Abortion opponents who believe that all fetuses have an absolute right to life would surely opt for some intervention. Otherwise, all the fetuses would die.
Their choice would thus be either to adhere to their absolutist position and be overwhelmed by a population explosion of overwhelming magnitude or else act to save only one or a few of the fetuses. The latter choice would be tantamount to abortion since all the fetuses are viable. It would, nevertheless, take someone very, very doctrinaire to opt to have the birth rate increase, at least initially, by a factor of 10 to 20.
This is obviously not a knockdown, airtight argument (although delivered to the right audience, it might result in knock downs). As already noted, however, it's not the usual boilerplate and may induce induce fresh thinking in some people.
The argument's point is that if certain contingent biological facts were to change, then presumably even ardent abortion opponents would change their position, suggesting that their position is itself contingent and not absolute. After this is acknowledged, the haggling over the details might proceed.
Professor of mathematics at Temple University, John Allen Paulos is the author of best-selling books including "Innumeracy" and "A Mathematician Plays the Stock Market." His "Who's Counting?" column on ABCNews.com appears the first weekend of every month.