Here are a couple sites I found on the topic, I reccommend looking deep into the details. On the surface, ir appears no more dangerous than possibly taking OTC medicine, or smoking through pregnancy, but no of those produce desireable effects.
Was it a planned baby? Planned or unplanned, Maybe mentioning the possibility of raising a handicapped child because of her actions could reverse the situation. If she intends on having more children, possibly the long term effects on her reproductive health could be a deterrent. Also even if her kids never smoke marijuana, there is a possiblilty of them ALSO having reproductive problems when they come of age, due to genetic abnormalities and kifelong hormonal imbalances.
There is not a lot of research on it, but just the possibilty would be enough for me. I really have nothing against marijuana, legal or not, just concern for the baby. Honestly it is no more intoxicating than beer, and no more harmful than a cigarette, and what makes them dangerous is people who misuse it, or possibly getting a hold of tainted product. Thats not saying beer and cigarettes are not harmful, just that it's not really our place to tell someone no when those things are available legally.
Calling the police on it may be a bit harsh, depending on the amount she has, and whether she confesses she could do jail time. Also there is the possibility of them taking her child away. The plus side, they will likely put her in a rehab program and give her the opportunity to prove herself and win her child back. Maybe it could change her life
http://www.a1b2c3.com/drugs/wom06.htm
http://www.members.optusnet.com.au/~apfdfy/Unborn.html