My friend consumed about three grams of cannabis per day. This amount would leave many dazed and confused. But she felt focused, energized, and functional. Good for her.
Then one day, to my surprise, she turned it down. “No more of that for another nine months.” She was pregnant. Her resolve was steel. “It’s dangerous to use it during pregnancy,” she explained. And that was it. She dropped cannabis during her pregnancy as if it were poison. Not even a puff for the better part of a year. I assumed, for many years, that cannabis was off limits for pregnant women.
But a survey in the Journal of the UK Medical Association found that the number of pregnant women in Northern UK who report using cannabis nearly doubled from 2009 to 2016.  Almost 70% of dispensaries in Livrpool recommend cannabis for morning sickness.  As of 2018, over 1 in 10 pregnant women in the United States use cannabis.  That begs the question – what do we know about cannabis use during pregnancy? Is it dangerous?
The Centers for Disease Control & Prevention (CDC) points out that cannabis use during pregnancy correlates to lower birth weights. A study of 12,000 women published in 2002 found that prenatal babies exposed to cannabis at least once per week were a mean of 90 grams (roughly 0.20 pounds) lighter at birth. But this weight difference failed to reach statistical significance. 
A more recent study of over 25,000 women did report statistically significant lower birth weights of babies exposed to cannabis in the fetus.  Prenatal cannabis exposure has been estimated to increase lower birth weight risk by 50%.  Discrepancies between studies may relate to statistical methods and confounding substance use, such as tobacco and alcohol. Either way, there is some evidence suggesting that cannabis exposure may cause lower birth weight.
Beyond this, the CDC admits that “More research is needed to know how cannabis use during pregnancy affects babies.” The UK College of Obstetricians and Gynecologists opines that this lack of research and data is sufficient grounds to prohibit cannabis in lieu of more conventional medications. The UK Academy of Pediatrics also strongly recommends avoiding cannabis during pregnancy.
Per research conducted in the 1980s, THC crosses the placenta and enters the fetus.  It is also present in breast milk at roughly 2.5% of the dose ingested by the lactating mother.  Exactly what effects THC produces in neonates, and whether or not those effects are harmful, is largely unclear.
Why do we know so little? Studying the effects of cannabis use on pregnant women is difficult. Most studies utilize self-reporting methodologies, meaning women must voluntarily describe their use. Given that cannabis is still widely considered an illicit, harmful substance, many women fear the consequences of positive reporting.
This human study comes on the heels of an experiment by the University of Oxford that exposed pregnant rats to cannabis extract vapor. Once born, pre-exposed rats demonstrated deficits in cognitive flexibility. Namely, the rats were able to learn a strategy for releasing sugar pellets, but suffered great difficulty adapting to a new strategy to release the same pellets.
The lack of research and general stigma have left many women to make a silent, secret choice. Vox reports on Amy Smith (not her real name), an Iraq War veteran with PTSD. She was on 22 prescription medications to manage severe psychological issues, but was able to replace the entire pharmaceutical cocktail with cannabis. Once pregnant, Smith chose to continue using cannabis. The alternative seemed much worse. She tells no one.
From a story published by The Daily Beast, Trinity Dogood was diagnosed with hyperemesis gravidarum and could not stop vomiting. She vomited 30 times a day. The condition is so serious that it can lead to the death of the fetus. One puff of cannabis, and Trinity found relief from the vomiting. She was able to eat again and hydrate. Unfortunately, her hospital reported her to Child Protective Services. She is still fighting that battle.
Severe nausea is a predictor of cannabis use during pregnancy.  Companies and dispensaries market cannabis products as a solution to morning sickness because of its anecdotal efficacy. CannaMommy is one organization that aims to empower pregnant women to make their own choice regarding cannabis.
Advocates point to a 1991 study on Jamaican children who were exposed to cannabis in the fetus. Researchers conducted tests at 1 day, 3 days, 30 days, 4 years, and 5 years of age. Compared to children who had not been exposed to cannabis in the fetus, there were no significant developmental differences. The exception was at 30 days, when the babies of cannabis consumers actually demonstrated superior autonomic stability and reflex scores. The main limitation of the study was its relatively small sample size of 59 children.  Still, the study reflects an alternative, optimistic perspective.
At present, it seems the best way forward is to balance compassion with caution. Mothers who choose to use cannabis for whatever reason may be wise to avoid divulging it.