Why girls...?
The theory behind aircrew on a flying tour having more girls than boys is backed up by some facts - the sex of a child is determined by the sperm that fertilises the egg, and therefore if there is a hostile environment then a population survives better with a higher proportion of females than males. The female carrying sperm tend to handle a poor environment better - hence more girls are born in a hostile environment. There are many factors in determining the hostile environment which produces more surviving female than male sperm in aviation: low oxygen levels; low pressure; changes in G; more exposure to UV radiation at altitude; electromagnetic fields/radiation caused by increased electrical installations - etc etc.
Boys can be born, but girls are more likely. If you want a boy, then beware of coming home from a flying detachment where the second bang is the door closing behind you when you get home!
W. (Two daughters - both born on a flying tour).