Children with SGA who are younger than gestational age are born with length and/or weight below the length and/or weight that should have been achieved during pregnancy, and some of them may still grow slowly after birth, i.e., their height tends to remain below the third percentile and rarely exceeds the fiftieth percentile. Chatelain et al. reported that 49% and 37% of patients with normal IUGR and Russell-Silver syndrome had peak serum hGH below 10ug/ml after drug stimulation. In addition, some researchers measured the 24-hour hGH secretion profile of children with IUGR and found that the spontaneous secretion of hGH was reduced by about half compared to normal children, so their serum IGF-1 levels were relatively low and eventually their growth was limited, while the karyotype analysis was normal. After birth, most children younger than gestational age can catch up with the height of normal children if they are fed properly, have no or few illnesses, and have a regular and happy life. In the 2-3 years after birth, especially in the first 6 months, the growth rate of children younger than fetal age is accelerated, which is very important for the final height, so how many of these children can catch up with the normal height? According to research, 40% of the first 6 months catch up with normal infant height, 25% catch up with normal child height before 3 years old, and 20% catch up with normal child height after 3 years old, but about 15% of the younger-than-fetal-age children grow up to be still short. Some studies have also concluded that most small for gestational age children catch up with normal infants within 6-12 months after birth, and 10%-30% do not have accelerated growth after birth, and these children grow to an average height of 162 cm for boys and 147 cm for girls. A Swedish study concluded that 80% of younger-than-normal children born at less than gestational age had a lower-than-normal height at age 18, making short stature at birth more important than low weight. Younger-than-gestational-age children are often more neurodevelopmentally mature than preterm children of the same weight. However, due to inadequate intrauterine nutrition, they appear thin in the neonatal period, with dry, pale, inflexible and even cracked and peeling skin, and a sunken abdomen due to emaciation. These newborns are chronically hypoxic in utero, which leads to short size in addition to severe hypoxia, and in severe cases, may also contaminate the amniotic fluid due to fecal discharge, making the fetal fat and skin yellow, and may also have or cause respiratory distress and bruising due to amniotic fluid aspiration. The incidence of congenital malformations in children younger than gestational age is 10-20 times higher than that of normal newborns. Children with congenital malformations often have them, often associated with congenital diseases and chromosomal aberrations and other etiologies. In addition to the above manifestations, about 1/3 of children younger than gestational age develop hypoglycemia in the first three days of life, which is manifested by small muscle tremors, hypothermia, lethargy, convulsions, and recurrent apnea. This is due to insufficient storage of liver glycogen in their liver and rapid absorption and utilization of sugar by tissues in the body, causing an oversupply of blood sugar leading to hypoglycemia. The intellectual development of children younger than gestational age is mostly normal, and their intellectual development is related to the etiology that causes intrauterine growth retardation. If the cause is intrauterine infection, severe malnutrition or chromosomal abnormalities, it is possible to cause impaired intellectual development. Some children younger than gestational age are born without any abnormalities other than low weight, which may be due solely to the mother’s short stature.