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. For women in the 18 to 24 age bracket, marijuana usage spiked from 12.5 percent to 21.8 percent in that same period.
Across all age groups, marijuana use during pregnancy increased from 4 percent at the start of the study to 7 percent by the end.
Experts believe that the use of marijuana could potentially harm a fetus's brain development, cognition and birth weight, The New York Times noted, and its main psychoactive ingredient, tetrahydrocannabinol (THC), could be transferred to a baby via breast milk.
"Of concern" to the researchers was that fact that marijuana use was particularly common among younger pregnant women. A total of 3.3 percent of women older than 34 also reported to using the drug during their pregnancy in 2016, an increase from 2.1 percent in 2009.
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Many pregnant women are turning to the drug to treat morning sickness or anxiety associated with pregnancy.
While studies on pregnant women who use marijuana have been limited, the researchers state that prenatal use of marijuana may impair fetal growth and neurodevelopment.
Barbara Yankey, a researcher at Georgia State University, told Reuters marijuana use may be on the rise because of the recent legalization of its recreational use "has made people think of the drug as less unsafe, even during pregnancy".
But doctors in the U.S. warn that drinking any alcohol while pregnant could come with medical risks, such as the possibility of miscarriage, stillbirth, or physical and behavioral problems in the baby. "They absolutely don't smoke, drink or use any drugs during pregnancy", said Horsager-Boehrer, who is an editor of the Your Pregnancy Matters blog. "They know there are potential risks involved with many decisions they make involving medication exposure, alcohol use and smoking, but they decide those risks are acceptable, especially if the risks are not well-defined or conclusive", she said.