Traces of cannabis remain in breastfeeding mothers' breast milk for almost a week, according to new research.
The study, published in Pediatrics, found that tetrahydrocannabinol (THC), the primary psychoactive component of marijuana, was present in 63 per cent of breast milk samples analysed, for up to six days after mum's last reported use.
While it is known that marijuana readily crosses the placenta and enters the developing brain of the fetus, until now, the question of whether babies are exposed to marijuana through breastfeeding has been poorly understood.
To explore this issue, researchers at the University of California San Diego School collected samples from 50 breastfeeding mothers who reported using cannabis either daily, weekly, or sporadically.
Of the women studied, 88 per cent used marijuana daily, with inhalation the most common method of ingestion (64 per cent). And while the researchers didn't collect plasma samples from the mothers' infants, they did estimate how much THC a hypothetical three-month-old baby, weighing six kilograms, might be exposed to.
"We found that the amount of THC that the infant could potentially ingest from breast milk was relatively low," said Dr Chambers, "but we still don't know enough about the drug to say whether or not there is a concern for the infant at any dose, or if there is a safe dosing level."
The authors do note, however, that if a child is exposed to low levels of THC through daily breastfeeds there is a risk of various cannabinoids accumulating. "Because the brain rapidly develops during the time period when, ideally, a child's main source of nutrition is human milk, brain development may be altered by THC exposure," they write.
According to Dr Chambers, the findings are a stepping stone for future research, but many questions remain. "Are there any differences in effects of marijuana in breast milk for a two-month-old versus a 12-month-old, and is it different if the mother smokes versus eats the cannabis?" she said. "These are critical areas where we need answers as we continue to promote breast milk as the premium in nutrition for infants."
But while further studies are needed, a clinical report in the same issue of Pediatrics, which examined current rates of marijuana use among pregnant and lactating women, provides a clear recommendation for women.
"Present data are insufficient to assess the effects of exposure of infants to maternal marijuana use during breastfeeding," the report notes. "As a result, maternal marijuana use while breastfeeding is discouraged. Because the potential risks of infant exposure to marijuana metabolites are unknown, women should be informed of the potential risk of exposure during lactation and encouraged to abstain from using any marijuana products while breastfeeding."
Mary O'Connor, who co-authored the report, adds: "There's still a lot we don't know about how marijuana affects a baby's rapidly developing brain. But, based on what we know now, we're advising women who are pregnant or nursing that the safest choice for their child is to avoid marijuana."
It's advice echoed in current guidelines from the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG), which also cite previous studies on the effects of cannabis during pregnancy and breastfeeding. "There is evidence of neuro-developmental deficit or delay in the neonates and children of cannabis users in pregnancy and lactation, including cognitive deficit, visuo-spatial dysfunction, impulsivity, inattention and depression in children of women who have used cannabis in pregnancy or lactation," they write.
"Due to this, as well as the maternal and fetal exposure to the adverse effects of smoking, women who are pregnant or contemplating pregnancy should be encouraged to discontinue cannabis use."