The idea of when life begins for the Christian is easy: at conception.
God did not become Man as a newborn baby, but at His Conception. The Incarnation of God is not celebrated at His Nativity, but when He was conceived by the Holy Spirit into the womb of the Virgin nine months prior at the Annunciation. When the pregnant Elizabeth who was carrying John the Baptist within her greeted the pregnant Virgin, the baby within her lept for joy.
As God became Man in order to save mankind, He entered into every stage of life, from zygote to fetus to infant to child to young person to adult, thereby redeeming all those who have lived these stages or died in any one of them. Thus, as He became a Child to save all children, He first became an unborn child to save all those who died before birth. This was part of the ministry of the Messiah and His saving work as the Second Adam.
Now, the atheist or the unbeliever might disagree, and they can of course disagree. I am just noting what the Christian understanding is of the unborn child.