I don't think Kay would have ever 'made up' the abortion just to hurt Michael during that hotel room scene. In fact, I don't think she had ever originally intended to tell him about the abortion, and finally did only because he was so resistant to her leaving with the children.

Which was in itself a huge misjudgement, because as Michael said in his own words...what else would she expect him to do?

Anyway...she DID have the abortion it was not a lie. That's not how Kay ticks.

Apple