We know she got pregnant sometime before Monica and Chandler's wedding. Which was supposedly May 15th.
So we know that Rachel and Ross had sex before the wedding. Let's say April, because Rachel does not take a pregnancy test until the night before Monica's wedding.
Later on we find out that Ross is the father and when, supposedly, the ~deed~ happened.
Let's fast forward here. It is suddenly Christmas time and Rachel laments to Phoebe about how "erotically charged" she is. Phoebe realizes that Rachel is about 4 months pregnant and going through the "Evander Holyfield phase"
The Friends timeline shoots forward to Valentine's Day. Rachel is still pregnant.
Still, Rachel has not given birth. Even though she goes past her due date...
Rachel then complains that is is "One. Hundred. Degrees outside."
Then again, Monica celebrate's her and Chandler's two-month anniversary with a bachelor party he did not get. That episode aired after the Halloween episode. So, who's to say they didn't really get married in August.
That is all I have for you. Thank you for reading. I hope you found my Friend's investigation worthwhile.