Mother for whatever reason decides she doesn’t want to have the embryo implanted.

Who knows, maybe they can’t afford kids. Or her and the father are about to break up. Or she has found out she’s at risk of complications.

I think I am just suffering lack of information.
I assumed the contract is to be an after birth thing and not something that makes sure that the mother has to bear the child.
Besides, if the implantation is not done, hasn't He not actually done the procedure and can choose another (although hard to do so in time)?
Does the embryo have some kind of compatibility with the mother, for implantation to be successful?

In case He has the option to find another chap for the process in the above cases, I won't consider the contract extending to this time.