Like 99% of women have hit menopause by 55, and the number of births for mothers over 50 with no medical intervention in the conception process is extremely low. So low that Wikipedia has an article detailing specific women it’s happened to.
This wouldn’t be a huge deal, except a major plot point is that it was an accidental pregnancy neither of them planned for. So it’s not like she was getting any kind of fertility treatment.
While not impossible it just makes Jacks conception extremely unlikely