Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

Cannabis Use During Pregnancy Linked to Low Birth Weight

Dear Doctor: Several studies have shown a correlation between marijuana use and low birth weight, and I understand that marijuana today has been cultivated to be much stronger than what was used in earlier years. Should this be a concern for women who are pregnant?

Dear Reader: We’ve written before about the use of marijuana, also referred to as cannabis, during pregnancy, specifically in relation to morning sickness. And while it’s true that cannabis is effective in easing nausea, we recommend against its use during pregnancy.

Cannabis is a complex plant that contains hundreds of chemical compounds, including those with psychoactive effects. Science’s understanding of cannabis’s properties, as well as its long-term effects on the human body, is in its early stages. What we do know is that chemicals in cannabis can pass through a woman’s body and into her fetus, and that may affect development.

Several recent studies have drawn a connection between cannabis use during pregnancy and low birth weight, a term that refers to infants born weighing less than 5 pounds, 8 ounces. Although low birth weight infants can be healthy, they are also at increased risk of complications such as an inability to fight infection, having trouble eating and not gaining weight.

Cannabis use in pregnancy also has been linked to increased risk of preterm birth. A recent study from the Colorado School of Public Health analyzed health data from 3,200 women and found that cannabis use during pregnancy was associated with a 50% higher risk of low birth weight. Close to 90% of women in the survey who had used cannabis during pregnancy also breastfed their babies, adding another unknown to the equation.

As you mention, cannabis products today are 6 to 7 times more potent that they were in the 1970s, according to some estimates. Meanwhile, cannabis use among women who are pregnant has increased more than 60% between 2002 and 2014, according to the National Survey on Drug Use and Health. Current research suggests that 4% to 7% of pregnant women now use cannabis, often in the early months of pregnancy to deal with morning sickness. In a study published earlier this year, which also found a link between cannabis use and low birth weight, 22% of the 2,173 pregnant women screened positive for cannabis use.

Our advice against cannabis use during pregnancy is not based on any specific study. Rather, it’s because too little is known about how the chemical compounds in cannabis may affect a developing fetus or a child’s later development. In our opinion, that lack of knowledge makes the potential risks of cannabis use during pregnancy too great at this time.

(Send your questions to, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)