A fetal growth scan is not a standard, routine screening procedure offered to every pregnant person. It is a targeted diagnostic ultrasound used to assess fetal size and well-being when a specific medical concern or risk factor is present. The primary purpose is to estimate the baby’s size and track its growth trajectory, ensuring the developing fetus receives adequate nutrition and oxygen. While a comprehensive anatomy scan is typically offered to all expectant parents, the growth scan is reserved for pregnancies that require closer monitoring.
Defining the Fetal Growth Scan
The fetal growth scan is a specialized, non-invasive ultrasound examination. It is most often performed during the third trimester, usually beginning around 28 to 32 weeks gestation. This imaging technique uses sound waves to create a real-time picture of the fetus inside the uterus, serving as a detailed check of the baby’s development and overall condition.
During the scan, the sonographer takes several precise measurements of the baby’s anatomy. These standard measurements, known as fetal biometry, include the Head Circumference (HC) or Biparietal Diameter (BPD), the Abdominal Circumference (AC), and the Femur Length (FL). The Abdominal Circumference is particularly sensitive to the baby’s nutritional status as it reflects the size of the liver.
These individual measurements are mathematically combined using established formulas to calculate the Estimated Fetal Weight (EFW). The EFW is plotted on a specialized growth chart to determine the baby’s percentile compared to other fetuses of the same gestational age. This percentile assesses whether the baby is growing at an expected rate. The scan also evaluates the amount of amniotic fluid and assesses blood flow in the umbilical cord and other fetal vessels using Doppler flow studies, which helps evaluate placental function.
When Growth Scans Are Indicated
A growth scan is typically ordered when routine prenatal screening or medical history suggests a higher risk of the fetus being too small or too large. In low-risk pregnancies, standard practice involves measuring the fundal height (the distance from the pubic bone to the top of the uterus) at each appointment after about 20 weeks. If the fundal height measurement is significantly smaller or larger than expected, a growth scan is warranted to determine the actual fetal size.
Targeted scans are indicated based on specific maternal risk factors present before or during the pregnancy. Pre-existing conditions such as chronic high blood pressure, diabetes, chronic kidney disease, or certain autoimmune disorders can compromise the placenta’s ability to deliver nutrients and oxygen, necessitating closer fetal monitoring. Advanced maternal age (generally 40 years or older) or a very low or very high Body Mass Index (BMI) also place a person into a higher-risk category for growth concerns.
A history of previous complicated pregnancies is a strong indication for serial growth monitoring. This includes a prior birth of a baby who was small for gestational age, stillbirth, or intrauterine fetal demise. Current concerns, such as the detection of too little amniotic fluid (oligohydramnios) or too much (polyhydramnios), also trigger the need for a growth scan.
Pregnancies with multiple gestations, such as twins or triplets, require serial growth scans due to the increased risk of unequal growth or restriction. Scans may also be required if a specific placental abnormality was noted on the second-trimester anatomy scan. These indications move the pregnancy from routine care to a scan pathway, often replacing the less precise fundal height measurements.
Interpreting Results and Next Steps
The results of the growth scan are interpreted by comparing the Estimated Fetal Weight (EFW) to established growth charts for that gestational age. A fetus with an EFW below the 10th percentile is classified as Small for Gestational Age (SGA). This diagnosis often leads to a further distinction of Intrauterine Growth Restriction (IUGR) or Fetal Growth Restriction (FGR), implying the baby has not reached its full growth potential, often due to placental issues.
If IUGR is diagnosed, management becomes more intensive. The focus is on balancing the risks of early delivery against the risks of continuing the pregnancy in an unfavorable environment. This typically involves frequent follow-up scans to monitor growth velocity and Doppler studies to assess blood flow. Non-Stress Tests (NSTs) and Biophysical Profiles (BPPs) may also be performed to monitor the baby’s health and activity levels.
Conversely, a fetus with an EFW above the 90th percentile is classified as Large for Gestational Age (LGA), which may lead to a diagnosis of macrosomia. This finding is common in pregnancies complicated by uncontrolled diabetes and carries implications for delivery planning. The concern is the potential for birth trauma, such as shoulder dystocia, which may influence the decision to plan for an elective cesarean delivery.
Regardless of whether the baby is trending toward SGA or LGA, subsequent management is highly individualized. It depends on the severity of the finding, the gestational age, and the results of the Doppler studies. For example, a baby with severe IUGR might require delivery between 37 and 38 weeks, while a lower-risk SGA diagnosis might allow delivery closer to 39 or 40 weeks. The scan results guide the healthcare team in developing a comprehensive surveillance and delivery plan.