An analysis of urine samples from roughly 300,000 California women finds that more than 7 percent used marijuana while pregnant.

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.

That's according to the new report from Kaiser Permanente Northern California, a major regional healthcare group.

"Use among [pregnant] females younger than 18 to age 24 years increased the most," added the team led by Kelly Young-Wolff, a Kaiser researcher in Oakland.

Among this youngest group, use of pot during pregnancy rose from 12.5 percent in 2009 to nearly 22 percent by 2016.

Rates of use were highest among the young, and dropped steadily as age rose during pregnancy, the researchers said.

There was one important caveat to the survey: Urine samples were taken at eight weeks of gestation, so pot use might have occurred before many of the women realized they were pregnant. Pot can linger in the system for up to 30 days, the researchers noted.

But the researchers believe that urine samples are likely to provide the most accurate picture of marijuana use in pregnancy because people tend to hesitate about admitting such behaviors.

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