicine or recreation, and many people believe it is relatively safe. But the impact cannabis has on pregnancy has been understudied.

For the new study, researchers analyzed urine samples from more than 9,000 pregnant people between 2010 and 2013 to determine whether cannabis was used at any point during pregnancy, at how many weeks of gestation it was used and the amount.

The team measured THC, the psychoactive substance in cannabis, at three different periods roughly tracking with trimesters and used that data to calculate total cannabis exposure throughout the entire pregnancy. Their findings were published in JAMA on Tuesday.

The authors determined that pregnant people who used cannabis experienced unfavorable birth outcomes at rates of 25.9 percent, compared with 17.4 percent among those who did not use cannabis.

Low birth weight and cannabis use had the strongest association out of all the adverse outcomes, the study found. Low birth weight is defined as weighing less than 5 lbs., 8 ounces at birth. This can lead to a range of health complications and long-term risks, including an increased likelihood of chronic conditions later in life.

Cannabis users were also more likely to experience preterm births, stillbirths and hypertensive pregnancy disorders, which is an umbrella term for conditions characterized by high blood pressure; these can include preeclampsia, which can be a potentially life-threatening condition for the pregnant person and the baby.

The highest risks were associated with ongoing cannabis use throughout pregnancy and for those who had higher levels of exposure, said lead study author Torri D. Metz, an associate professor of obstetrics and gynecology and vice chair of research of obstetrics and gynecology at the University of Utah Health. This suggests that patients who discontinue cannabis use early in pregnancy, particularly within the first trimester, can significantly reduce the risk of adverse pregnancy outcomes.

"Looking at any adverse outcome, we saw that people who had stopped use within the first trimester didn't actually have a statistically significant increased risk [vs.] those who had continued use," Metz said.

One of the other factors the research accounted for was tobacco use. Many people are aware of tobacco's negative health effects, while many also perceive cannabis as safe. That's led to a more favorable view of cannabis use compared with tobacco, including during pregnancy.

"One of the questions in previous studies has been, 'How much of this is related to nicotine?' because there's a lot of co-use of cigarettes and cannabis," said Smita Das, a board-certified psychiatrist at Stanford University who specializes in addiction and who was not part of the study. "The authors are able to adjust for cigarette smoking status," she said, adding that "this study is an excellent quantitative assessment of cannabis use."

Experts say the study adds to a growing body of evidence that no amount of cannabis is safe during pregnancy, and that health-care providers need to work on better messaging about cannabis, as well as helping patients identity other, pregnancy-safe alternatives for their symptoms.

"It's important that we get the message out there that cannabis use in pregnancy is not recommended," said Kathryn Gray, an associate professor and director of maternal-fetal medicine research at the University of Washington School of Medicine. "That should be a conversation that's happening during preconception clinic visits, so when people are thinking about getting pregnant or when they come in for the first prenatal visit, that should be something that is routinely assessed."