when abortion is to be considered murder?

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I was goin thru a discussion about woman rights to abort, then I thought that having a fetus would be considered murder at what stage? I know that before the second trimester there's little chance that there will be any pain "felt" from the fetus. But even in the last embryonic stage there are detectable movements... when is it that we can say that something growing in the womb of a gravida begins to be alive thus if aborted to be killed? Please, let me know what you think, but armed if possible with scientific data on thing like when the brai is developed, the fetus can hear, see, move voluntarely feel pain, dream etc. Thank you.