The number of pregnant women using marijuana, especially if they have morning sickness, is growing—possibly influenced by a perception that it’s safe now that it’s been legalized in many states. In fact, a recent study by Kaiser Permanente of Northern California found that among pregnant women with mild nausea and vomiting, 8.4% were using marijuana in the first trimester…and among those with severe nausea and vomiting, 11.3% were using marijuana in the first trimester.

The danger: There is no proof that prenatal marijuana use doesn’t affect the developing baby…while evidence is mounting of serious harms. Tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, crosses the placenta. How THC affects a developing baby is not completely understood—more research is needed on the health effects of prenatal marijuana use. That’s why there is no medically approved use for marijuana during pregnancy (or breastfeeding)—in fact, guidelines recommend against such use.

It can be hard to tease out the effects of marijuana from other lifestyle factors common among women who use marijuana—such as cigarette smoking and use of alcohol or other drugs. Nor is anyone going to set up a controlled study in which some pregnant women are given marijuana. So researchers have to rely primarily on retrospective studies. But the compelling evidence from these, including a recent clinical report from the American Academy of Pediatrics, suggests that using marijuana during pregnancy is associated with lower birth weight and neurodevelopmental and behavioral problems in childhood.

It is also known that THC passes through to breast milk. Unfortunately, there is not much known about what effect this has on the nursing baby because there have been very few studies. One survey done in Colorado, where marijuana is legal, found that 18% of women who have ever used marijuana also used it while breastfeeding.

Another factor: The potency of THC in marijuana is increasing, and the potential harms may be greater now than in the past. Also, today’s marijuana crops are more likely to be grown with pesticides, herbicides, rodent killers and fertilizers, adding other possible toxins to the mix.

Bottom line: Women experiencing morning sickness should ask their doctors for ways to reduce nausea that are known to be safe for their babies.

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