Watch the episode where Rachel's pregnancy is causing her to be attracted to a lot of guys. When she's telling Phoebe about her problem, Phoebe asks her if she's in about her fourth month of pregnancy. Rachel responds that she is, when in fact, she's actually about eight months in. We know this because Monica and Chandler get married on May 15, and we don't hear the story about how Rachel got pregnant until after they get back from their honeymoon. So assuming that their honeymoon lasts about a week, that puts the date at around May 22. Well Rachel and Ross tell the gang that it happened six weeks earlier, which means Rachel got pregnant in the first week of April. Four months from then would be early August, but the episode where Rachel is attracted to all of the guys takes place after the Thanksgiving episode, which puts us in late November or early December. Therefore, she's about 8 months pregnant.