There is no scientific, definitive, unquestionable way of determining without question or on some grand high authority when a fetus is or is not a "child". That is an entirely opinion based issue where facts, both anecdotal and scientific, can be used to argue on either side of the issue. While there is a definition for it legally under the law, its far from uncommon for a Libertarian to disagree with the precedence the law states now. Viewing the situation in such a way that the ONLY way to view it as an "unborn fetus" rather than an "unborn child" means you're not attempting one iota to actually honestly address the issue from the variety of legitimate view points and thus can't make an honest call on whether or not one could be libertarian and be pro-life. THEIR personal belief that it isn't a child is no better or worse than your personal belief that it is or isn't a child. Thus, by NOT acting you could just as much be failing to protect the rights of another life as you could be to infringing upon their rights. Which leads the libertarian that believes it to be a child to come into the quandry of do you gamble on the childs potential rights, which if you're wrong means its violated by their death, or do you gamble on the parents potential rights, which if you'er wrong means its violated by their right to bodily soveriegnty. It is perfectly reasonable to suggest that the gamble is more apt to be made in defference to the first situation as "death" would be the worst consequence of violating an individuals rights as possible.