I think it depends on how early they tell you. I was told at 13wks that DS was a boy and if the sonographer wasnt a specialist ob to i dont think i would have been confident until later scans. Im lucky that i get u/s every 3 weeks so i can keep checking lol. I was told at my last scan (25wks) that we are having a girl so i think the longer they have to develope the better iykwim.