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A newborn, whether delivered preterm, term, or postterm, whose weight is less than that of 90% of newborns of the same gestational age at birth (below the 10th percentile) is considered small for gestational age.

• Newborns may be small because their parents are small, the placenta did not function normally, or the mother has a medical disorder or has used drugs or consumed alcohol during the pregnancy.
• Unless they are born with an infection or have a genetic disorder, most small-for-gestational-age newborns have no symptoms and do well.
• Some small newborns remain small as adults.

There are several causes for this condition. In many cases, newborns are small simply because of genetic factors, such as having small parents (less commonly, a specific genetic syndrome associated with small stature may be involved). In other cases, the placenta (the organ that connects the fetus to the uterus and provides nourishment to the fetus) may have functioned poorly so that the fetus did not receive adequate nutrients, impairing growth. A poorly functioning placenta may occur if the mother has high blood pressure, preeclampsia, kidney disease, or long-standing diabetes. A viral infection, such as cytomegalovirus infection acquired before birth, may be responsible. Fetal growth may also have been impaired if the mother smoked or used alcohol or illicit drugs during the pregnancy (see Drug Use During Pregnancy).

Small-for-gestational-age newborns who are not premature do not have the complications related to organ system immaturity that premature newborns of similar size have. They are, however, at increased risk of the following problems:

• Meconium aspiration (see Problems in Newborns: Meconium Aspiration Syndrome)
• Excess red blood cells (polycythemia—see Problems in Newborns: Polycythemia in the Newborn)
• Low blood sugar levels (hypoglycemia—see Hypoglycemia)
• Difficulty regulating body temperature (see Problems in Newborns: Difficulty Regulating Body Temperature)
• An impaired immune system

Unless they have a genetic syndrome or viral infection, most small-for-gestational-age newborns have no symptoms. If the fetal growth was impaired because of poor placental function and inadequate nutrition, growth may accelerate when newborns are provided with good nutrition after delivery. Depending on the cause and severity of growth impairment, some small-for-gestational-age newborns remain small as children and adults.