I feel sad that the OP was attacked by several individuals.
He is clearly worried about the fetus, and has spend time and energy doing research to see if smoking marijuana seems like a very risky thing to do while pregnant.
He is obviously very concerned with the health of his baby, and is far, far more responsible than the thousands of people who continue using serious drugs while pregnant without asking any questions. Many people have no leeway to ask such questions, and are only able to continue their lifestyle, focused on themselves, not their fetus.
It is ideal to use no drugs while pregnant (and, BTW, yes, I do include sugar in my list of drugs). There is no question that this is what we would all do, if we had the appropriate willpower, the knowledge, and the emotional and mental circumstances.
However, life does not always allow for us to be perfect little angels.
Do you have leeway in your heart to understand that not everyone is working according to your ideals? That circumstances can vary, and that some people might be doing their absolute best to smoke marijuana (instead of, for example, using crack) while pregnant? That marijuana is unlikely to harm the fetus, and that it might help the mother greatly?
A woman smoking marijuana while pregnant is EXTREMELY different than one who uses crack, heroin, tobacco, or alcohol, at least to me.
She is very roughly on par with one who uses caffeine, for example, at least to me, depending on the exact situation. Drinking 5 coffees a day is worse, to me, than smoking 2 hits of pot through a water pipe, in that I would estimate that it is more likely to have detrimental effects on the fetus.
Marijuana is a relatively benign drug. My wife and I also researched it while she was pregnant, and determined that light use, like she was considering (smoking one small hit once or twice during the pregnancy) was unlikely to harm the fetus. My wife gave up her 20+-year caffeine habit for the sake of her pregnancy, but she wanted to know if smoking weed would be an acceptable way to relax once or twice. Ultimately, she rejected the idea, saying that she didn't need to smoke in order to relax, but she wasn't adverse to the idea of smoking occasionally after examining the literature on marijuana and pregnancy, and weighing the potential damage against the expected benefits of smoking.
It is very, very easy for people to judge others without actually going through the experience themselves.
Been pregnant yourself? Or your partner? Did you stop all drug use for 9 months, without using anything even a single time?
If not, you have no right to judge other people, especially someone who is making an effort to find out the facts about health and drug use.
It is especially easy for a man to say this, while he continues his drug use throughout his partner's pregnancy, laying down the moral law that she should stop while he continues.
If you have been pregnant, and stopped all drug use, that is wonderful. You are very responsible. However, not everyone is able to do this. You do not know the particular situation of the woman involved. Therefore, I would say that you still have no right to judge her. In fact, I would ask that you would give her the benefit of the doubt, and assume that she is using weed for a good reason, and has determined to do so based on research.
As for wishing horrible things on the innocent fetus, I would consider that to be a more heinous crime than smoking marijuana while pregnant (in my own personal lawbook). I cannot imagine ever wishing something so horrible on anyone, especially an innocent life. I shudder to think of how angry and small hearted you must be to do such a thing.
OP, I wish your baby the best.

I also suggest, as I have already said in this thread, that your partner abstain from using all drugs while pregnant.
However, ultimately, she has to make the choice herself about what benefits she might gain and what potential dangers there are to smoking.
As has been mentioned, vaporizing and eating weed are preferable to smoking it.
Not using it at all is ideal.
(But, if she is incredibly stressed, or nauseous, for example, then using a small amount of weed might be worth it, to her. I hope that you can be supportive of her needs and decisions.)
Peace, all.