A bunch of cells that may lead to a baby with half a heart is not the same as a baby with half a heart.
That's the point.
My vegetarianism has limits which are very difficult to justify.
I don't eat prawns for example.
Not because I have a personal feeling or scientific evidence which suggests that prawns can suffer in the same way that a cow can or a human but because I have to draw a line somewhere and it's just easier to draw it not eating animal flesh.
Some plants demonstrate a more obvious response to their environment than some prawns.
A prawn in the wild is probably more capable or at least as capable of suffering as a human embryo of the same size.
A fully grown human baby is a different matter, arguably an adult has more capacity for suffering than a child (because it can perceive a position where it wouldn't be in pain more than a child).
I would sacrifice the prawn-like life of an undeveloped fetus to prevent a fully grown baby suffering the futile pain and suffering of trying to live on half a heart for under 24 hours.
I would certainly have less compunction extinguishing that life to allow another fully formed human to live and thrive for more than 24 hours.
Such an act sparing the suffering of someone I know having a dead baby representing the last chance she has at having a son prised out of her arms is a bonus in my view.
It depends on if you see the personality as pre-packaged or an emergent property.
If it's pre-packed every sperm and ovum are indeed sacred and every ejaculation is a disaster with only one possible survivor.
If it's an emergent property, attaching more value to even a developed human fetus than any other animal is a to some degree just sentimental.
Most people are somewhere in the middle.