The abbreviation AC on a prenatal ultrasound report stands for Abdominal Circumference. This standard biometric reading is one of the most informative assessments used during the second and third trimesters to track the baby’s development. Understanding what AC represents and how it is obtained is a helpful step in monitoring the health and growth trajectory of the developing fetus. The measurement provides insight into whether the baby is growing at an appropriate rate for its gestational age.
The Meaning of AC (Abdominal Circumference)
The Abdominal Circumference captures the perimeter of the fetus’s midsection at a defined cross-sectional level. This measurement reflects the size of the fetal abdomen, which is highly influenced by the liver. The liver is the largest organ in the fetal abdomen and serves as the primary storage site for glycogen and essential nutrients.
The AC is considered a leading indicator of the baby’s nutritional status and overall growth in the womb. When the fetus receives adequate nutrients through the placenta, the liver stores glycogen, causing the abdomen to grow. Conversely, if the fetus is malnourished, the liver size often decreases, resulting in a smaller abdominal circumference.
The AC is particularly sensitive to changes in the fetal environment, making it a valuable tool for monitoring well-being. Unlike measurements of the head or femur, which are less affected by short-term changes, the AC can change more quickly in response to placental function. Deviations in abdominal size often signal a change in the baby’s growth pattern before other measurements do.
How the AC Measurement is Obtained
Obtaining an accurate Abdominal Circumference measurement requires the sonographer to capture a specific transverse view of the fetal abdomen. The required image must be taken perpendicular to the baby’s spine, creating a circular cross-section. Several anatomical landmarks must be visible in this single plane to ensure the measurement is valid and reproducible.
These landmarks include the fetal stomach bubble, which indicates the correct height for the measurement. Also required is the junction of the umbilical vein as it enters the left portal sinus, forming a characteristic J- or L-shape in the liver. The sonographer must obtain a perfectly symmetrical view, avoiding oblique cuts that distort the shape and lead to an inaccurate reading.
Once the correct image is captured, the measurement is taken by tracing the outer perimeter of the fetal abdomen along the skin line. This tracing method is generally preferred over calculating the circumference from two diameters because it tends to be more precise. Achieving this precise, uncompressed circular image can be challenging due to fetal movement and breathing.
AC’s Role in Assessing Fetal Growth and Weight
The Abdominal Circumference is a foundational element in calculating the Estimated Fetal Weight (EFW), which is the primary way doctors assess the size of the baby in utero. EFW uses complex mathematical formulas combining the AC with other biometric measurements, such as the biparietal diameter (BPD), head circumference (HC), and femur length (FL). Because the AC is the measurement most influenced by fat and glycogen storage, it is the single most important factor in the EFW calculation.
The resulting EFW is plotted on a growth chart to determine the baby’s percentile rank for its gestational age. This percentile provides context for the baby’s size relative to others at the same stage of development. Clinicians are concerned if the AC or EFW falls below the 10th percentile, which is a sign of Fetal Growth Restriction (FGR). FGR suggests the baby is not growing as expected, often due to poor placental function, and usually necessitates closer monitoring and potential intervention.
Conversely, an AC or EFW above the 90th percentile indicates macrosomia, meaning the baby is Large for Gestational Age (LGA). This finding is often associated with conditions like maternal diabetes. Maternal diabetes causes the fetus to receive excess glucose, leading to increased fat and glycogen storage and a larger abdomen. Both FGR and LGA can impact the delivery planning, with LGA potentially leading to birth complications like shoulder dystocia.
The AC measurement is also used as a direct screening tool due to its strong correlation with the baby’s actual birth weight. Monitoring the AC over time allows doctors to track the growth velocity. Identifying growth patterns outside the normal range ensures timely investigation and intervention for the best possible outcome.