Some of the sites use the chinese lunar calendar -- here's one that does.
http://www.thelaboroflove.com/chart/pred.html
That can mess you up -- I was 35 when I conceived the child I'm carrying now but the Chinese lunar calendar calls me 36. So if you don't use the lunar calendar, my child comes up a girl, if you do, it comes up a boy. (We don't know the gender yet).
It was right for my first child, though - a son.