Maternal Vitamin D Osteoporosis Study (MAVIDOS) is a large study conducted recently at 3 different big centres (Sheffield, Southampton and Oxford). Results from this study have shown that giving a higher dose of vitamin D to pregnant women every day from 14 weeks of pregnancy increased the strength of the bones in their infants. In the proposed study Investigators aim to show how vitamin D supplementation during pregnancy affects the response of bone to vibration in children whose mothers participated in the MAVIDOS study in Sheffield. These children will be 4 years of age when they participate in this study.