Abortion is prohibited to all mankind by the Hebrew text of Genesis 9:6. The so-called “Ten Commandments” are specific to Jews and do not negate the commandments given to all mankind previously.
I don't think your Genesis verse will work. Otherwise it would prohibit war, capital punishment, killing slaves, and disobedient children... no, I think it's time to face the cold, hard facts: abortion wasn't an issue back then and your all-seeing all-knowing God didn't foresee this situation enough to specifically prohibit it. The Jews are right. It's a non-issue that Christians have simply glommed onto in their efforts to battle sexual freedom. Losing battle.