Generally speaking, two weeks after birth, infants need to start taking calcium and cod liver oil supplements, which are mainly composed of vitamin A and vitamin D. Calcium supplements are mainly calcium gluconate or calcium carbonate granules. Infants tend to grow and develop very rapidly from two weeks onwards, and the lack of timely supplementation of vitamin A and vitamin D will often result in vitamin A deficiency and vitamin D deficiency rickets, which is very detrimental to the health of children, for example, after suffering from vitamin D deficiency rickets, there will be more head sweating, chicken chest, rib beads and other symptoms. Vitamin A deficiency often causes night blindness and dry skin, and children’s breast milk, or infant formula often does not contain enough vitamin A and D. Therefore, most infants need to start calcium and cod liver oil supplements in the first two weeks of life.