The One With The Free Porn - S4-E17
Corrected entry: When the doctor is explaining to Phoebe about the triplets she says that "giving birth to three babies is not that different than giving birth to one". However, most triplets are born prematurely and by Caesarian section (which the doctor doesn't mention).
Correction: This mistake is self correcting. The Doctor was expressing her professional opinion that 'giving birth' (i.e. naturally, not by caesarian) to three babies was not much different to giving birth to one. The likelihood of a premature birth could easily have been discussed at an earlier stage.