Cannabis use by mothers during pregnancy and while nursing may disrupt and slow the reproductive development of their daughters and even granddaughters, a new Washington State University study suggests.
The research, published in the journal Toxicological Sciences, found female mice exposed to cannabis while pregnant and nursing gave birth to smaller offspring that experienced delayed puberty and irregular reproductive cycles. The strongest effects appeared in the first generation, with signs diminishing in later generations.
"Our study was specifically designed to gain a better understanding of the long-term consequences of cannabis use by pregnant and nursing mothers on reproductive function and how it might affect subsequent generations," said Kanako Hayashi, a professor in the WSU College of Veterinary Medicine's Center for Reproductive Biology who led the study. "While many people think cannabis is safe to use while pregnant or nursing, our data and other studies suggest otherwise."
Cannabis, which is now legal in 24 states and Washington, D.C., is the most widely used recreational drug in the United States, with more than 48 million people reporting use in the past year.
The American College of Obstetricians and Gynecologists advises against cannabis use during pregnancy and breastfeeding, citing links to low birth weight, impaired brain development and other risks, but national surveys show use has risen among pregnant people in recent years, spiking to as high as 25% during the COVID-19 pandemic. Nearly 70% of women also said they believe cannabis poses little or no risk during pregnancy, even as cannabis products have become more potent, according to a study published in the American Journal of Obstetrics and Gynecology.
To mirror human use, Hayashi and her team exposed female mice to vaporized cannabis extract twice daily from the start of pregnancy until their offspring were weaned at 21 days old. They then tracked outcomes across three generations.
The researchers found cannabis exposure had no effect on the mothers' pregnancies, as gestational length, litter size and survival rates were all normal. The first-generation litters, which were directly exposed in the womb and during nursing, however, were born smaller, reached puberty later and experienced disrupted reproductive cycles. Although they eventually grew to a normal size as adults, their early delays were clear signs of disrupted development.
The effects were much milder for second generation females, which were daughters of the first litters. This generation still had some irregular cycles, with fewer and longer cycles than normal, but they were otherwise healthy and able to get pregnant, carry litters and give birth without issues.
By the third generation, the differences had disappeared. The females grew and developed normally, had regular cycles and showed no reproductive problems compared with the control group.
"Our conclusion is that prenatal cannabis exposure directly affects early development," Hayashi said. "Those effects are clear in the first generation but become limited and much less severe in later generations.
"Cannabis can make people feel relaxed and relieve stress, which is why some may use it during pregnancy or nursing," she continued, "but it's important to consider how those choices may affect children and even grandchildren."
The study found no evidence that cannabis exposure during pregnancy induces preterm birth or stillbirth, although Hayashi noted some women who use cannabis during pregnancy may be more likely to take other substances.
"In this study we only looked at cannabis exposure, but the realistic situation is the pregnant women who take cannabis, some probably drink and smoke. Some might take cocaine and other drugs," Hayashi said, "and we don't yet know how those combinations might affect reproductive development."
Hayashi's lab is now studying how cannabis exposure influences male fertility and reproductive development. Early findings suggest male offspring may experience more severe reproductive effects.
