(Reuters Health) - Even though there are still a lot of unknowns about the effects of marijuana exposure in the womb and from breast milk, research to date still suggests that pregnant and nursing women avoid cannabis, U.S. doctors recommend.

“This recommendation is based on evidence for potential harm including decreased birthweights of fetuses exposed to marijuana and concern for an impact on neurological development based on longitudinal human studies,” said Dr. Torri Metz of the University of Utah School of Medicine in Salt Lake City.

“More recent data also identifies a possible association between marijuana use and neonatal intensive care unit admission and stillbirth,” Metz, coauthor of the evidence review of marijuana use in pregnancy and while breastfeeding, said by email.

As more U.S. states legalize marijuana, concern is mounting in the medical community that many people, including pregnant women, may mistakenly assume that using the drug is risk-free, the researchers note in Obstetrics & Gynecology.

The American College of Obstetricians and Gynecologists discourages doctors from prescribing or suggesting the use of marijuana for medicinal purposes while women are trying to conceive, pregnant or nursing their babies.

“There are no studies in which women who used marijuana for a condition are compared to those who did not in order to evaluate the efficacy of marijuana use for treating medical conditions in pregnancy,” Metz said. “As such, we do not have any evidence of a benefit of marijuana use in pregnancy.”

Marijuana used by pregnant women can cross the placenta to reach babies in the womb, and it can also pass into breast milk and reach nursing babies, previous research has shown.

“Active ingredients in marijuana like THC pass through the placenta directly exposing the fetus, which can impact the infant’s overall development, particularly brain development,” said Dr. Cynthia Rogers of Washington University School of Medicine in St. Louis.

“The brain has receptors for these active ingredients in marijuana very early on in gestation. It is therefore possible that exposure to marijuana during pregnancy alters fetal brain development because these brain receptors are exposed to these ingredients at higher levels than the developing brain is expecting,” Rogers, who wasn’t involved in the evidence review, said by email.

“What we still do not yet know from infant studies is exactly how exposure to marijuana affects human infant brain development at birth or throughout childhood with continued exposure either through lactation or through second-hand smoke exposure,” Rogers added.

Another limitation of research to date is that much of it has been done in animals.

Many studies in humans, meanwhile, have relied on women to accurately recall and candidly report on their drug use, which may not offer a reliable picture of exactly how exposure to the drug effects their babies.

Scientists also don’t know how different types and amounts of cannabis use might affect pregnant women and their babies.

“Much of what we know about prenatal marijuana exposure comes from earlier studies that were conducted when marijuana was not as strong as what is currently available, so we are probably under-estimating the effects of prenatal marijuana use on offspring development,” said Gale Richardson of the University of Pittsburgh School of Medicine.

All of this makes it difficult for doctors to give women evidence-based advice, particularly when it comes to one of the most common reasons pregnant women use cannabis: nausea.

Cannabis can indeed ease nausea, but most of the research in this area has focused on cancer patients who have nausea as a side effect of chemotherapy.

“Although many pregnant women report using recreational marijuana to treat nausea and vomiting, marijuana use itself may cause nausea and vomiting, especially in women who have been using it daily for a long period of time,” Richardson, who wasn’t involved in the evidence review, said by email.

SOURCE: bit.ly/2zYNpoI Obstetrics & Gynecology, November 2018.