Small for Gestational Age (SGA) is a classification used in maternal and infant care to identify newborns who are smaller than expected for their time developing in the womb. This classification allows clinicians to assess potential risks to the baby’s health both before and after birth. Determining if a baby meets this classification requires a precise, two-step methodology involving the collection of specific physical measurements and a statistical comparison to a reference population.
Defining Small for Gestational Age (SGA)
Small for Gestational Age (SGA) is a statistical description based solely on the size of the baby, not a diagnosis of an underlying medical problem. An infant is classified as SGA if their birth weight, or estimated fetal weight before birth, falls below the 10th percentile for their specific gestational age and sex. The 10th percentile is the chosen cutoff point because it statistically separates the lowest 10% of the population from the remaining 90%. By definition, 10% of all newborns will fall into this category, but only a fraction of these are pathologically small. This threshold functions as a screening tool, flagging infants who may be at a higher risk for complications like hypothermia, hypoglycemia, or perinatal asphyxia.
The Core Inputs for Measurement
The calculation of Small for Gestational Age relies on two essential and accurate pieces of data: the baby’s size measurement and the precise gestational age. Without reliable figures for both inputs, any subsequent determination of SGA is unreliable.
Weight measurement is obtained differently depending on whether the baby is still in the womb or has been born. Postnatally, a precise birth weight is taken on a calibrated scale shortly after delivery. Prenatally, clinicians use ultrasound measurements to calculate an Estimated Fetal Weight (EFW) using biometric data.
Biometric Measurements for EFW
Clinicians use several biometric measurements to calculate the Estimated Fetal Weight (EFW):
- Biparietal diameter (BPD)
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length (FL)
The abdominal circumference is highly reflective of the baby’s size and nutritional status. These measurements are plugged into mathematical formulas, such as the Hadlock formula, to generate the EFW.
The second input is the accurate Gestational Age (GA), which is the total number of weeks the baby has been developing. GA is ideally confirmed early in the pregnancy using an ultrasound measurement of the crown-rump length (CRL) or based on the date of the mother’s last menstrual period (LMP).
Applying the Growth Percentile Curve
The process of calculating SGA involves comparing the baby’s unique weight measurement against a standardized reference population, which is visualized as a growth percentile curve. This curve is a graph that plots the average weight of thousands of healthy babies at every week of gestation, with lines indicating various percentiles, such as the 10th, 50th, and 90th. The 50th percentile line represents the average weight for that particular gestational week.
To determine the SGA classification, the baby’s weight and corresponding gestational age are plotted as a single point on this graph. For instance, if a baby has an estimated fetal weight of 1,500 grams at 32 weeks, that specific coordinate is located on the curve. Clinicians then observe where this plotted point falls in relation to the 10th percentile line.
If the baby’s plotted point falls on or below the line marked as the 10th percentile, they are statistically classified as Small for Gestational Age. Some clinical centers use customized growth curves, which adjust the population standard based on maternal characteristics like height, weight, ethnicity, and parity, providing a more personalized standard for the baby’s growth potential.
SGA Versus Fetal Growth Restriction
Once a baby is classified as Small for Gestational Age, the next clinical step is to determine if this is a benign condition or if it indicates a more serious underlying pathology known as Fetal Growth Restriction (FGR). While all babies with FGR are SGA, only a portion of SGA babies actually have FGR. The distinction is crucial because FGR implies a pathological failure to achieve a baby’s genetically determined growth potential, often due to issues with the placenta or maternal health.
FGR is associated with significantly higher risks of stillbirth and long-term neurodevelopmental problems, necessitating more intensive monitoring. To differentiate between a constitutionally small but healthy baby (SGA) and a pathologically growth-restricted baby (FGR), clinicians use additional tools. These tools include Doppler ultrasound studies, which evaluate the blood flow in the umbilical artery and other fetal vessels, looking for signs of placental insufficiency.
Abnormal Doppler results, combined with low amniotic fluid levels, suggest the baby is experiencing compromise and is more likely to be classified as FGR. This distinction guides management, with FGR babies requiring increased surveillance, potential early delivery, and specialized care, while constitutionally small SGA babies often need only routine follow-up. The classification of SGA is the initial flag, and the subsequent clinical assessment determines the appropriate management plan.