An increasing number of women are using marijuana while pregnant, possibly for morning sickness or anxiety, a research letter published in JAMA Tuesday suggests.

From 2009 to 2016, marijuana use among pregnant women increased from 4.2% to 7.1% researchers found using a database of about 318,000 pregnant females from a California health care system. Those 18 and younger increased use from 12.5% to 21.8% during that time. Pregnant women ages 18 to 24 increased use from 9.8% to 19%. 

The findings come from self-reported surveys, and "likely underestimate use," researchers say. 

Findings from a January study published in JAMA also found marijuana use among pregnant women rose from 2.37% in 2002 to 3.85% in 2014. The January study, which used data from the annual National Survey on Drug Use and Health, also found younger women, those between 18 and 25 years old, are most likely to use marijuana. 

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Some pregnant women are turning to marijuana to treat morning sickness, Nora D. Volkow, director of the National Institute on Drug Abuse, wrote in an editorial published online with the January study. Pregnant women should avoid using marijuana or other cannabinoids because “there is cause for concern.”

While some studies exist, not a lot is known about marijuana's effects on an unborn baby. Research cited by the Centers for Disease and Control shows pregnant women who use marijuana could put their child at risk of low birth weight and developmental problems.

The American Congress of Obstetricians and Gynecologists discourages marijuana use for women who are planning to become pregnant, pregnant and breastfeeding mothers.