Through the accumulation and summary of clinical data and experimental studies, it has been found that there are many causes of smaller-than-gestational-age babies besides nutritional factors. Short maternal stature may lead to small for gestational age babies. Mothers with short stature have a higher chance of having small babies. One study found that mothers in the small-for-gestational-age group had an average height of 156 cm, while mothers in the normal newborn group had an average height of 161 cm, a highly significant difference. Multiple gestations often produce small for gestational age children. Twins and triplets are smaller. Sometimes with twins, one is of normal weight and the other is a smaller than gestational age baby. This may be due to inequitable blood supply to the placenta, with the one with more blood supply being larger and the one with less blood supply being smaller, or it may be due to blood transfer between the two fetuses. Chronic intrauterine hypoxia is also an important cause of small for gestational age babies. For example, vaginal bleeding before 28 weeks, severe gestational hypertension syndrome, severe diabetes mellitus with vascular disease, chronic cardiovascular or renal disease, as well as the mother’s location on a plateau or smoking (more than 5 cigarettes per day), can reduce the placental blood flow and fetal chronic hypoxia and nutritional deficiency, resulting in poor fetal growth and small size at birth. Abnormalities of the placenta or umbilical cord also affect fetal growth, and the fetus is supplied with nutrients and oxygen by the placenta and umbilical cord blood. If the placenta has hemangioma, small placenta or placental dysplasia, placental vascular obstruction, umbilical cord vascular abnormalities, insufficient blood flow to the uterus, etc. can affect nutrition and oxygen supply and lead to intrauterine growth retardation. Intrauterine infection is another important cause of fetal growth retardation. The pathogens from the mother through the cervix or blood stream first invade the placenta, causing inflammation, and then further invade the embryo or fetus causing fetal growth retardation and malformation. Common pathogens include various viruses (such as rubella virus, cytomegalovirus, herpes virus, coxsackie coxsackie virus, orphan virus and hepatitis B virus) and Toxoplasma gondii (transmitted by contact with dogs and cats) that can cause less-than-fetal-age infants. In addition, certain chromosomal aberrations, genetic and congenital diseases can also cause children to be born short.