While I'm generally in support of marijuana, you don't mess around with pregnancy. Anything you're taking recreationally to feel good needs to at the very least take a 9 month vacation, you cannot be selfish enough to be getting stoned and drunk while carrying a developing child. The risk/reward scale is really unbalanced here.
The particles accumulated over a very long period of time could potentially lead to ill effects in the mother which are passed down to the child. Also, one would have to take into account the possible use of pesticide to grow hemp plants. Though indirect, that could cause issues with a greater probability than the Marijuana itself.
Marijuana, like tobacco, can cause cancer in the smoker and the unborn by poisoning the smoker with heavy metals. These heavy metals can build up to dangerous level and cause cancer, as well as other degenerative illnesses. This process may take years but it doesn't diminish the fact that smoker's likelyhood of developing cancer is greatly increased. So is a smoking mother's unborn child.
Heavy metals contained in marijuana smoke, like cigarette smoke, can accumulate in the mother. These heavy metals can then be transferred to the fetus and cause birth defects like tumors in the unborn child. Marijuana is said to have beneficial effect, but so does tobacco. The bad far out weighs the good.