The U.S. House of Representatives on Friday passed a bill to end the federal prohibition on cannabis but that doesn't mean weed is legal in the United States, and it doesn't mean that it is safe for pregnant and breastfeeding mamas to partake.

According to researchers at the University of North Carolina School of Medicine, when cannabis is used in early pregnancy it can cause major malformations in both the face and brain in a developing embryo, similar to those caused by fetal alcohol syndrome.

The study was published in Nature's Scientific Reports. The researchers gave cannabinoids to pregnant mice, which closely model early human pregnancy.

"The development of the embryo in this time period is very similar across all vertebrates," says Dr. Scott Parnell, one of the lead researchers on the study. "In this study we also test a synthetic cannabinoid in zebrafish that yielded similar growth deformations as the natural [cannabioids]. Having the same results across animal models reinforces our findings."

The American Academy of Pediatrics has previously recommended against cannabis use by pregnant and lactating women, and notes that because there cannot be randomized controlled trials on the effect of marijuana on pregnant humans we don't know exactly how cannabis will impact a developing baby, but previous animal studies suggested prenatal exposure to marijuana could impact neurodevelopment, growth and behavior. That research was limited and this new study is the first to show this connection to major malformations in both the face and brain after a one-time exposure during early pregnancy.

The publication of this study came after the Food and Drug Administration released a statement strongly advising against CBD and THC during pregnancy or while breastfeeding.

And the FDA's warning came more than a year after one from the American Academy of Pediatrics released its first set of guidelines on cannabis use during pregnancy and breastfeeding, counseling against the use of cannabis for expecting and nursing mothers, and as that use continues to rise.

As NPR reports, research suggests more and more pregnant women are turning to cannabis to treat morning sickness, and Reddit's archive shows many moms have sought advice on this topic in subreddits. Many Redditors have bemoaned a lack of research, while others have counseled conservative use. But the advice of a researcher is more valuable than that of anonymous Redditors, and Parnell tells Motherly that THC and even CBD should be avoided during pregnancy.

"Morning sickness most often occurs during the first trimester, when the developing face, brain, and other organs are initially being formed and are very sensitive to environmental factors. This study suggests that CBD during this time may not be safe and alternative morning sickness remedies should be used," Parnell explains.

According to the research, both THC and CBD may negatively impact prenatal development. "This is especially so when mixed with even low amounts of alcohol," Parnell explains.

Critics will point out that this was an animal study, and question whether lab mice are truly comparable to pregnant humans, and more research needs to be done regarding the topical application of CBD in pregnant and breastfeeding women. But just like with alcohol, when it comes to cannabis, the experts say the best course of action when pregnant is to abstain.

We spoke with Diana Spalding, our resident midwife, Digital Education Editor, and writer of The Mother Guide to Becoming Mama: Redefining the Pregnancy, Birth, and Postpartum Journey, to get her take. "Battling morning sickness during pregnancy can be absolutely miserable; especially if you have hyperemesis gravidarum. I really do understand the intense desire to feel better. Ultimately, I think the stress of worrying about potential side effects of cannabis use in pregnancy makes it not worth it. There are a lot of treatment options out there, so talk with your provider—and keep trying until you find something that works."

Spalding continues "A 2018 study led by the Boston University School of Public Health found that marijuana use in men and women does not seem to decrease chances of getting pregnant. ACOG, however, states that there are "concerns regarding impaired neurodevelopment [in babies], as well as maternal and fetal expo- sure to the adverse effects of smoking," and advises hopeful parents to quit their use of marijuana. A 2017 study found that using marijuana during pregnancy increased the risk of newborn illness and death.Many women also wonder about the safety of cannabidiol, or CBD. The trouble is, CBD is relatively new to the mainstream, which means we don't know a lot about its impact on conception and pregnancy."

If you're struggling with morning sickness, ask your doctor or midwife for advice on medications that are safe for pregnant women.

[A version of this post was originally published November 5, 2019. It has been updated.]