Small for Gestational Age
Introduction
Small for gestational age (SGA) is a term used to describe babies who are smaller than usual for the number of weeks of pregnancy. These babies have birth weight below the 10th percentile. This means they are smaller than many other babies of the same gestational age. Many babies normally weigh more than 5 pounds, 13 ounces by the 37th week of pregnancy. Babies born weighing less than 5 pounds, 8 ounces are considered low birth weight.
The terms intrauterine growth restriction (IUGR) and SGA, although often used as synonyms, are not interchangeable. SGA infants have not necessarily experienced IUGR and, conversely, infants with documented IUGR are not inevitably born SGA. Unlike SGA, IUGR always refers to a pathological process that results in decelerating fetal growth velocity. Serial ultrasound assessment (of fetal anthropometric traits, umbilical cord flow, and amniotic fluid) is necessary to confirm IUGR.
Causes
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
- Symmetric: The newborn is proportionately small, that is, its weight, length, and head size are similarly low or small.
- Asymmetric: Only weight is affected.
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.
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
- High blood pressure (hypertension)
- Long-standing diabetes
- Chronic kidney disease
- Heart disease or lung disease
- Lupus
- Severe anemia
- Sickle cell disease
- Abnormalities of the uterus, for example, a uterus that has two parts (bicornuate uterus)
Pregnancy risk factors
- 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
- Preeclampsia
- Early separation of the placenta (placental abruption)
- Use of alcohol or cigarettes
- Use of certain drugs such as amphetamines, anticonvulsants, certain cancer drugs, cocaine, or opioids
- Severe malnutrition
Fetal risk factors
- Birth defects that involve the brain, heart, or kidneys
- Certain infections in the fetus, including Zika virus, cytomegalovirus (CMV), or rubella (German measles)
- Genetic abnormalities, such as trisomy 18
Drugs
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.
Epidemiology
The incidence of small for gestational age (SGA) varies among populations and increases with decreasing gestational age.
Nevertheless, it appears that SGA is more prevalent in resource-limited countries. In 2012, data from the Child Health Epidemiology Reference Group (CHERG) based on 14 birth cohorts and using International Fetal and Newborn Growth Consortium for the 21st Century (INTERGROWTH-21st) birth weight (BW) standard showed that SGA was observed in 19.3 percent of live births in low and middle income countries. In this report, 22 percent of neonatal deaths occurred in infants born SGA.
Pathophysiology
In SGA, the nutrient supply to the fetus is compromised. The fetus, in order to increase its chance of survival, responds by reducing its overall size, preserving brain growth, accelerating lung maturation, and increasing red blood cell production. The fetus redirects blood flow from less vital organs to the brain, heart, adrenal glands, and placenta. Total body fat, lean mass, and bone mineral content are reduced, resulting in a wasted appearance in infants with severe SGA. Nitrogen and protein content are lower because of reduced muscle mass. Glycogen content is decreased in skeletal muscle and liver because of lower fetal plasma glucose and insulin concentrations.
Possible Complications
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.
- 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.
Long-term complications
Recent studies suggest that intrauterine growth restriction increases the likelihood of problems during adulthood, including heart disease, high blood pressure, and stroke.
Possible Treatment
Once SGA has been detected, the management of the pregnancy should depend on a surveillance plan that maximizes gestational age while minimizing the risks of neonatal morbidity and mortality. This should include steroid administration when at all feasible, based on the monitoring and delivery strategies discussed below. Fetal lung maturity studies by amniocentesis, in fetuses greater than 34 weeks’, may additionally influence delivery timing.
The goal in the management of SGA, because no effective treatments are known, is to deliver the most mature fetus in the best physiological condition possible while minimizing the risk to the mother. Such a goal requires the use of antenatal testing with the hope of identifying the fetus with SGA before it becomes acidotic. Developing a testing scheme, following it, and having a high index of suspicion in this population when results of testing are abnormal is important. The positive predictive value of an abnormal antenatal test result in fetuses with SGA is relatively high because the prevalence of acidemia and chronic hypoxemia is relatively high.
