More women - especially younger women - are testing positive for marijuana use during pregnancy in Northern California, according to a new study.

The research letter, published in the Journal of the American Medical Association, analysed the results of urine tests administered during standard prenatal care of 280,000 women enrolled in the Kaiser Permanente Northern California health-care system.

It found that from 2009 to 2016, the percentage of women who tested positive for marijuana at roughly 8 weeks into pregnancy rose from 4 percent to 7 percent.

Marijuana use was particularly prevalent among those younger than 18 (22 percent), and women between ages 18 and 24 (19 percent).

The rate of marijuana use among pregnant women age 25 and older was considerably lower, at less than 5 percent.

In part because of the plant's mixed legal status, the risks of pot use during pregnancy aren't fully understood.

Evidence indicates that maternal marijuana use may impair foetal growth and brain development, but conclusive links between marijuana and prenatal complications haven't been conclusively established the way those have been for, say, alcohol or tobacco.

Marijuana use may be less harmful during pregnancy than that of alcohol or tobacco, in other words, or it may turn out to be more harmful. But at present we simply don't know either way.

The study was limited to Northern California, a region that's not exactly representative of behaviours and attitudes toward marijuana nationwide.

After California legalised medical marijuana in 1996, a robust marijuana growing industry took hold in the Emerald Triangle region of the northern part of the state.

Marijuana cultivation and use are woven into the culture of Northern California like no place else in the country.

It's also possible that for many women in the study, the findings reflect marijuana use that occurred before they knew they were pregnant.

Marijuana is detectable in the urine up to 30 days after the last use, and many women do not realise they are pregnant until several weeks into their pregnancy.

"Prenatal use before vs after women realised they were pregnant could not be distinguished," the study's authors note.

Still it's clear that nationwide, changing attitudes toward the plant are spilling over into the realm of pregnancy. Marijuana has been shown to be effective at treating nausea, and prior research indicates that some pregnant women are using it to treat morning sickness.

A study published earlier this year found that 4 percent of pregnant American women reported using marijuana in the past month of their pregnancy in 2014, up from 2 percent in 2002.

By comparison, the rate of past-month alcohol consumption is roughly 10 percent among pregnant American women.

In part because of the lack of knowledge about marijuana's effects during pregnancy, the American College of Obstetricians and Gynaecologists has a simple recommendation for moms-to-be contemplating pot use: don't do it.