Marijuana is the illicit drug most commonly used by pregnant woman, and now its use is increasing, a new study finds.

According to urine samples from about 300,000 women in California, more than 7 percent of them had marijuana in their systems while they were pregnant, the researchers found.

This is likely bad news for fetuses, as "initial evidence suggests that prenatal marijuana may impair fetal growth and neurodevelopment," the researchers wrote in the study. [25 Odd Facts About Marijuana]

In the study, the researchers divided the pregnant women into different age groups. Overall, the rate of pregnant women using marijuana increased, growing from 4.2 percent in 2009 to 7.1 percent in 2016, according to the research, which was published online Dec. 26 in the journal JAMA.

During the study period, the use of pot rose in every age group, but its sharpest increase occurred in women younger than 24 years of age. In women younger than 18, it rose from 12.5 percent to almost 22 percent, and in women ages 18 to 24 years, it increased from almost 10 percent to 19 percent during the study period.

Fewer older women used pot while pregnant, the study found. For instance, the rate of women ages 25 to 34 years old who used pot rose from about 3 percent to about 5 percent, while the rate for women older than 34 increased from about 2 percent to about 3 percent from 2009 to 2016.

However, it's possible that some of the women took pot before they realized they were pregnant. Marijuana can stay in the body for up to 30 days, and the women gave urine samples when they were eight weeks (56 days) pregnant, the researchers said.

The reasons behind the increase may be explained by society's budding acceptance of the substance. Moreover, California legalized medical marijuana in 1996, and recreational marijuana will be available in the state on Jan. 1, 2018.

However, it appears that women may not understand the risk of using marijuana while they are pregnant. Another survey conducted between 2007 and 2012 found that 79 percent of women reported they thought there was "little to no harm in prenatal [marijuana] use," the researchers wrote in the study.

The study was published online Dec. 26 in the journal JAMA.

Original article on Live Science.