That is actually such a reassuring theory. Like they did have some fertility problems but it looked so much worse thanks to Janice (Chandler’s “slow swimmers” got super-reluctant to come into a world with her voice in it, who can blame them.) And Monica’s stress can’t have helped. I have friends who got so stressed about getting pregnant that they basically gave up….calmed down and then got pregnant. So I think stress definitely plays into it. Plus that whole year had been so stressful for them with Chandler in Tulsa, then changing jobs, Monica trying to support Rachel/Emma/Ross - that won’t have helped.
There’s no doubt in my mind they got pregnant post-show. (And also that they were then super-careful about not favouring biological versus adopted kids). Interestingly Marta Kauffman said they almost revealed Monica was pregnant in the finale (instead of Erica having twins) and Kevin Bright said in an interview that he saw them having a biological kid. (They only really did the infertility/adoption thing, because both Pheebs and Rachel had pregnancy storylines). So yeah, no question that Mondler had a biological kid too.