It appears men may be genetically pre-disposed to fathering either girls or boys.
British researchers studied the family trees of nearly a thousand families, dating back to 1600. They found evidence that men with brothers are more likely to have sons, and those with sisters appear to have more daughters.
This indicates a gene is passed down through generations that determine whether a man's sperm will be more likely to produce more x- or Y-chromosomes.
X-chromosomes produce baby girls, while Y-chromosomes of course mean a boy is on the way.