This question was discussed before here:

http://www.thegodfathertrilogy.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=004229#000005

Here is my opinion, again:

"I always thought that Kay really did have a miscarriage, and later lied to Michael about having an abortion. She wanted to hurt him so he would let her leave.

It would have been very very difficult for Kay to find a doctor to perform an illegal abortion. Kay was practically under "house arrest" at Tahoe. Hagen wouldn't let her out of the compound. Therefore, how could she go out and find a Doctor - WITHOUT HAGEN'S KNOWLEDGE - in the pre-Roe v. Wade days? I wonder if the mob ever had any control over illegal abortions? And, what doctor would perform an illegal abortion on the wife of Michael Corleone!!!???

Also, I don't think Hagen would lie to Michael. Therefore, Hagen also believed Kay had a miscarriage. Otherwise, we must believe that naive Kay was smart enough to fool Tom Hagen!!"