Under normal circumstances, infants can start taking vitamin D supplements from birth until they are about 2 weeks old, after which time they should continue to take into account their specific habits, environment, climate and other factors. Vitamin D3 is a kind of vitamin D, a fat-soluble vitamin, and is closely related to sunlight, also known as the “sunshine vitamin”. Infants are in a period of rapid growth and development and have a high demand for vitamin D. Infants aged 0-6 months, whether breast-fed or formula-fed, need to be given 400 international units per day, while premature babies, low birth weight babies and twin babies are at high risk of vitamin D deficiency and can be supplemented with about 800-1000 international units per day until 3 months after birth. Parents are advised to take their babies out in the sun when the weather is sunny to ensure 2 hours of sunshine a day, which can effectively promote calcium absorption and prevent the occurrence of vitamin D deficiency rickets.