I am not sure of exact statistics but in my Husband's family there are hardly any girls. His sister being the only grandaughter (she did have a baby girl herself though). My side of the family there are seems to be a no set boy or girl trend.

A friend of mine has 2 sisters and they have all had girls....no boys at all in their immediate family. So maybe something to do with what the female is receptive too and what genes she has does come into play.

My SIL (who was a twin herself but never knew because she was adopted) had twins and was told that she would have an 80% chance of another set of twins so didn't go again.

I have heard that there are more boys born than girls in the world but lifestyle choices (more risky behaviour) means more of us Females survive.