My family have a theory:
My mum had 5 girls, for simpleness I will call them (in age order, starting with eldest) T, A, K, N (me), and S.
T has no children.
A had 1 MC, one boy child, 1 MC.
K has 1 boy child, 1 MC, 1 boy child, 3 MC's, 1 boy child.
N (me) has 2 boy children, 1 MC.
S has no children.
So from 5 girls (so far) we have produced 6 perfectly healthy boys, and had a total of 7 MC's, two of which are confirmed as being girls.
The theory goes that because we a large family of girls, we all carry too many girly hormones, that attack a female baby.
So sorry about your friends loses, it is devasting to go through the once, 7 times is unimaginable.