Secondhand marijuana smoke while pregnant

What does the evidence say about secondhand marijuana smoke during pregnancy?

1. About 50% of THC survives combustion when marijuana is smoked, with 6 to 53% released into the surrounding air, so passive inhalation can occur for anyone in proximity to a smoker, and secondhand exposure has been shown to leave detectable THC in bodily fluids. 1
2. Because secondhand exposure has been shown to leave THC in bodily fluids, researchers say it is reasonable to assume secondhand exposure can also affect breast milk and a nursing infant, and they recommend avoiding passive marijuana exposure during breastfeeding given the potential for harm. 1
3. In a study of young children in New York City, a marijuana metabolite (COOH-THC) was detectable in the urine of about 21% of children tested, and children living in attached housing where smoking was allowed anywhere on the property had more than double the detection rate of children in smoke-free housing, 34.8% versus 13.0%. 2
4. A study comparing pregnancies with and without a maternal cannabis use disorder found a higher rate of infant death in the first year of life (1.0% versus 0.4%) and noted that its data could not account for postnatal factors like secondhand marijuana or tobacco smoke, which the researchers still flagged as important, unmeasured contributors to sudden unexpected infant death risk. 3
5. With direct maternal use, THC passes into breast milk and can stay detectable there anywhere from 6 days to 6 weeks, the kind of prolonged presence that helps explain why researchers approach any route of infant THC exposure, direct or secondhand, with the same caution. 4

Is secondhand marijuana smoke safe in each trimester?

• First trimester. There's no dose known to be safe, and early organ development is when many exposures carry the most theoretical concern, so this is a good trimester to be direct with the people you live with about smoking outside, away from windows and vents.
• Second trimester. The passive-exposure math doesn't change as pregnancy progresses: roughly half of the THC in a joint or blunt survives combustion, and some of that reaches the surrounding air, so proximity to a smoker matters more than which trimester you're in.
• Third trimester. By the third trimester it's worth planning ahead for the newborn phase too, since the same avoidance recommendation carries into breastfeeding and early infancy, and secondhand marijuana smoke has been detected in the urine of young children living in homes where smoking was allowed indoors.