Primary Prevention
Although numerous protocols have been suggested for antenatal monitoring of SGA fetuses, the mainstay includes a weekly nonstress test (NST). Additional modalities may include amniotic fluid volume determination, biophysical profiles, and/or Doppler assessments. Other more complex protocols have been proposed. The protocol for antenatal testing suggested by Kramer and Weiner is one example. It relies heavily on the use of UA Doppler testing because severely abnormal Doppler findings (absent or reversed end-diastolic flow) can precede an abnormal fetal heart rate by several weeks. Harman and Baschat suggested a different proposed antenatal testing strategy. This protocol integrates multiple venous and arterial Doppler measurements and the biophysical profile score (BPS); this strategy may be used at institutions where these measurements are routinely obtained by qualified technicians
Prognosis
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.
Risk factors
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
- High blood pressure (hypertension)
- Long-standing diabetes
- Chronic kidney disease
- Heart disease or lung disease
- Lupus
- Severe anemia
- Sickle cell disease
- Abnormalities of the uterus, for example, a uterus that has two parts (bicornuate uterus)
Pregnancy risk factors
- 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
- Preeclampsia
- Early separation of the placenta (placental abruption)
- Use of alcohol or cigarettes
- Use of certain drugs such as amphetamines, anticonvulsants, certain cancer drugs, cocaine, or opioids
- Severe malnutrition
Fetal risk factors
- Birth defects that involve the brain, heart, or kidneys
- Certain infections in the fetus, including Zika virus, cytomegalovirus (CMV), or rubella (German measles)
- Genetic abnormalities, such as trisomy 18
Signs or Symptoms
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.
Studies
Active Not Recruiting
Number of studies: 8
Completed
Number of studies: 89
Enrolling by Invitation
Number of studies: 1
Not Yet Recruiting
Number of studies: 5
Recruiting
Number of studies: 13
Results Available
Number of studies: 12
Results Not available
Number of studies: 120
Suspended
Number of studies: 0
Link
Terminated
Number of studies: 8
Withdrawn
Number of studies: 1
Typical Test
Physical examination
The baby with SGA is often identified before birth. During pregnancy, a baby’s size can be estimated in different ways. The height of the fundus (the top of a mother’s uterus) can be measured from the pubic bone. This measurement in centimeters usually corresponds with the number of weeks of pregnancy after the 20th week. If the measurement is low for the number of weeks, the baby may be smaller than expected.
Although many SGA babies have low birthweight, they are not all premature and may not experience the problems of premature babies. Other SGA babies, especially those with IUGR, appear thin, pale, and with loose, dry skin. The umbilical cord is often thin, and dull-looking rather than shiny and fat.
Other diagnostic procedures may include the following:
Ultrasound. Ultrasound (a test using sound waves to create a picture of internal structures) is a more accurate method of estimating fetal size. Measurements can be taken of the fetus’ head and abdomen and compared with a growth chart to estimate fetal weight. The fetal abdominal circumference is a helpful indicator of fetal nutrition.
Doppler flow. Another way to interpret and diagnose IUGR during pregnancy is Doppler flow, which uses sound waves to measure blood flow. The sound of moving blood produces wave-forms that reflect the speed and amount of the blood as it moves through a blood vessel. Blood flow through blood vessels in both the fetal brain and the umbilical cord can be checked with Doppler flow studies.
Mother’s weight gain. A mother’s weight gain can also indicate a baby’s size. Small maternal weight gains in pregnancy may correspond with a small baby
Gestational assessment. Babies are weighed within the first few hours after birth. The weight is compared with the baby’s gestational age and recorded in the medical record. The birthweight must be compared to the gestational age. Some doctors use a formula for calculating a baby’s body mass to diagnose SGA.
References:
https://www.msdmanuals.com/home/children-s-health-issues/general-problems-in-newborns/small-for-gestational-age-sga-newborn
https://emedicine.medscape.com/article/261226-overview#a5 https://www.ncbi.nlm.nih.gov/pubmed/9429784
https://clinicaltrials.gov/