The idea is that a woman repeatedly getting pregnant and then killing the child is putting a lot of strain on society, both in terms of resources and in terms of comfort. We allow a lot of privileges for pregnant women and new mothers, with the expectation that they're trying to bring new people into society, something we encourage.

I'd think that that the bulk of the resource cost of a newborn is the physiological cost (and medical risks) the mother endured during pregnancy. The general societal cost seems small in comparison.