I dont know if you already know, but I know of a fool proof method told to me when I was pregnant by a old German lady. You take the age you were when you got pregnant then add the month you are due. Like if you were 22 when you got pregnant and due in October ( 10) 22+10= 32.. Girl!
Odd number is a boy, even is a girl..
Now if it was your birthday the month you got pregnant then its the reverse. .. I have no clue why it works, but it does- you can even do it on past children, yourself, etc..