The estimated half-life of THC in breast milk is 17 days, according to the study results, with a projected time to elimination of more than 6 weeks.
The clinical importance of the remaining THC is up for debate, according to some experts.
"To limit THC effects on fetal brain development and promote safe breastfeeding, it is critical to emphasize marijuana abstention both early in pregnancy and post partum," write Erica M. Wymore, MD, MPH, an assistant professor of pediatrics and neonatology at the University of Colorado in Aurora, and colleagues. The group published their results online March 8 in JAMA Pediatrics.
And while the study was a pharmacokinetic analysis rather than a safety investigation, Wymore told Medscape Medical News that the detectable levels of THC suggest any use is of concern and no safety thresholds have been established. "We wish we had more data on the potential effects on the neurocognitive development of children, but for now we must discourage any use in prepregnancy, pregnancy, and breastfeeding, as our national guidelines recommend," she said.
Therefore, the findings support current guidelines discouraging any cannabis use in mothers-to-be and breast-feeding mothers issued by national organizations, including those from the American Academy of Pediatrics, the American College of Obstetricians and Gynecologists, and the Academy of Breastfeeding Medicine.
Furthermore, the difficulties many mothers face in abstaining from marijuana, a commonly used drug in pregnancy, and the persistence of THC in maternal milk led the authors to question the feasibility of having women who use marijuana simply discard their breast milk until THC is cleared.
"We report challenges in abstention and prolonged excretion of THC in breast milk greater than 6 weeks among women with prenatal marijuana use," they write. "These findings make the recommendations for mothers to discard breastmilk until THC is undetectable unrealistic for mothers committed to breastfeeding."
However, not all experts are equally concerned about low THC concentrations in breast milk. Neonatal pharmacologist Thomas R. Hale, PhD, a professor of pediatrics at Texas Tech University School of Medicine in Lubbock, said a previous study by his group showed that THC levels in maternal milk peaked within 60 minutes of a moderate dose of inhaled marijuana and fell to quite low levels over the next 4 hours.
The highest concentration in maternal milk occurred shortly after the peak in plasma.
"So you can see that just because a mom is drug screen-positive, the clinical dose transferred to the infant is probably exceedingly low," he told Medscape Medical News.
Hale also stressed that judgments about drugs in this context should weigh the risk of the drug against the risk of not breastfeeding. "All of us caution women not to use cannabis when pregnant or breastfeeding," Hale said. "But when the decision has to be made as to whether a mom breastfeeds or not if she is drug screen–positive, a lot of other factors must be analyzed to make such a decision."
For the study, Wymore and colleagues screened 394 women who gave birth between November 1, 2016 and June 30, 2019. Of those, 25 women, with a median age of 26 years, were eligible and enrolled. Inclusion criteria included known prenatal marijuana use, intention to breastfeed, and self-reported abstinence. Prenatal use primarily involved inhaling cannabis more than twice a week.
Of the 25 enrolled mothers, 12 who self-reported marijuana abstinence were in fact found to be abstinent according to the results of plasma analysis. Those who continued to use the substance were younger than the overall sample, with a median age of 21, and were less likely to have attended college (23%) than abstainers (58%).
The researchers prospectively collected data on self-reported marijuana usage and paired maternal plasma and breast milk samples several times a week. All participants had detectable THC in breastmilk throughout the study. Initial median THC concentrations were 3.2 ng/mL (IQR, 1.2 - 6.8) within the first week after delivery. These increased to 5.5 ng/mL (IQR, 4.4 - 16.0) at 2 weeks and declined to 1.9 ng/mL (IQR, 1.1 - 4.3) at 6 weeks. In terms of ratio, the milk:plasma partition coefficient for THC was approximately 6:1 (IQR, 3.8:1 - 8.1:1).
Hale noted that although THC was detectable in milk, the levels were exceedingly low. "This is where the risk assessment comes in. There's a lot of hysteria in the cannabis field right now, and we're going to need time and a lot more studies to really be able to predict any untoward complications."
Wymore, however, countered that THC levels were low only in those who abstained and that her concerns relate not just to postpartum breastmilk levels but the health effects on children of mothers' cannabis use over the course of prepregnancy, pregnancy, and lactation. "[Hale's] message makes it difficult for clinicians to counsel mothers since it goes against national guidelines," she said. "We need to be consistent."
But Wymore and other experts acknowledge the dilemma faced in that breast milk clearly offers substantial benefits for infant and child health. "The risks of an infant's exposure to marijuana versus the benefit of breast milk must be considered," said Amy B. Hair, MD, assistant professor of pediatrics and neonatal medicine at Baylor College of Medicine in Houston, Texas, who was not involved in the Colorado study. "And it's unrealistic, as the study suggests, for mothers to discard breast milk for 6 weeks."
Nevertheless, calling the findings of THC persistence after abstinence "troublesome," Hair said the legalization of marijuana in some states gives the public the impression it's safe to use marijuana even during pregnancy and lactation. "Research studies, however, are concerning for potential detrimental effects on brain growth and development in infants whose mothers use marijuana during pregnancy and breastfeeding," she said.Wymore stressed that more US cannabis dispensaries must engage in rigorous point-of-sale counseling to women on the potential harms during pregnancy.
This is the case in Canada, she noted, where recreational and medicinal cannabis has been legal since 2018 and more than 90% of outlets (vs two thirds of their US counterparts) advise women not to use cannabis during pregnancy or lactation, even for nausea.
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