Numerous chemicals in marijuana, like tetrahydrocannabinol, known as THC, could pass through a mother's system to her baby.
"There is an increased perception of the safety of cannabis use, even in pregnancy, without data to say it's actually safe", said Dr. Torri Metz, an obstetrician at Denver Health Medical Center who specializes in high-risk pregnancies, according to The New York Times.
More than one-fifth of American teenagers are using marijuana during pregnancy, a new study finds. The women were treated at Kaiser Permanente facilities that had screening for marijuana use as part of standard prenatal care.
The researcher said the amount of marijuana consumed and how often a person uses it is a factor.
Among teen mothers under age 18, marijuana use during pregnancy surged from about 13 percent in 2009 to nearly 22 percent in 2016, researchers found.
The study also reported an increase in pot usage among older pregnant women ages 25 to 34.
However, the institute says more research must be done to determine which effects are related to marijuana exposure and which are related to environmental factors that could be associated with the mother's marijuana use, such as poverty or other drug use.
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And with younger women, it appears mother tokes best.
An expert speculates that the rise might be partially explained by the increased availability of legal marijuana in recent years.
Use of the drug, found to impair an infant's brain development and function, is on the rise with pregnant women of all ages.
Californians voted to legalise recreational use of the drug in 2016.
California became the first US state to legalize medical marijuana in 1996. It is now legal in 29 states and Washington, DC. Use among women between the ages of 18 and 25 almost doubled during that time, from 9.8 percent in 2009 to 19 percent in 2016.
Along with advising against marijuana use, doctors have long advised pregnant women to avoid drinking alcohol during pregnancy.