As FrostedMiniWheats123 notes in their very thorough Reddit post, Rachel is basically pregnant for an entire year. During the Season 8 opener, “The One After I Do,” it’s revealed she’s been pregnant for at least a few weeks. The episode takes place after Monica and Chandler’s wedding, which was set to occur on May 15. From here, “Friends” does a haphazard job of reminding us Rachel is supposed to be pregnant as the season unfolds. For example, when we cut to Season 8, Episode 5, “The One with the Halloween Party,” Rachel is meant to be approximately five months pregnant — or, to put it another way, well into her second trimester — and yet she is barely showing.
Even if we allow for the fact that she is a slim woman, it strains credulity that Rachel wouldn’t have even the slightest of baby bumps by this point in her pregnancy. By Episode 9, “The One with the Rumor,” Rachel is nearly done with her second trimester and still isn’t showing, nor are any comments made by any of the characters about her pregnancy at this point in the season.