My experience in the US has been to make it part of the negotiations after the interview and when you are in direct contact with the hiring manager. I've known several people in the US who successfully convinced their new employer to treat them as "fully vested" for the purposes of p/maternity leave or even just extra time off for a honeymoon. Yoy might not even have to take unpaid leave.

An aside- good paternity leave is the best retention incentive I've encountered. I've twice been ready to leave my megacorp, and both times I've stayed because I had a kid on the way and wanted the paternity leave. Things always got better in the meantime. Besides, I was pretty useless the month after the kid came anyways.

PS- Congrats! You should be a happy man.