In 1935, the American Academy of Pediatrics proposed that any live birth with a weight of 2500g or less is considered premature. In the subsequent clinical application process, people realized that there can be a large difference between gestational age and actual fetal weight when fetal growth is restricted. Therefore, in 1961, the World Health Organization established a standard for gestational age for premature birth, defining births of those who gave birth at less than 37 weeks of gestation as premature, but did not set a lower limit. There is no unified definition of premature birth at home and abroad. At present, my country defines premature birth as births of 28 to less than 37 weeks of gestation, calculated from the first day of the last menstrual period. People divide premature births into three categories based on gestational age: 1. Extreme premature births occur between 20 and 28 weeks of gestation, accounting for 5%. 2. Early premature births occur at <32 weeks of gestation, accounting for 10%. 3. Premature births between 32 and 36 weeks of gestation are considered mild premature births, accounting for 85%. Some scholars further divide 32-36 weeks into two subgroups, calling premature births before 32-34 weeks moderate premature births, and premature births between 34 and 36 weeks mild premature births.