Small for gestational age is a term used to describe a baby who is smaller than the usual amount for the number of weeks of pregnancy. SGA babies usually have birthweights below the 10th percentile for babies of the same gestational age. This means that they are smaller than many other babies of the same gestational age.
SGA babies may appear physically and neurologically mature but are smaller than other babies of the same gestational age. SGA babies may be proportionately small (equally small all over) or they may be of normal length and size but have lower weight and body mass. SGA babies may be premature (born before 37 weeks of pregnancy), full term (37 to 41 weeks), or post term (after 42 weeks of pregnancy).
Although some babies are small because of genetics (their parents are small), most SGA babies are small because of fetal growth problems that occur during pregnancy. Many babies with SGA have a condition called intrauterine growth restriction (IUGR). IUGR occurs when the fetus does not receive the necessary nutrients and oxygen needed for proper growth and development of organs and tissues. IUGR can begin at any time in pregnancy. Early-onset IUGR is often due to chromosomal abnormalities in the fetus, maternal disease, or severe problems with the placenta. Late-onset growth restriction (after 32 weeks) is usually related to other problems.
Some factors that may contribute to SGA and/or IUGR include the following:
High blood pressure
Chronic kidney disease
Heart or respiratory disease
Substance use (alcohol, drugs)
Factors involving the uterus and placenta:
Decreased blood flow in the uterus and placenta
Placental abruption (placenta detaches from the uterus)
Placenta previa (placenta attaches low in the uterus)
Infection in the tissues around the fetus
Factors related to the developing baby (fetus):
Multiple gestation (for example, twins or triplets)
When the fetus does not receive enough oxygen or nutrients during pregnancy, overall body and organ growth is limited, and tissue and organ cells may not grow as large or as numerous. Some of the conditions that cause SGA and IUGR restrict blood flow through the placenta. This can cause the fetus to receive less oxygen than normal, increasing the risks for the baby during pregnancy, delivery, and afterwards.
Babies with SGA and/or IUGR may have problems at birth including the following:
Decreased oxygen levels
Low Apgar scores (an assessment that helps identify babies with difficulty adapting after delivery)
Meconium aspiration (inhalation of the first stools passed in utero) which can lead to difficulty breathing
Hypoglycemia (low blood sugar)
Difficulty maintaining normal body temperature
Polycythemia (too many red blood cells)
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.
Specific treatment for SGA will be determined by your baby's doctor based on:
Your baby's gestational age
The fetus' overall health, and medical history
Your opinion or preference
Babies with SGA may be physically more mature than their small size indicates. But they may be weak and less able to tolerate large feedings or to stay warm. Treatment of the SGA baby may include:
Temperature controlled beds or incubators
Tube feedings (if the baby does not have a strong suck)
Checking for hypoglycemia (low blood sugar) through blood tests
Monitoring of oxygen levels
Babies who are SGA and are also premature may have additional needs including oxygen and mechanical help to breathe.
Prenatal care is important in all pregnancies, and especially to identify problems with fetal growth. Stopping smoking and use of substances such as drugs and alcohol are essential to a healthy pregnancy and can reduce the risk for sudden infant death syndrome (SIDS) and other sleep-related infant deaths. Eating a healthy diet in pregnancy may also help.