How long after birth does a premature infant pass the critical period?

Generally, it is difficult to estimate how long a premature infant will remain in the critical period; the earlier the birth, the longer the critical period tends to be.
Premature infants refer to newborns born before 37 weeks of gestation. Due to differences in delivery timing, the duration of the critical period varies accordingly—the earlier the birth, the longer the critical period usually lasts. If a premature infant is close to 37 weeks at birth, has a relatively high birth weight, and shows normal body temperature, breathing, nutrition, and no signs of illness or other abnormalities, this typically indicates that the critical period has been passed.

If the infant is born at an earlier gestational age, the respiratory center and various organs are often underdeveloped, and the nervous system is less mature, making the infant more susceptible to complications such as intracranial hemorrhage, neonatal asphyxia, hypoxic-ischemic encephalopathy, and infectious diseases. Therefore, the critical period may last longer. The infant is generally considered out of danger only when developmental indicators normalize, body weight reaches at least 4 catties (approximately 2 kilograms), and no other abnormal symptoms are present.