This is a randomized, placebo-controlled trial of vitamin D supplementation with 20mcg cholecalciferol (to achieve a total intake of 25mcg/day), with or without 500mg calcium to assess vitamin D requirements in lactating women and to ascertain whether vitamin D supplementation at levels sufficient to achieve defined thresholds of maternal serum 25-hydroxyvitamin D will increase the vitamin D content of maternal milk. The study will also report serum 25-hydroxyvitamin D in maternal-cord dyads over a 12-month period and describe the relationship between them. Information on maternal iPTH levels, anthropometry, diet and sun exposure will also be reported.