I'll reference the previous (nearly identical) question's answers:
"When men strive together and hit a pregnant woman, so that her children come out, but there is no harm, the one who hit her shall surely be fined, as the woman’s husband shall impose on him, and he shall pay as the judges determine. But if there is harm, then you shall pay life for life, eye for eye, tooth for tooth, hand for hand, foot for foot, burn for burn, wound for wound, stripe for stripe.
For You formed my inward parts; You wove me in my mother's womb.
If man chooses to define when life begins at any point later than conception, then it is logically viable to choose to define life "beginning" at any arbitrary age. For example, if the definition is "viability", then it is reasonably arguable that someone is not "viable" until they are old enough to provide for themselves - therefore, a 6 year old is not "viable" since it would die on its own.
By accepting conception as the moment life begins, you accept the plain, simple reading of God's word over a 'nuanced' reading as being correct. Reading nuance into the verse is not good exegesis, but is rather eisegesis - a dangerous path to be on when considering the Bible is God's word, and not man's!
If you want to look at the medical technology and knowledge of the day in which the Bible was written (and, really, until very recently), it could not be known that a woman was with child until weeks or months into the pregnancy - except by inference of potential symptoms (lack of menses, morning sickness, etc). The law issued to the Israelites regarding what would happen to a man if he struck a pregnant woman and harm came to the baby should show clarity into God's view of how important life is - and how 'early' He considers it to exist!
sidebar: at conception the new person is already a multi-celled organism - amoebae are single-celled organisms, and no one would deny they are alive