Hi Debra, nice question!
When you mention ‘life’ in the context of conception and birth, you are talking about life in a biological / physiological / medical context.
The problem with the question “When does life begin?” in this context is this: we do not have an acceptable medical definition for the more fundamental question, “What is life?” How then can we adequately answer when this thing called ‘life’ begins?
As a person who has dealt with numerous end-of-life issues and brain-death, the other question that I have struggled with medically is “When does life end?” We don’t have a satisfactory answer for when life begins or ends because we do not have a satisfactory answer for what life is in the first place.
Many attempts have been made to answer this fundamental question - ‘what is life’ in biological or medical terms. In modern science, when DNA was discovered and its mysteries unravelled, DNA was called the basis for life…until we discovered organisms without proper DNA. Earlier philosophers called ‘breath’ the evidence of life, or ‘humours’ or other ethereal imagined or real things. Today, in medical terms we say a person has life as long as there is electrical activity in the heart, or EEG waves in the brain or active oxygen exchange in the lungs etc. But all these attempts have proved inadequate in actually defining what life itself is. And that is because ‘life’ itself is something that is beyond the scope of material and biological investigation and we need to enter into the philosophical and metaphysical realm of discussion to answer this.
And that is where the problem lies. When we do not know what life is, it is anybody’s guess as to when it begins or when it ends, which is why we have such vociferous proponents in favour of and against contentious issues such as abortion, euthanasia, pro-life, pro-choice etc. Some say life begins at conception (when the zygote is formed), or when the embryo develops spontaneous heart beats and circulation, some say at separation from the mother at birth, and some modern naturalistic medical specialists go so far as to say only in late childhood when consciousness is supposedly formed (and we still don’t know for sure what consciousness is!!). There is no universally accepted consensus on this though many claim there is. It is still a matter of faith, educated guesses and derived conjecture even in medical circles! The accepted conventions for beginning and end of life are formulated only for the purpose of defining terms for medical procedures and decisions, not for arriving at a definitive answer.
The Biblical understanding of life is that it begins in the womb (perhaps at conception, or even before it), it proceeds from and is a gift of God. Even if we do not understand it fully and can still debate and quibble, two things are certain from a Christian viewpoint - 1. life is precious and 2. we do not have authority over it, either to give or take it. Christ said of himself, “I am the way, the truth and the life” (John 14:6). He alone has authority to give and take it. We are only caretakers of this wonderful and mysterious gift.
Psalm 139: 13-16
For you created my inmost being; you knit me together in my mother’s womb.
I praise you because I am fearfully and wonderfully made; your works are wonderful,
I know that full well.
My frame was not hidden from you when I was made in the secret place,
when I was woven together in the depths of the earth.
Your eyes saw my unformed body; all the days ordained for me were written in your book before one of them came to be.