In the sentence "life begins at conception", the word "life" is a "mass noun", which can mean either "the characteristics which differentiate living things from dead ones" (that is, that the thing in question is alive), or "the period during which a human being exists". Pro-lifers use this ambiguity (possibly unwittingly) to help to "prove" they are correct, by claiming that science has proven the latter, when they have only proved the former.

If they said "a life begins at conception, that is the "count noun" form of the word life, which only means "the existence of an individual human being" (which is actually synonymous with the latter definition of the mass noun life). Since there is no ambiguity, you'll note that pro-lifers can't use this to deceive people, so they will only ask when debating "does life begin at conception", not "does a life begin at conception.

This is also why some pro-choicers, when asked if "life begins at conception", will reply that "it is alive, but not a life, in an attempt to differentiate between the two possible meanings.