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, smoked, 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.
Gestational age refers to how far along the fetus is. The gestational age is the number of weeks between the first day of the mother's last menstrual period and the day of delivery. This time frame is often adjusted according to other information doctors receive, including the results of early ultrasound scans, which give additional information regarding the gestational age. A baby is estimated to be due (the due date) at 40 weeks of gestation.
At a gestational age of 40 weeks, boys who weigh less than about 6 pounds 9 ounces (3 kilograms) are small for gestational age. Girls who weigh less than about 6 pounds 3 ounces (2.8 kilograms) are small for gestational age. Doctors use published growth charts or computer apps to evaluate babies at other gestational ages.
(See also Overview of General Problems in Newborns.)
Most newborns who are moderately small for gestational age are normal babies who just happen to be on the smaller side. However, some have had their growth restricted by various factors. Growth restriction can be classified as
In symmetric growth restriction, the cause probably occurred early in the pregnancy when it would affect all of the cells in the newborn's body. Asymmetric growth restriction probably results from problems that occur later in pregnancy because some tissues develop sooner than others and not all would be affected equally.
Risk factors for growth restriction include those involving the mother's underlying health, and those involving the pregnancy and/or the fetus.
The risk of having a small-for-gestational-age (SGA) baby is increased for mothers who are very young or very old or who have had other SGA babies.
Medical disorders in the mother that increase the risk of having an SGA baby include
Having more than one fetus, for example, twins or triplets (Twins grow at the same rate as single fetuses until about 32 weeks. After that, twins grow more slowly and may be SGA at birth. For triplets, slower growth begins at about 28 weeks.)
Use of assisted reproduction to conceive the pregnancy
Early separation of the placenta (placental abruption)
Despite their size, small-for-gestational-age newborns usually look and act similar to normal-sized newborns of similar gestational age. Some small-for-gestational-age newborns appear thin and have less muscle mass and fat, and some have sunken facial features (called wizened facies). The umbilical cord can appear thin and small.
During pregnancy, growth-restricted fetuses are at increased risk of miscarriage or stillbirth. At birth, small-for-gestational-age newborns who are born full term 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:
Perinatal asphyxia: This complication is a decrease in blood flow to the baby's tissues or a decrease in oxygen in the baby's blood before, during, or just after delivery. It may result from a problem with the placenta before or during delivery.
Meconium aspiration: Growth-restricted fetuses may pass meconium (dark green fecal material that is produced in the fetus's intestine before birth) in the amniotic fluid and take forceful gasps and breathe (aspirate) the meconium-containing amniotic fluid into their lungs.
Low blood sugar (glucose) levels (hypoglycemia): This complication often occurs in the early hours and days of life because the small newborn does not have enough stored carbohydrates to use for energy and is unable to adequately process the carbohydrates it does have.
Excess red blood cells (polycythemia): Small-for-gestational-age babies may have a higher blood count than usual, and too many red blood cells cause the blood to become too thick, which may slow blood flow. Newborns with polycythemia have a reddish complexion and are sluggish. Polycythemia also can contribute to hypoglycemia, respiratory distress, and hyperbilirubinemia (causing jaundice, in which the skin turns yellow).
Difficulty regulating body temperature: This complication occurs because small-for-gestational-age newborns have less fat and body weight to keep them warm and they do not have enough carbohydrates to use for energy.
Increased risk of infection: Small-for-gestational-age newborns may have an impaired immune system, which increases their risk of developing infections in the hospital.
During pregnancy, doctors measure the distance on the woman's abdomen from the top of the pubic bone to the top of the uterus (fundus). This measurement, called a fundal height measurement, corresponds roughly with the number of weeks of pregnancy. For example, the normal fundal height for a woman who is 32 weeks pregnant is about 30 to 34 centimeters. If the measurement is low for the number of weeks, the fetus may be smaller than expected.
Ultrasonography can be done to assess the size of the fetus and estimate the weight of the fetus to confirm the diagnosis of small for gestational age. Ultrasonography may also be helpful in establishing the cause of the growth restriction and how it has affected the fetus. Depending on the findings, doctors may do genetic testing or magnetic resonance imaging (MRI) to determine the underlying cause.
After birth, small for gestational age is diagnosed by assessing the gestational age and the weight of the baby. Doctors measure the baby's length and head circumference to categorize the growth restriction as symmetric or asymmetric. Diagnostic tests, including ultrasonography, x-rays, MRI, tests for infection, blood tests, and genetic tests, may be needed to find the cause of the growth restriction.
Prognosis varies greatly depending on what caused the infant to be small for gestational age and whether complications developed.
Infants who have a moderately low birthweight usually do well unless they have an infection, genetic disorder, or perinatal asphyxia. Most catch up their growth during the first year of life and have a normal adult height.
Infants who are particularly small because of illness in the mother are at risk of complications but usually do well. Some small babies remain small as adults and others are within the normal range.
Infants whose growth was restricted because their mother used alcohol while pregnant are likely to have long-term developmental and behavioral problems.
The outcome for SGA infants exposed to illicit drugs during pregnancy is complicated. It is difficult to predict the prognosis because pregnant women who use illicit drugs often have other social and economic problems that affect their child's development.
There is no specific treatment for small-for-gestational-age newborns, but underlying conditions and complications are treated as needed. Growth hormone injections are sometimes given to certain SGA infants who remain quite small at 2 to 4 years of age. This treatment must be given for several years and must be considered on a case-by-case basis.
Newborns with polycythemia are given intravenous (IV) fluids, and newborns with hypoglycemia are treated with frequent feedings or IV glucose.
All pregnant women should receive good prenatal care and should avoid alcohol, tobacco, and illicit drugs (such as cocaine and heroin) while pregnant.