I've said it before, but it bears repeating--that the pro-lifers define "conception" as what makes a baby is a rhetorical device to reinstate the belief that a baby is made by a man and merely borne by a woman. There are many steps in the process of turning raw material into a baby, but only one is bandied around by pro-lifers as the point that something turns from raw material into a baby, and amazingly enough that step is the only one that involves a man. Anything pre-conception (or, with the morning after pill, pre-implantation) that prevents bearing a child isn't baby-killing, but anything after a man has planted his seed, if you will, is the moral equivalent of murder. Ejaculation has become the end-all and be-all to pro-lifers of what makes something a baby.
Prior to Roe v. Wade and the formation of the radical anti-choice movement, most women and midwives regarded quickening -- the time when the woman starts to feel the foetus moving inside her -- as the start of new life and when this thing forming inside her really becomes a baby. But religious figures, doctors, and other assholes who had a problem with women having a say in their reproduction pushed the view common among anti-choicers today that life begins at the instant sperm and egg combine.