Bart Ehrman is worth listening to. (I only have a set of audio lectures of his; he may be worth reading as well!) However, he put forward what must be the worst argument I have ever seen against the existence of a miracle.
He was discussing the idea of the virgin birth, and he dismissed it by saying, "We know for a fact that women who haven't had sex don't have babies."
Yes, in the normal course of events they do not. That is why, if such a thing ever happened, it would be termed… miraculous.