Sicilian Babe raises a significant point when she writes, "Could she have ... lied to Michael about it in order to hurt him?"

Not only does she confess to the abortion but, notably, knowing Michael's desire for a boy, she adamantly and repeatedly emphasizes that the aborted child was a son:

"I didn't want your SON Michael. I wouldn't bring another one of your SONS into this world. It was an abortion, Michael... It was a SON Michael -- a SON -- and I had it killed... "

While it's certainly true that she had the abortion, I've wondered how Kay was so certain that she aborted a son. Could she have said that, even if she didn't know for sure, just to deliver an extra blow to Michael?

Personally, I believe Kay was not deliberately lying; she truly felt she was carrying a son. She had been pregnant with Anthony, so if her symptoms were identical to what she experienced with him, then her motherly intuition would tell her that this was another boy. I don't think FFC wanted us to question that. Narratively, the irony is just too perfect: The powerful Michael Corleone being denied the only things he truly desired, one of them being a son.

But since we are Godfather obsessives, we can't help but take an interest in any extra information we can get about a scene. And for the sake of that interest, I'd like to submit that Kay couldn't be medically certain that the fetus was male. There is ultrasound, but it was just being developed in the ob/gyn field in the 1950s. (And besides, even with today's equipment, an accurate sex determination can't be made before the mother is 20 weeks along; Kay was only 12-14 weeks, or 3 and a half months, as Tom indicated.) The other pre-natal sex determiners, amniocentesis and CVS, were only concepts in the mid-1950s (late 1960s for CVS).

And what about taking a look at the aborted fetus? While sex can be visually differentiated at 12-14 weeks, the methods of abortion at that stage, suction or scraping (curettage), invariably destroy the fetus. Labour-inducing abortions in which the fetus is expelled are only done in the 2nd trimester (and weren't begun until the 1970s anyway).

So there's our technical Godfather information for the day. But regardless, Kay's driving home the point that she aborted a son was the final twist in the knife during the abortion confession scene. As Sicilian Babe indicated, Michael had hurt Kay; and now she was showing him how it feels to be hurt back.