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 Gestational age Problems in newborns may develop Before birth while the fetus is growing During labor and delivery After birth About 9% of newborns need special care after birth due to prematurity, problems... read more 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 Overview of General Problems in Newborns Problems in newborns may develop Before birth while the fetus is growing During labor and delivery After birth About 9% of newborns need special care after birth due to prematurity, problems... read more .)
Causes of SGA Newborns
Most newborns who are moderately small for gestational age are healthy babies who just happen to be on the smaller side. However, some are small because their growth in the womb was restricted by various factors. Growth restriction is when a baby in the womb (a fetus) does not grow as expected.
Growth restriction can be classified as
Symmetric: The newborn is proportionately small, that is, its weight, length, and head size are similarly low or small.
Asymmetric: Weight, length, or head size are not equally affected (for example, head size may continue to grow as expected, but weight or length may be lower than expected).
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.
Not all infants whose growth was restricted in the womb are small for gestational age.
Risk factors for growth restriction include those involving the mother's underlying health, and those involving the pregnancy and/or the fetus.
Maternal risk factors
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
Abnormalities of the uterus, for example, a uterus that has two parts (bicornuate uterus)
Pregnancy risk factors
Having more than one fetus Multiple Births The term multiple births refers to the presence of more than one fetus in the uterus. The number of twin, triplet, and other multiple births has been increasing during the last two decades.... read more , 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 Assisted Reproductive Technologies Assisted reproductive technologies involve working with sperm and eggs or embryos in a laboratory (in vitro) with the goal of producing a pregnancy. (See also Overview of Infertility.) If treatment... read more to conceive the pregnancy
Use of alcohol Alcohol during pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more or cigarettes Cigarette (tobacco) smoking during pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more
Use of certain drugs such as amphetamines Amphetamines during pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more , antiseizure medications Seizure Disorders During Pregnancy Most women who have a seizure disorder that is well-controlled by antiseizure drugs are able to safely give birth to a healthy baby. If these women get enough sleep and take antiseizure drugs... read more , certain cancer medications, cocaine, or opioids Opioids during pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more
Fetal risk factors
Birth defects Overview of Birth Defects Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. They are usually obvious within the first year of life. The cause of many birth... read more that involve the brain Overview of Brain and Spinal Cord Birth Defects Birth defects of the brain and spinal cord can occur in early or late fetal development. Typical symptoms include intellectual disability, paralysis, incontinence, or loss of sensation in some... read more , heart Overview of Heart Defects About one in 100 babies is born with a heart defect. Some are severe, but many are not. Defects may involve abnormal formation of the heart's walls or valves or of the blood vessels that enter... read more , or kidneys Kidney Defects There are several different birth defects that affect the kidneys (the two organs that filter waste from the blood to make urine). These defects are not usually apparent at the doctor's examination... read more
Certain infections in the fetus, including Zika virus Zika Virus Infection Zika virus infection is a mosquito-borne viral infection that typically causes no symptoms but can cause fever, rash, joint pain, or infection of the membrane that covers the white of the eye... read more , cytomegalovirus Cytomegalovirus (CMV) Infection Cytomegalovirus infection is a common herpesvirus infection with a wide range of symptoms: from no symptoms to fever and fatigue (resembling infectious mononucleosis) to severe symptoms involving... read more (CMV), or rubella Rubella Rubella is a contagious viral infection that typically causes mild symptoms, such as joint pain and a rash, but can cause severe birth defects if the mother becomes infected with rubella during... read more (German measles)
Genetic abnormalities Gene abnormalities Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... read more , such as trisomy 18 Trisomy 18 Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities. Trisomy 18 caused by an extra chromosome 18. Infants... read more
Symptoms of SGA Newborns
Despite their size, small-for-gestational-age (SGA) newborns usually look and act similar to normal-sized newborns of similar gestational age. Some SGA 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 Miscarriage A miscarriage is the loss of a fetus before 20 weeks of pregnancy. Miscarriages may occur because of a problem in the fetus (such as a genetic disorder or birth defect) or in the woman (such... read more or stillbirth Stillbirth Stillbirth is death of a fetus after 20 weeks of pregnancy. Stillbirth may result from a problem in the woman, placenta, or fetus. Doctors do blood tests to try to identify the cause of a stillbirth... read more . At birth, SGA newborns who are born full term do not have the complications related to organ system immaturity that preterm newborns of similar size have. They are, however, at increased risk of the following problems:
Perinatal asphyxia Perinatal asphyxia Birth injury is damage that occurs as a result of physical pressure during the birthing process, usually during transit through the birth canal. Many newborns have minor injuries during birth... read more : 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 Meconium Aspiration Syndrome Meconium aspiration syndrome is trouble breathing (respiratory distress) in a newborn who has breathed (aspirated) a dark green, sterile fecal material called meconium into the lungs before... read more : 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 Hypoglycemia Hypoglycemia is abnormally low levels of sugar (glucose) in the blood. Hypoglycemia is most often caused by medications taken to control diabetes. Much less common causes of hypoglycemia include... read more ): 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 Polycythemia in the Newborn Polycythemia is an abnormally high concentration of red blood cells. This disorder may result from postmaturity, diabetes in the mother, twin-to-twin transfusions, in which blood flows from... read more ): SGA 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 Jaundice in the Newborn Jaundice is a yellow color to the skin and/or eyes caused by an increase in bilirubin in the bloodstream. Bilirubin is a yellow substance formed when hemoglobin (the part of red blood cells... read more (causing jaundice, in which the skin turns yellow).
Difficulty regulating body temperature Difficulty regulating body temperature A preterm newborn is a baby delivered before 37 weeks of gestation. Depending on when they are born, preterm newborns have underdeveloped organs, which may not be ready to function outside of... read more : This complication occurs because SGA 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: SGA newborns may have an impaired immune system, which increases their risk of developing infections in the hospital.
Recent studies suggest that intrauterine growth restriction increases the likelihood of problems during adulthood, including heart disease, high blood pressure, and stroke.
Diagnosis of SGA Newborns
Before birth, measurement of the uterus and ultrasonography
After birth, assessment of gestational age and size and weight of the baby
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 Ultrasonography Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more 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 Length and Height Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this rapid infant... read more and head circumference Head Circumference Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this rapid infant... read more 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 for SGA Newborns
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.
Treatment of SGA Newborns
Treatment of underlying causes and complications
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 Polycythemia in the Newborn Polycythemia is an abnormally high concentration of red blood cells. This disorder may result from postmaturity, diabetes in the mother, twin-to-twin transfusions, in which blood flows from... read more are given intravenous (IV) fluids, and newborns with hypoglycemia are treated with early (within 1 hour after birth) and 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.