Well, that scene raises a couple of points:

First, Kay shouts with vehemence: "And it was a boy, Michael!" Kay was in her first trimester (when Michael asked Tom, after he got back from Havana, if it was a boy, Tom replied, "Michael, after three months?"). Amneocentisis was unknown in the US in 1959. Kay couldn't have known for sure if it was a boy. She spit that out at Michael to hurt him.

Second, how did Kay get the abortion? Although abortion was illegal in the US before Roe v. Wade (1973), any woman or girl with a few hundred bucks could have gotten one from a real doctor, in a medical office or clinic. But, Kay was a virtual prisoner at the Tahoe estate while Michael was away. Kay might have told Tom that "something wasn't right" with the pregnancy. He would either have arranged for her Dr. to see her at Tahoe, or had her taken under guard to a hospital. Either way, the Dr.'s identity would be known, and he would have had to certify that she had a "miscarriage." Think Michael would have let him get away with that? For that matter, do you think that any Dr. in his right mind, knowing who Kay's husband was, would have stuck his neck out?