I think with the amount of literature and physician's advice out there to treat a woman's pregnant body like a temple it is a no-brainer that anything has an effect upon it, let alone a drug like marijuana. It may not be as drastic as many think, but it enters the bloodstream and therefore will come into contact with the fetus. How can it not have some effect?
Yes, marijuana can effect pregnancy. It kills brain cells and is not really good for anyone but why would you subject an unborn infant to that in the first place? They don't have a choice but to deal with the harm the mother is doing to her body and that is not fair.
It can do way more than just have the affects of marijuana. It is a gateway drug to many other and worse drugs. If a mother is risking a pregnancy on smoking a joint than they would most likely hurt it with doing something that could be much worse and really hurt the baby.
Since everything that a pregnant woman ingests affects the fetus in some way, yes, marijuana affects pregnancy. So far, I have not heard of any evidence that suggests that marijuana is not advised for pregnant mothers, but since the first few months are so important for the child's development it is advisable to stay away from anything that changes a mother's state of mind in any way. Since marijuana is a drug, and does cause certain changes in the brain due to the activation of receptors (much like medical drugs like morphine), it is possible that exposure of marijuana to a developing fetus can harm it in some way, just as many prescription drugs as well as alcohol and smoking are harmful to it.
My wife tried this becuase we used to do this all the time. Our baby is was a super college athlete and is going to the MLB. He is a 2nd baseman. He got drafted by the Akron Aeros and he is the whole team. He is a common marijuanan baby and he is a complete beast. Do what you have to do to be rich right?