How long it takes to reach normal infant level after birth depends on the cause, degree and treatment after birth, and the exact time cannot be generalized. It can be caused by a variety of factors, such as genetic factors, fetal malnutrition, intrauterine distress or other congenital developmental abnormalities. Children born younger than gestational age, usually due to genetic factors, may have difficulty catching up with normal people after birth if both parents are short or have genetic disorders. In contrast, children younger than gestational age who are born with fetal malnutrition and intrauterine distress can quickly regain their weight for their age with adequate nutrition and treatment. In addition, patients with congenital developmental abnormalities may be born with poor physical fitness and have difficulty catching up with normal individuals, but some of them can recover after active treatment. In conclusion, full-term less-than-fetal-age infants should have their care enhanced during the neonatal period and be started as early as possible.