I don't think anything binds a relationship more than a kid.
There's about nothing as much as to thoroughly
ruin a relationship as much as having a kid just the same.
Not that I think this will happen to Djok and Jelena, just a critical side remark.
Why wait? Does Djokovic still have some doubts?
If they want to have kids, why should they postpone it, indeed.
In contrary to the ordinary Tom, Dick and Harry (or Wendy, Jane and Suzy, for that matter), they won't have to worry about any logistic or financial consequences.
My best wishes to the happy to-be family, of course!