Kim this theory is an interesting one and one that we have discussed at length.

DH's father was one of 3 boys then a girl, DH was one of 3 boys then a girl, and now DH has fathered 3 boys and (praying the u/s dude got it right!) now a girl! Quite freaky if you ask me...

(DH's oldest brother doesn't have any kids. His other brother has a boy then a girl and his younger sister is PG with her second girl so it's just us that has followed this pattern)