There's been a rise in the number of pregnant women who are using marijuana to soothe anxiety and morning sickness.
The study analyzed data from 280,000, gathered from 2009 to 2016.
While the letter does not discuss any potential problems from pot use during pregnancy, the Centers for Disease Control and Prevention (CDC) advises pregnant women to avoid it. Among women 18-24, marijuana use climbed from 9.8 percent to 19 percent.
Overall, there was an increase in cannabis use among study subjects, from four percent to seven percent, over the seven-year period.
Though little clear evidence exists on the effects of marijuana on developing human fetuses, animal studies have revealed that it may disrupt brain development.
And with younger women, it appears mother tokes best.
Worryingly, if women are not reporting their marijuana use in their medical records, their doctors are unable to counsel them about the risks of using marijuana, or what alternative reliefs for symptoms like nausea might be available.
"Marijuana is [one of] the most commonly used illicit drugs during pregnancy, and its use is increasing", the study says. As of January 1, recreational marijuana will be legal in the state for people over 21.