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 American Medical Association found that the number of pregnant women in Northern California who report using cannabis nearly doubled from 2009 to 2016.  Almost 70% of dispensaries in Colorado 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.