Cannabis Use During Late Pregnancy Linked to Lower Birth Weights, Study Finds

While it’s well known that smoking cigarettes during pregnancy can affect a baby’s health, there’s less scientific consensus around the prenatal effects of smoking cannabis.

But a new study, published in the Medical Journal of Australia, has found that mothers-to-be who continue consuming cannabis after 15 weeks of pregnancy are more likely to give birth to babies with a lower birthweight and a shorter head circumference.

The 15-week threshold

To get their results, the researchers from Australia, New Zealand, and the UK sifted through the data from 5,610 pregnant women who were part of a long-running cohort study from 2004 to 2011.

The participants disclosed how often they consumed alcohol, cigarettes, cannabis, and other illegal drugs to research nurses.

In total, 314 women (5.6 percent) said they used cannabis in the three months prior to or during their pregnancy. Of those 314 participants, 97 (31 percent) stopped consuming the drug before they became pregnant, while 157 (50 percent) stopped sometime during the first 15 weeks of pregnancy. Sixty women (19 percent) continued to consume cannabis beyond the 15-week threshold.

And, according to the researchers, it’s this 15-week pregnancy marker that makes all the difference.

Compared with babies of mothers who had never used cannabis, infants of those who still consumed the drug after 15 weeks had lower mean values for birthweight, birth length, head circumference, and gestational age at birth. Except for gestational age, the defects were greater for women who used cannabis more than once a week.

“The observed reduction in neonatal birthweight was comparable with that associated with continued tobacco use during pregnancy,” the researchers wrote in their paper.

Notably, neonatal outcomes for babies of women who quit cannabis before or during early pregnancy weren’t significantly different from those for infants of women who had never used cannabis.

But while the effects of sustained cannabis use during pregnancy were evident, the mechanisms underlying them are little less clear.

In their paper, the researchers suggest that the lower birthweights may be related to the carbon monoxide generated by smoking cannabis, but further studies will be needed before any firm conclusion can be made.

Future research could also account for how much cannabis participants consume – a limitation of the current study.

Pregnancy and marijuana

The proportion of women using cannabis to relieve their morning sickness symptoms is on the rise in areas with legal recreational access.

But, while it’s been known since the 1980s that THC can cross the placenta and reach the fetus, its prenatal effects have been sparsely documented in comparison to the effects of other substances, such as cigarettes.

To fill this knowledge gap and help inform mothers-to-be, several recent studies have investigated just exactly what happens to a fetus when exposed to cannabis.

One recent study found that, when exposed to THC, bovine eggs are significantly less likely to result in a viable pregnancy.

Another study published last year found that prenatal exposure to cannabis may alter the fetal brain’s endocannabinoid system, which could lead the child to develop psychosis when they’re older.  

Fortunately, these THC-induced psychotic symptoms may be curable. A separate study from last year discovered that rats born with psychosis could be successfully treated with a dose of pregnenolone, an FDA-approved drug and diet supplement.

But, speaking to Analytical Cannabis last year, the study’s lead author advised that the best method for preventing such symptoms is avoiding cannabis all together while pregnant.

“Forty percent of Americans would think that it's okay to smoke marijuana when you're pregnant, and our study clearly says that is not okay at all,” said Miriam Melis, an associate professor of pharmacology at the University of Cagliari, Italy.

“Would you give a joint to a kid?” she asked. “I think you wouldn’t. So why would you need give it to a fetus?”