How Smoking Weed During Pregnancy Might Affect the Fetus

According to a recent study published in the Journal of the American Medical Association, the percentage of women who are smoking weed in the first two months of pregnancy has increased significantly since 2002. The study's findings are based on medical records of 280,000 women in California who were asked about marijuana use at a prenatal appointment and then given a cannabis toxicology test during their first trimester. In 2002 2.4 percent of pregnant women self-reported marijuana use, compared to 3.9 percent in 2012.

Notably, the study mentions that not all of the women who tested positive for cannabis in their first trimester knew they were pregnant when they last smoked (women typically don't know they're pregnant until they're one month along). While it's still not fully clear how smoking weed during pregnancy can affect a developing fetus, there are a few preliminary (and somewhat inconclusive) studies on the subject. Most of the early research has been disputed by more recent work, but – for anyone potentially concerned about smoking weed while unknowing (or knowingly) pregnant – here's what's known so far about weed and pregnancy.

1. Babies born to women who smoked during pregnancy are smaller. A 2012 study of pregnant women in Brisbane, Australia published in Pediatric Research found that, on average, babies born to women who reported smoking weed during pregnancy were about 375 grams lighter than babies born to women who didn't smoke while pregnant. When researchers controlled for other factors that may be at play (like socioeconomic status, mother's weight, and alcohol or cigarette use), they still found that the risk of low birth weight for babies born to mothers who smoked weed was statistically significant.

2. The risk for preterm birth could be higher. The same 2012 study found a number of other health risks for moms who smoked during pregnancy, one of which was an increased risk for preterm birth.

3. These combined factors put babies at a higher risk of being admitted to the NICU. The combination of increased risk for low gestational weight and preterm birth puts babies born to women who smoked weed during pregnancy at greater odds for NICU (neonatal intensive care unit) admittance.

4. There isn't enough evidence to put concern for marijuana use during pregnancy above tobacco use. While there's definitely more research on the effects of tobacco use during pregnancy (because it's just been widely legal for a lot longer), the preliminary research on marijuana use doesn't raise enough alarm to warn against weed with the same intensity as tobacco for pregnant women. A 2013 study published in Obstetrical & Gynecological Survey researched the effects of marijuana use on lactation, under the guise that marijuana will soon become more widely available in the U.S., and didn't find conclusive evidence to support treating marijuana and tobacco use the same way.

5. But that doesn't mean it's totally cool to smoke weed if you're pregnant or breastfeeding. A study from 2015, published in American Journal of Obstetrics and Gynecology, found that marijuana freely crosses the placenta and is found in breast milk. The effects of how marijuana in the placenta and in breast milk may effect a baby aren't fully clear, but researchers believe there could be adverse affects on neurological development, and even pose a higher risk for preterm or stillbirth.

6. Emerging research suggests adverse affects on cognitive function. The same 2015 study references the growing body of research on marijuana and developing fetuses, and says some studies show marijuana use during pregnancy causing neurodevelopment issues like hyperactivity and poor cognitive function. None of the research thus far is wholly conclusive –but as marijuana becomes legal (both medically and recreationally) in more states, research on the effects it could have on fetuses and newborns continues to increase. But for now, most studies recommend not smoking anything while pregnant.

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