Generally speaking, preterm babies can start taking AD and D₃ around 15 days after birth, although only one of them at a time is sufficient, the exact dosage is related to the preterm baby’s constitution. AD refers to vitamin AD, which consists mainly of vitamin A and vitamin D, with about 500 IU of vitamin D. D₃ refers to vitamin D₃, with about 400 IU. If both vitamins are taken at the same time, the dose of vitamin D in the body will be elevated, which will affect the growth and development of the preterm baby, so it is best not to take them together. Typically, most preterm babies can be started on vitamins AD and D₃ around 15 days of age, taking one capsule a day, alternating every other day. During the process of taking AD and D₃, parents should maintain good communication with the doctor and be careful to monitor the health condition of their preterm babies so as not to cause various discomforts such as nausea and vomiting.