Vitamin D Supplementation of Lactating Mothers

The investigators would like to test the hypothesis that maternal vitamin D supplementation during lactation in dose 1200IU/d (400IU from multivitamins + 800 IU cholecalciferol) is more effective than 400IU/d (400IU from multivitamins + placebo)to built appropriate maternal vitamin D status and could be beneficial for maternal and infants bone mineralization and body composition (proper proportion of muscle and fat tissues in body weight) and is safe for both. The investigators also want to confirm that vitamin D supplementation of exclusively breastfed infants in dose 400IU/d is adequate to build appropriate vitamin D status independently of mother's vitamin D supplementation up to 1200 ID/d. Additionally the investigators hypothesize that because of changes in lifestyle there will be no substantial seasonal differences in vitamin D status of pregnant women at the delivery and their newborn infants.