Well the most likely widely used but least effective form would be prayer and offerings to gods and goddesses. After that a clear second would be herbs and potions. There quite some poisons out there that would end a pregnancy. Ending a pregnancy isn't particularly hard, the difficulty is in doing it in a save way.

The Greeks are known to have used Silphium for this purpose. It has since sadly been lost to us. Most likely it's simply gone extinct but we can't know for sure.