That is the question.
In fact, in 1973 when abortion became a legal right in the United States, not being able to discern when life begins was a major convincing factor in the fateful decision:
"We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer...we do not agree that, by adopting one theory of life, Texas may override the rights of the pregnant woman that are at stake."
Here's what strikes me in the above paragraph: "When those trained in the respective disciplines of...theology are unable to arrive at any consensus..."
As Christians, the question of when life begins should be as easy a question as "who are we created in the image of?" If we read our bibles, we know that God knitted us together in the womb, that human beings are created in His image, and that the murder of an innocent is prohibited. We have made complicated these simplicities. I am not suggesting that the circumstances around the truth are simple, many times they certainly are not. What I am saying is that to the person who knows Jesus, the basic truth of when life begins is simple.
The below article nails it:
Argument For The Silent: A Biblical Case Against Abortion
"...if the unborn are human beings, killing them is forbidden because it is an attack on the image of God... It is a scientifically certain fact that the event of conception (or, perhaps more narrowly, fertilization) is the first event in the history of the human being. Prior to conception, what exists are a mother’s egg and a father’s sperm. When fertilization is complete a new human being is brought into existence that is genetically distinct from the mother and father and that exists as a distinct entity. Therefore, from the moment of conception what exists is a human being with potential for growth and full realization." Read the full article here.
"You shall not murder." Exodus 20:13