Tom was watching over the family.
Kay couldn't even go to the market, was turned back at the Gate on Tom's orders and Michael's request for their safety. New England trip is off too

She didn't necessarily have to get away by herself, to get an abortion If...
Even an escorted Kay would not have buttonmen present during Kay's medical examination / consultation with her Gynaecologist

Don't know... We have debated before abortion or miscarriage?

Some of us including myself, believe it was miscarriage perhaps during the Tahoe shooting trauma and Kay out to spite:
It was a son Michael, a son and I had it killed --

Besides No doctor especially the family doctor would dare perform an abortion on Michael Corleone's wife

The bigger mystery is Michael just accepting it