A prenatal growth scan is a specialized, non-invasive ultrasound examination performed during the later stages of pregnancy, usually in the second or third trimester. This diagnostic tool uses high-frequency sound waves to create images of the developing fetus inside the womb. Its primary purpose is to monitor the baby’s growth rate and overall size to ensure development is progressing as expected for the gestational age. The scan offers quantitative data on the baby’s physical dimensions and helps healthcare providers assess fetal well-being.
Why a Growth Scan Is Necessary
A healthcare provider may recommend a growth scan if there are specific factors that suggest a baby might be growing too slowly or too quickly. The most common indication is a discrepancy in the fundal height measurement, where the size of the uterus does not align with the number of weeks pregnant. A history of previous pregnancies with a small birth weight or complications like placental issues also often prompts serial monitoring.
Maternal health conditions, such as pre-existing diabetes, high blood pressure, or kidney disease, are further reasons for a referral, as these can affect the placenta’s function and nutrient delivery. Additionally, patients carrying multiples, like twins or triplets, are routinely scheduled for these scans to track the growth of each fetus individually. The timing of these scans is often staggered, with some high-risk pregnancies beginning monitoring as early as 28 weeks.
The Procedure and Data Collection
The growth scan procedure is similar to a standard ultrasound, involving the application of a clear, water-based gel to the abdomen. A sonographer then moves a handheld transducer across the skin, which sends sound waves through the body and captures the returning echoes to form an image. The entire process typically takes about 20 to 30 minutes, during which the technician focuses on obtaining precise measurements.
The provider collects four primary biometric measurements used for assessing growth. These specific data points are then used in complex formulas to estimate the baby’s weight.
- Biparietal Diameter (BPD), a side-to-side measurement of the head.
- Head Circumference (HC), which measures the perimeter of the skull.
- Abdominal Circumference (AC), measured around the baby’s midsection.
- Femur Length (FL), which measures the longest bone in the body.
How Estimated Fetal Weight is Calculated
The result of the scan is the Estimated Fetal Weight (EFW), which is a calculation derived from the four biometric measurements, not a direct weight. The ultrasound machine uses established mathematical formulas, such as the Hadlock or Shepard equations, combining the HC, AC, BPD, and FL measurements to generate a weight estimate. The abdominal circumference is often considered the most influential measurement because it best reflects the baby’s nutritional status.
The EFW is plotted on a growth chart to determine the baby’s percentile for their gestational age. A percentile ranks the baby’s size relative to other babies at the same stage of pregnancy, with the 50th percentile being the average. An EFW below the 10th percentile may suggest Fetal Growth Restriction (FGR), while an EFW above the 90th percentile may indicate the baby is Large for Gestational Age (LGA). Note that even the most accurate formulas carry an expected margin of error, often up to 15%.
Next Steps Following Growth Scan Results
If the growth scan determines the baby is within the normal range (10th and 90th percentiles), routine prenatal care continues without changes. When a baby measures outside this range, the provider discusses follow-up actions based on the deviation’s severity. For suspected FGR or LGA, the first step is usually to schedule serial growth scans every two to four weeks to track the trajectory. Additional testing may include a Doppler ultrasound, which assesses blood flow to check the placenta’s function, or non-stress tests to monitor the baby’s heart rate and movement. These combined results help the provider formulate a management plan, including discussions about the ideal timing and method of delivery.