“Studies on marijuana use during pregnancy are inconsistent and inconclusive. But cannabis is not known to be teratogenic—that is, to cause birth defects—in humans. The bulk of scientific evidence suggests that risks posed to developing fetuses are relatively minor and babies exposed to marijuana in utero still fall within normal ranges of outcomes.
A 2020 review looked at longitudinal studies on “the impact of prenatal cannabis exposure on multiple domains of cognitive functioning in individuals aged 0 to 22 years” and found that “evidence does not suggest that prenatal cannabis exposure alone is associated with clinically significant cognitive functioning impairments.” Researchers did note some differences—”those exposed performed differently on a minority of cognitive outcomes (worse on < 3.5 percent and better in < 1 percent)" — although "cognitive performance scores of cannabis-exposed groups overwhelmingly fell within the normal range." A 2016 review of studies on potential ties between in utero marijuana exposure and adverse birth outcomes—things like low birth weight and preterm delivery to miscarriage and stillbirth—found "maternal marijuana use during pregnancy is not an independent risk factor for adverse neonatal outcomes after adjusting for confounding factors." Instead, any increases in adverse outcomes appeared "attributable to concomitant tobacco use and other confounding factors."" ... ""The best new evidence on this comes from a 2019 study out of Canada," Oster writes. Matching women who used cannabis with demographically similar women who didn't, researchers did "find evidence of worse birth outcomes among the cannabis users," including "an increased risk of prematurity and NICU transfer. The increases are moderate but statistically significant: preterm birth occurred in 10% of cannabis users and 7% of non-users." An August 2020 study from the same authors found marijuana use correlated with slightly higher incidences of intellectual disability and learning disorders, as well as higher chances of having autism spectrum disorder. "The percent increase is large—about 50%—and significant," Oster points out, though the researchers do note that the overall incidence rate is still small. Though the researchers tried to demographically match participants between groups, it can still be hard to totally compensate for the ways marijuana users may differ from non-users." ... "The biggest problem is that it's hard to isolate specific factors like marijuana consumption. The population of women who not only use marijuana during pregnancy but are also willing to admit to researchers that they do may differ from those who don't." ... "A review of evidence published in February 2020 "points to the possibility of lower birth weight, diminished IQ and more behavior problems among children whose mothers used cannabis during pregnancy, but notes it is very difficult to separate the marijuana use from other demographics or other variables,"" ... " With the limited evidence available, it may make sense for most pregnant women to avoid marijuana to minimize possible risks to their offspring. But the best choice for one woman and her baby won't be the best choice universally. For women who have extreme morning sickness that makes getting adequate nutrients through food and vitamins difficult, and for whom marijuana mitigates nausea, using cannabis might make sense. Likewise, women with certain mental health conditions helped by marijuana may deem it safer than their usual prescription drugs."