Since mormons believe in a pre-existence or in a premortal life, life has already begun. In the preexistence, spirits are waiting for a physical body. They achieve this physical body upon birth. So one can claim that human life begins upon birth.
Let me put it this way: Women have miscarriages all the time. Does this mean that life has ended upon miscarriage? Probably not. Since we existed before we came to this earth as spirit children of Heavenly Father and Heavenly Mother, we wait for that physical body and when that physical body is achieved the spirit ‘continues to live and function in the mortal body’.
In Jer. 1:5 we read: ‘Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations’. This makes clear that God knew Jeremiah in the pre-existence. And in Eccl. 12:7, it reads: 'and the Preacher asserted ‘The spirit shall return unto God who gave it’
And so, when does life begin? Upon birth when the body receives the spirit meant for that body.
But then again, I could be wrong.