What determines when life begins, SCIENCE  

Life starts at conception, the human embryo is human being at fertilization.

At fertilization, the human being emerges as a whole, genetically distinct, individuated zygotic living human organism, a member of the species Homo Sapiens.  That is the science!

Jeremiah 1:5

"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."

If the soul, the 'person' exists prior to the physical vessel, the body, then it is separate from the body. Various beliefs say various things about that, from prior to conception (Jeremiah 1:5) to the first birthday (or later). If it is a 'potential' person so the sonsequences of the decision are speculative and one gets into all manner of difficulties... like spilling your seed.

Are you circumcised? Did removing your foreskin kill anybody? No, it was a part of your body just as the pregnancy is so, until an undetermined point.

Science is silent or at the very least contradictory and/or unproven as to when the process results in a person with standing in the law. I will argue absolutism in every respect on this point whether the opinion is conception, first breath or some point in life. What is inarguably so is that it is not my decision, I am not pregnant nor a woman. Allow those bearing the consequences to make the decision.