I was totally convinced that DD2 was a boy. Utterly sure. The moment i began to wonder even a tiny bit was when my MW sent me a text telling me she'd caught a boy (her 4th in a row) the morning of the day i started labour. We were due 13 days apart but the babies came within 22 hours of one another! Anyway at that point i thought "hmmm, 5 boys in a row? Maybe this is a girl!" but i was still pretty sure it was a boy until she was out. I remember taking her from the mw between my thighs to bring her to my chest for a cuddle (i was on all fours to give birth) and holding with one hand between her legs and one round her chest and thinking THAT'S NOT A BOY! I wasn't disappointed, though i spent a few hours thinking it over honestly in my head, as i had been really completely convinced it was a boy.
I'm thinking this one is a boy just now because my sex drive is through the roof (normally i'm totally NOT interested during pregnancy) and i'm wondering if it's extra testosterone circulating...?