Ive heard of this! I have done it for all my friends, and even tried with my parents. everytime it is right! all you do is take age you are when you conceived, and add it together with the number month that it is due. For instance, if you are 25 when you conceive the baby, and you are due in october(10), then your number is 35, which is a girl. Odd numbers for girls, and even numbers for boys!