Small for Gestational Age (SGA) is a term used by healthcare providers to classify a developing fetus or newborn baby. It is a finding based purely on size, indicating the baby is smaller than expected for their time spent in the womb. Understanding this classification is important because it alerts healthcare teams to monitor the pregnancy and the baby closely for potential issues. This information clarifies the clinical meaning of SGA and distinguishes it from related conditions.
Defining Small for Gestational Age
Small for Gestational Age is a statistical classification defined by an estimated fetal weight or birth weight that falls below the 10th percentile for a specific gestational age. This means the baby is lighter than 90% of other babies at the same stage of development. The classification is made by plotting the baby’s size onto standardized growth charts.
SGA is a descriptive label of size, not necessarily a diagnosis of disease or a growth problem. A large portion of babies classified as SGA (50 to 70%) are constitutionally small but otherwise healthy. These babies are following a growth pattern appropriate for their genetic potential, often reflecting the smaller stature of their parents.
The classification becomes a concern when SGA is used to identify Fetal Growth Restriction (FGR), sometimes called Intrauterine Growth Restriction (IUGR). FGR is a functional diagnosis, implying that a disease process, such as placental dysfunction, has prevented the fetus from reaching its natural growth potential. A baby can be SGA but not growth restricted, or conversely, a baby may have FGR but still be born above the 10th percentile.
FGR is a more serious finding because it indicates a failure to thrive in utero due to a lack of nutrients or oxygen, which carries higher risks. Healthcare providers use the SGA label as a screening tool to identify fetuses who may be experiencing FGR. The likelihood of pathological growth restriction is higher when the estimated fetal weight falls below the 3rd percentile.
Causes and Contributing Factors
The reasons a fetus may be classified as SGA are varied and grouped into factors related to the mother, the placenta, the fetus, or the environment. Understanding the underlying cause determines the appropriate monitoring and management plan.
A common source of growth restriction is issues with the placenta, the organ responsible for delivering oxygen and nutrients to the fetus. If the placenta does not function efficiently (placental insufficiency), the fetus will not receive adequate nourishment to grow properly. This often results in growth that is normal early in pregnancy but slows down in the later stages.
Maternal health conditions can directly impact fetal growth by affecting placental function or blood flow. Pre-existing conditions such as chronic high blood pressure, advanced diabetes, and kidney problems are associated with SGA. Lifestyle choices, including poor maternal nutrition, low pre-pregnancy weight, or the use of tobacco, alcohol, or illicit drugs, can also impair the fetus’s ability to grow.
Less common causes relate to the fetus itself, including genetic anomalies, chromosomal abnormalities, or congenital infections. In these cases, the fetus may be constitutionally small due to an inherent condition rather than a supply issue. Environmental factors, such as living at a high altitude or having naturally small parents, can also contribute to a baby being classified as SGA without underlying disease.
Prenatal Detection and Monitoring
Detection of a potential SGA fetus begins with routine prenatal appointments and simple measurements. Around the midpoint of pregnancy, the provider measures the Symphysis-Fundal Height (SFH), which is the distance from the pubic bone to the top of the uterus. This measurement is plotted over time on a growth chart to ensure the uterus is expanding at the expected rate.
If the SFH measurement is consistently below the expected range or shows a sudden slowing of growth, a specialized ultrasound is ordered to assess the baby’s size. The ultrasound provides an Estimated Fetal Weight (EFW) using measurements of the head circumference, abdominal circumference, and femur length. These measurements are plotted on customized growth charts that consider maternal characteristics to improve accuracy.
Once an SGA fetus is identified, monitoring shifts to specialized surveillance to determine if the small size is due to Fetal Growth Restriction. The primary tool for this enhanced monitoring is the Doppler flow study, which assesses blood flow through the umbilical artery and other fetal vessels. Abnormal Doppler results suggest compromised placental function, indicating a higher risk of FGR and fetal distress.
The surveillance plan includes regular non-stress tests (NSTs) and biophysical profiles (BPPs) to monitor the fetus’s well-being and movement patterns. Management focuses on increased maternal rest, optimizing maternal health conditions, and closely tracking the fetal condition. The timing of delivery is balanced between the risks of early delivery and the ongoing risks associated with a compromised environment in utero. Delivery is often recommended around 37 weeks of gestation in cases of confirmed FGR.
Health Implications for the Newborn
For newborns classified as SGA, health implications vary depending on whether the small size was constitutional or due to pathological growth restriction. Constitutionally small babies who were not growth restricted have outcomes similar to those of appropriately sized newborns. However, babies whose SGA status is linked to Fetal Growth Restriction face several immediate challenges after birth.
In the short term, these newborns are at an increased risk for metabolic and thermal difficulties. They often have low stores of glycogen, which can lead to hypoglycemia (low blood sugar levels) in the first hours and days of life. They may also struggle with hypothermia, as lower body fat makes it difficult to regulate their temperature.
Other immediate concerns include a higher likelihood of perinatal asphyxia (a decrease in oxygen supply during birth). They also have an increased risk of polycythemia, where the body produces too many red blood cells in response to chronic low oxygen exposure in utero. These short-term complications often necessitate care in the neonatal intensive care unit (NICU) for monitoring and treatment.
Looking at long-term implications, most SGA babies who are otherwise healthy show a rapid period of “catch-up” growth within the first two years of life. However, approximately 10% of these children will remain short and may require specialist evaluation. SGA babies, particularly those with FGR, may also have an increased susceptibility to neurocognitive and behavioral issues.
The experience of growth restriction in utero is thought to “program” the body’s metabolism, increasing the risk of adult diseases. This programming effect links a small birth size to a higher risk of developing metabolic syndrome, hypertension, cardiovascular disease, and type 2 diabetes later in life. Long-term follow-up is recommended for SGA children to monitor their growth, development, and overall health.