Small for gestational age (SGA): An infant whose birth weight is below the 10th percentile of the average weight for the same gestational age. In China, infants who are full term but weigh less than 2500g are called small for gestational age, which is one of the more frequent type of small for gestational age infants. At present, there is no uniform diagnostic criteria for small for gestational age babies at home and abroad. Different countries or regions have different diagnostic criteria. Most of them consider a small for gestational age infant as a newborn with birth weight and/or length below the 10th percentile of normal reference value for the same gestational age; or a newborn with birth weight below -2 standard deviations or the 3rd percentile of normal reference value for the same gestational age. The former is commonly used in China as a diagnostic indicator for infants younger than gestational age. The FDA approved the use of rhGH for the treatment of small for gestational age infants in 2001, but not all children diagnosed as small for gestational age at birth need to be treated with rhGH. The majority of children younger than gestational age achieve catch-up growth between 6 and 12 months of age, and 90% of children younger than gestational age achieve catch-up growth by 2 to 3 years of age. However, it may take 4 or more years for preterm small for gestational age children to reach normal height range. Achieving catch-up growth has two meanings: ① length and weight exceed the 2s of normal children of the same age and sex; ② growth rate exceeds the average value of children of the same age and gestational age. If the growth rate exceeds the average value for children of the same age and gestational age, then the growth rate fails to catch up. Domestic and international experts do not agree on the age of initiation of treatment for children younger than gestational age. The US FDA recommends starting rhGH treatment in children younger than 2 years of age who have not achieved catch-up growth. The European EMEA recommends rhGH therapy for children over 4 years of age with a height SDS <-2.5, a growth rate lower than the mean for the same age, and a height SDS lower than 1 SD of the genetic target height SDS. The International Society for Pediatric Endocrinology and the GH Research Society recommend starting rhGH therapy for younger children aged 2 to 4 years without catch-up growth and with height SDS < -2.5; there is no consensus on whether rhGH therapy should be used for younger children aged 4 years or older without catch-up growth and with height SDS of -2 to -2.5, but most experts believe that height < -2.0 SDS can be considered for rhGH therapy. rhGH treatment. The indications for rhGH therapy for children younger than gestational age are: ① birth weight and/or length below the 10th percentile of normal reference values for the same gestational age and sex; ② height ≥ 4 years of age still below -2SD of the average height of normal children of the same age and sex. It is controversial whether GH secretion status needs to be evaluated before rhGH therapy for children younger than gestational age. The GH-IGF1 axis in children younger than gestational age is variable, and typical GHD is rare. Some children may have a reduced 24-h GH secretion rate and lower levels of IGF1 and IGFBP3 than normal children and children of gestational age with short stature. If the growth rate continues to decrease in children younger than gestational age and they show signs of GH deficiency or hypopituitarism, the function of the GH-IFG1 axis should be evaluated and other pituitary endocrine axis functions should be evaluated if necessary. Epidemiological data suggest that children younger than gestational age are at increased risk of cardiovascular disease, metabolic syndrome, and stroke in adulthood. rhGH treatment may be preceded by glucose metabolism testing, depending on the child's condition, to rule out combined glucose metabolism abnormalities, etc.