Interesting legal blogWatch
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If you allow an unborn foetus to be capable of being 'murdered' then you give credence to the pro-life position that all abortion is murder which is incredibly politically volatile.
The morality question depends largely on how you view the purpose of sentencing/punishment through the criminal justice system. If the purpose of punishment is seeing that justice is done e.g. attaching the word 'murderer' to the baby killer, then the manslaughter/GBH/1929 Act route is unsatisfactory. If the purpose isn't to label someone, then the law as it stands can achieve a very similar effect and still recognise that a baby has been killed.
As you say, the 1929 Act provides similar offences namely section 1 child destruction i.e. any person who with intent to destroy the life of a child capable of being born alive, by any wilful act causes a child to die before it has an existence independent of its mother, shall be guilty of child destruction. Life imprisonment is available here too.
I guess the question remaining is 'why does the label of murderer matter?' The person convicted would be guilty of 'child destruction' which is arguably equally as abhorrent a label to receive and would be equally sentenced.