What he said.
In the first place, why do you think that it's somehow a Christian belief that "human life begins at the moment of conception?" You certainly don't get that from Christ -- there is not one word about abortion anywhere in the Bible, New Testament or Old, no definition of human life, and no reference to the sanctity of fetuses or even, for that matter, the sanctity of human life in any form.And it's not because they didn't know about abortion and contraception in those days.
Christians didn't start to argue that abortion was even sinful, let alone tantamount to murder, until the late 19th Century. So how did they make this discovery? Do you believe that it suddenly occurred to God that he'd forgotten to mention it while he happened to be incarnated almost two millennia earlier, so he quickly provided the Pope and some Evangelical preachers with a new revelation? Or, if it's merely a conclusion you come to based on your overall understanding of ethics, then why aren't other people free to come to different conclusions?