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Oregon will start collecting survey data next year on marijuana use among pregnant women.

(The Associated Press)

Marijuana use among pregnant women has increased more than 60 percent in just over a decade, according to a study published Monday.

Use was highest among younger women, though overall relatively few pregnant women reported using marijuana, the analysis showed.

Published in the Journal of the American Medical Association, the study was based on an analysis of the National Survey on Drug Use and Health. The analysis looked at reports from about 200,000 women age 18 to 44 years.

In 2002, just over 2 percent of pregnant women reported using marijuana in the past month, compared with nearly 4 percent in 2014. Use was highest for those age 18 to 25, with nearly 7.5 percent partaking in the past 30 days.

Oregon health officials have not collected data on marijuana use among pregnant women, but they do know the state has a relatively high rate of use in the general population. For example, in the 18- to 25-year-old group, 25 percent use marijuana compared with 19 percent nationally, and 11 percent of those 26 and older use marijuana compared with 6 percent nationally.

Scientists also know that the main psychoactive component of marijuana, THC or tetrahydrocannabinol, can pass to the fetus through the placenta and to babies through breast milk. But it's not been subject to the kind of double-blind, placebo-controlled trials that are conducted on approved medications because it is classified as a Schedule 1 substance. That means the Drug Enforcement Agency considers it to have no medical use and carries a high potential for abuse.

To date, scientists have mainly conducted observational studies on the effect of marijuana. They have found it to be associated with negative effects on children, decreasing academic ability, cognitive function and attention. It's unknown how much and over what time period can cause a lasting effect.

"That's the piece that we don't have concrete data on," said Dr. Katrina Hedberg, Oregon's health officer and state epidemiologist.

To be safe, Oregon health officials urge pregnant women or those who are considering conceiving to avoid marijuana, similar to how they are warned off alcohol.

The amount, though, is probably key, Hedberg said.

"A half a glass of wine once or twice is very different than the fetal alcohol syndrome where the women are drinking a fifth of whiskey or whatever a day," Hedberg said. "Even for alcohol, it's really the amount and at what point in the baby's development."

Oregon health officials will start to collect survey data on the use of marijuana among pregnant women next year as part of an annual survey conducted by the state on pregnancy risks. The results won't be available for months.

But it's likely marijuana does have an impact on the fetus.

"If it's affecting your brain and goes through the placenta, it's going to affect your infant's brain," Hedberg said.

-- Lynne Terry