I. Definition: A newborn whose birth weight and/or length is below the normal reference value for the same gestational age -2SD or 3rd percentile. Incidence: 3%~10% in human fetuses, 7 or 5% in China. Causes: mother factors: such as the mother during pregnancy infections; get chronic diseases (hypertension, anemia); unhealthy lifestyle (smoking, alcoholism and drug abuse, etc.); malnutrition during pregnancy; other: affected by the mother’s age, height, weight and race and other factors. Placental factors: placental insufficiency, infarction, placental abruption, vascular malformation. Fetal factors: chromosomal or other genetic defects disease (Fanconi syndrome, Bloom syndrome, Down syndrome and Turner syndrome, etc.); congenital malformations, intrauterine viral or bacterial infections, multiple births. Third, clinical symptoms: Catch-up growth and short stature Fetal to early postnatal (within 1 year) pediatric growth, mainly by nutrients – insulin – insulin-like growth factor metabolism axis regulation; 1 year to 1.5 years old after the growth of young children, by the growth hormone / insulin-like growth factor axis (GH / IGF) regulation, the SGA of the Catch-up growth is mainly by the GH / IGF – I axis regulation; the majority of children with SGA ( About 85%) have postnatal spontaneous catch-up growth; catch-up growth begins immediately after birth and reaches a maximum within 6 months; most heights usually reach normal at 2 years of age; whereas about 10% of children with SGA do not show catch-up growth, resulting in heights in childhood and adulthood that are lower than the normal mean value of -2SD; short stature due to SGA accounts for 20% of short stature in adults. Intellectual and Psychological Abnormalities Children with SGA are generally short and thin, with facial protuberance, small jaws, small arms and legs and pelvis, and relatively large ratios of arms and legs to body length; they may have clumsy movements, backwardness in intellectual ability and bone age, and cognitive deficits (e.g., poor performance in school and work); psychosocial dysfunction (e.g., lack of self-confidence, poor self-awareness, social apprehension, and behavioral problems). Treatment: SGA was first treated with growth hormone in 1970; the U.S. FDA approved SGA as one of the indications for growth hormone in July 2001; the European EMEA approved it in March 2003; recombinant human growth hormone (rhGH) is used in the long-term treatment of children with short stature who fail to achieve catch-up growth at the age of 2 years in SGA. V. Recombinant Human Growth Hormone (rhGH) Purpose: Primary Goal: Promote prepubertal linear growth to normalize height in early childhood. Secondary goal: to maintain normal height in late childhood (puberty). Terminal goal: attainment of normal adult height.