Dizygotic twinning, birth weight and latitude

In Europe and the USA, both mean birth weight and age-standardized dizygotic twinning rates correlate positively and significantly with latitude. It seems possible that these two sorts of correlation have explanations in common via pituitary action. Factors which may be responsible for such action are total consumption of food and consumption of specific items of diet, for example, milk products and potatoes. Lastly, one might wonder whether the association of photoperiodicity with latitude is relevant.