Let’s suppose this is true, that the early embryo or foetus doesn’t feel pain (though there is evidence to the contrary). Is that a legitimate justification for allowing the killing of an unborn child? Are you permitted to plunge a knife into the heart of someone who is sleeping just because they won’t feel the pain?
Of course I hope your answer is “NO!” The issue here is confusing harm and hurt. If you knife someone while they are sleeping or in a coma they may not feel pain but they have certainly been harmed!!