Monthly age is the number of months an infant is born, and there are two methods of calculating monthly age: actual monthly age and corrected monthly age. 1. Actual months of age: refers to the days after the birth of the baby. 2. Corrected age: An indicator used to assess the growth and development of preterm babies and a method to calculate the actual age of preterm babies. Generally, children born prematurely need corrective age, because the fetus has not been in the mother’s body for a sufficient number of months, the body organs may not be well developed at birth, low weight, weak ability to live, and defects in various functions, so it is necessary to subtract the period of time from the birth of the child to the expected date of birth, and start calculating the actual age of the child according to the time of the child’s expected date of birth. Calculation of age at birth is useful for monitoring and evaluating the growth and development of infants after birth, and should be done under the guidance of obstetricians and gynecologists, as well as pediatricians.