It can be assumed that although under house arrest (which is how Kay saw it but not Tom), Kay would still be allowed, even if heavily guarded, to keep Dr. appointments during her pregnancy. Even if a Dr. would have to be summoned to the compound to conduct the examination(s), it's even easier to assume that a woman in Kay's financial and social position could provide the means to have an abortion performed once she made the decision that it is what she would have to do.

Though Roe v. Wade was certainly years in the future...this was the 1950's, NOT the 1850's. Women like Kay could get certain things done behind their husband's backs.

When the film was made, I don't think there was anything in the writer's/director's minds other than that Kay Corleone in fact DID have the abortion.

Apple