It's the classical trolley problem.

https://en.wikipedia.org/wiki/Trolley_problem

I don't want to put words in your mouth etamponi, but if we take the trolley problem to classically mean choosing between different numbers of deaths; then I wouldn't describe it as a trolley problem.

I imagine the typical car accident in which three 'car seat age' children are involved, and at least one is killed. Then I imagine the typical event where a family decide against a third child. I feel quite a substantial qualitative difference! I certainly wouldn't claim one offsets the other.

Personally I don't know if there's any multiplier I'd accept. I find reduction of suffering and trauma much more important to me than offset creation of life and opportunity.

I don't think fewer births qualifies for the classic problem