The One with the Sonogram at the End - S1-E2
Corrected entry: Ross already told his parents that Carol was a lesbian in Episode 1 "I told mom and dad last night, they seem to have taken it pretty well." But in the 2nd episode Ross tells his parents after dinner at Monica's like it was the first time he told them.
Correction: He told them the first time that they were getting divorced, but not why.