Monica and Chandler became Friends’ most stable romance for the latter half of the series - however, a Chandler joke from season 3 hinted at their major pregnancy struggles in season 9. Friends premiered in 1994 as a series that follows a group of six New York City friends as they navigate jobs, relationships, and friendships in their 20s and 30s. Though Ross and Rachel started as Friends’ major in-group romance, Monica and Chandler quickly took the title as they began moving in together, getting engaged, tying the knot, and eventually having kids.
Monica and Chandler got together at the end of season 4 at Ross’s wedding to the surprise of fans and the rest of the characters, who slowly found out in season 5. Their romance was teased early on in the show when they almost slept together in “The One with the Flashback” and when Chandler tried to convince Monica why he could be her boyfriend in “The One at the Beach.” Oddly enough, Monica and Chandler’s relationship originally wasn't supposed to last; they were meant to be a one-time hook-up until fans enthusiastically reacted. Although the most secure relationship in the series, the couple did eventually see a few setbacks in the later seasons.
In Friends’ season 3 finale, “The One at the Beach,” Chandler makes a one-off joke to the gang that actually hints at Monica and Chandler’s pregnancy setbacks in season 9. When Rachel comes to the taxi with a giant hat, Chandler makes jokes comparing it to a UFO, explaining the hat abducted him, conducted tests, and now he “can’t have children.” The joke wasn’t meant to be taken literally, but it takes on a new meaning when realizing that in season 9, Chandler finds out that he actually is infertile.
Monica was known for wanting to get married and have children from the start of the series, but Chandler's commitment issues made him hesitate. Once he got with Monica, however, his struggles with commitment changed and he was ready to marry and have kids. Once Monica and Chandler decided to officially start trying to have a baby after Ross and Rachel had Emma, they worried because they tried for a year with no luck. Finally, in late season 9, Monica and Chandler both went to a fertility doctor to see if there were any issues with their reproductive organs. They found out that Chandler’s sperm has low motility and Monica has an inhospitable environment, meaning the odds of them naturally conceiving a child are extremely low.
Once Monica and Chandler come to terms with not being able to have children, they go through the route of adoption, which takes up much of their storyline in the 10th and final season, showing how much they matured over the course of the series. Instead of just wanting to have a child because they think they want it, think they have to, or just want a child to look like them, they truly want to raise children together and give a great life to a child. Interestingly enough, Courteney Cox was actually pregnant in the 10th season, but they couldn’t write it in since Friends already decided Monica and Chandler would only adopt.