It's about 5% of pregnant women, while about 14% of women report smoking cannabis, and the rate is significantly higher under age 55:

https://news.gallup.com/poll/284135/percentage-americans-smo...

So probably about 20% of women who may become pregnant smoke marijuana, and 75% of them stop when pregnant, before adjusting for whether they are aware of the pregnancy. There are usually a few irresponsible individuals in any group.