Legality and safety (from a health perspective) are two different things. Cigarettes, alcohol, and opioids while all legal have health consequences. Some more concerning than others. This is especially true in pregnancy.
Chemicals (whether they are naturally occurring or not) may have effects on blood flow to the uterus or at a microscopic level in the placenta (affecting growth and/or brain and organ development) and/or may be a teratogen (cause birth defects). In addition, neuroactive chemicals may affect development of the brain and/or nervous system.
So what about marijuana in pregnancy? With growing legalization of pot there may be increased exposure in pregnancy, either unintentionally (a smoker who doesn’t know she is pregnant), via second-hand smoke, or a smoker who is intentionally smoking during pregnancy. Are there risks?
A new study from the Stillbirth Collaborative Research Network provides more insight. This is a population based case-control study where women between March 2006 and September 2008 who delivered a stillbirth had extensive testing from maternal blood, fetal blood from the umbilical cord, and placental tissue analysis. The samples were tested for cotinine (a nicotine metabolite) and a variety of substances. The cases were matched with control live births who had the same testing.
Smoking cigarettes was significantly associated with stillbirths (as expected). And the more cigarettes the higher the risk. Even second-hand smoke was a concern as women with positive cotinine levels and who reported not smoking had a significant increased risk of stillbirth.
Tetrahydrocannabinolic acid was used as the proxy for marijuana and its presence more than doubled the odds of a stillbirth. When controlling for cotinine levels (i.e. controlling for smoking and/or exposure to second-hand smoke) the odds dropped about 10% so there is a possibility that there was some confounding with cigarette smoke exposure. This confounding effect, even if robust, is unlikely to reduce the risk of marijuana exposure to zero, however more research is definitely needed to study the interaction of cannabis and cigarettes (especially as there is a high incidence of cigarette smoking among marijuana smokers).
There are relatively few animal studies looking at the effects of cannabis on pregnancy (likely due to the illegal nature of the drug). The few I found were not encouraging from a safety standpoint, in that exposure to marijuana smoke reduced oxygen delivery to the fetus. Reduced oxygen delivery is a definite risk for stillbirth via a variety of mechanisms.
The body of literature, such as it is, does not support the safety of marijuana during pregnancy and certainly based on this new study marijuana appears to be associated with an increased risk of stillbirth.