I still disagree, my wife's mom visited us. We did mention pregnancy, she was asked at the consulate and she was honest about her visit which was pregnancy. My wife's doctor gave us a letter saying that she needed help. There is no reason why someone should not visit their daughter to help her with her pregnancy. There is no help as good or great as one's own mom (no nurse can provide care as much as a mother!). There is nothing that says that your mom can't take care of you when you are pregnant, especially if you have another kid, even though you can put him/her in a day care, if you have a difficult pregnancy, who is going to help at nights? I have friends that have mentioned pregnancy and has worked. It is not about what reason you are visiting, it is about how strong is the chance that you would not stay back. Pregnancy or no pregnancy, they need strong proof that you will return. If they feel that you are not going to return because there are no strong ties back home, saying that you are just going on a visit for sight seeing actually weakens your case, atleast pregnancy has some merit. I am not suggesting that you just say that your mom is visiting to help you with prgenancy, provide proof that she is going to return and she has no reason to stay back. Someone mentioned that $ is what's important, if that was the case eveyone who has money to spend should get a visa and that's not true. As I mentioned, it is all about how well you document your case.
Well, it has worked for me but that doesn't mean it will work for everyone. You should evaluate your options and make an educated decision based on what you feel is right.