On this thread there does seem to be about 50/50 of people who say 'I just knew and I was right' and people who say 'I just knew and I was wrong'.
BIls gf told me that she knew instinctively she was having a girl and those who are 'right' will never believe the ones who say that you can't tell 'instinctively'.
Like Melrose says, its a 50/50 chance of being right.
IMO people might want, or prefer the thought of (even subconsciously), a girl or a boy, so they imagine the baby to be the one they want, and then if its the one they want, they are 'right'. Have you ever heard of anyone say 'I'm convinced I'm having a girl but we wanted a boy this time'. etc. No. I rest my case
And don't all post saying people shouldn't be picky, cos there are enough threads on here saying they are