Some of the sharpest increases in marijuana usage were among pregnant women under the age of 18 and between the ages of 18 to 24 years old. The largest increase in use was found in subjects younger than 18 years of age to 24-year olds.
What's more, rates of pot use in pregnancy have steadily risen over the years - from 4.2 percent of women in 2009 to 7.1 percent just seven years later.
"Marijuana is [one of] the most commonly used illicit drugs during pregnancy, and its use is increasing", the study says. For women in the 18 to 24 age bracket, marijuana usage spiked from 12.5 percent to 21.8 percent in that same period.
However, since marijuana is federally prohibited, there is little research that shows how marijuana affects fetal development during pregnancy, or its effect on women and babies during breastfeeding.
In 2018, marijuana will be legally available for recreational use, suggesting that that even more pregnant women may have access to and indulge in it.
For other age groups, the researchers found that marijuana use rose from 3.4% to 5.1% among women 25 to 34 and from 2.1% to 3.3% among women older than 34.
"Of concern" to the researchers was that fact that marijuana use was particularly common among younger pregnant women.
Women who participated in the study were asked to self-report their marijuana use in a survey and undergo urine testing. "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.