Only the age and the amount of childs of the women counts, the man's age and amount of childs does not count.I forget the ratio, but I think the more children you have and the older you get, the lower the chance of more. So if your character is ageing... eg A 30% increase in a 10% chance is 13% so not a big deal.
I'm up to 5 girls on my current playthrough. I'm considering scumming to get a few boys in the mix..
And a 40 year old women does not make many childs, a 18 year old women will make up to 6 - 8 childs.
And girls in this game can not grow your clan, they are currently completly useless in the process of child making.