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In 2009, only about four percent of pregnant women were using marijuana.

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. Among women 18-24, marijuana use climbed from 9.8 percent to 19 percent.

While marijuana effects on a fetus aren't clear, doctors do caution women that smoking could lead to lower birth weights and developmental problems.

California became the first US state to legalize medical marijuana in 1996.

The study, from the Journal of the American Medical Association (JAMA), notes that from 2009 to 2016, pot use among expectant mothers jumped from 4.2 percent to 7.1 percent.

And with younger women, it appears mother tokes best. For women in the 18 to 24 age bracket, marijuana usage spiked from 12.5 percent to 21.8 percent in that same period.

The legalization of marijuana may have contributed to the increase in use found by the study, KABC reported.

"Marijuana is the most commonly used illicit drug during pregnancy, and its use is increasing", the study said.

But, according to a Forbes analysis, at least three states - Vermont, Michigan and New Jersey - are likely to pass a legal weed bill in the coming year.