I think it counts from the moment of pregnancy with the said child. I remember other studies suggesting that obese mothers already predispose their children for illness while they're in the womb. And some other studies suggesting inheritance of habits and patterns that form in childhood. It all can affect their future already. I doubt that there is any link if parents were obese in their own childhood. They probably overcame their predispositions a will pass on good habits and enviornment to their children. Just my opinion. Might be wrong.