If it were me, I'd just say that I'm finding that with my pregnancy, I wasn't up to watching the two of them anymore. Use doctor's orders, to be sure, but the fact of the matter is, it's perfectly reasonable for you not to be up to the task of baby-sitting when you're heavily pregnant. If your friend doesn't understand, then she's not much of a friend. If your friend is a true friend, she would feel terrible to know that you were exhausting yourself to care for her child. Just tell her the truth, in a nice way.