You're IN your fifth month, that's why I gets confusing. Like a 2 month old baby is IN it's first year. So if you were looking it up in a book or a website or whatever, week 19 is going to be in the fifth month chapter. But I'd call that 4 1/2 months.
I'm having a hard enough time remembering how many weeks I am. Luckily my friend is 6 weeks ahead of me, so I keep asking her.
It definitely gets confusing -- lately I've been telling people I'm 16 weeks, and they say "Oh wow! Four months already!"
Well, not quite. People think of gestation as being "9 months" but it's really 40 weeks (well...40 weeks from LMP...so those first two weeks you're not *actually* pregnant), which most people would translate as ten months. So I usually tell folks how many weeks I am, and if they look confused or I see them trying to calculate how many months that is, I say "Out of about 40. So in four weeks I'll be about halfway."
Anyway, I try to just not even think in terms of months. It gets too confusing.