Would that work? It would be a strong incentive for effective contraceptive use, and some people who would have otherwise had a child later will already have one, etc.

No idea how it would all add up, but its not obviously true.

It’s not obviously true that a ban on abortion would lead to more children? Contraceptives aren’t 100% effective. The availability of contraception + abortion is absolutely going to block more children from being born than contraception alone.