Fetal abdominal circumference (AC) is a biometric measurement obtained during an ultrasound used to assess fetal size and estimate weight. The AC is sensitive to the nutritional status of the developing fetus and helps determine if growth is progressing appropriately for gestational age. When the AC falls below established norms, typically the 10th percentile, it suggests a potential growth issue requiring prompt medical attention. A low AC is a clinical finding that signals the need for further investigation to ensure the fetus is thriving.
Significance of Fetal Abdominal Circumference Measurement
The abdominal circumference is measured across the fetal abdomen at a specific level that includes the stomach, spine, and the deep portion of the umbilical vein. The AC largely reflects the size of the fetal liver, the main storage organ for glycogen and nutrients. A reduced AC often indicates the fetus is mobilizing stored energy reserves due to suboptimal nutrient supply.
The AC is used with the Head Circumference (HC) and Femur Length (FL) to calculate the Estimated Fetal Weight (EFW). The AC is often the first measurement affected in asymmetrical Fetal Growth Restriction (FGR) caused by placental problems. The HC and FL may remain preserved as the body prioritizes nutrient flow to the brain, a process called “brain sparing.” The ratio of HC to AC helps distinguish between a constitutionally small fetus and one whose growth is pathologically restricted.
Potential Reasons for Reduced Fetal Growth
A small abdominal circumference, often below the 10th percentile, suggests Fetal Growth Restriction (FGR). Causes are categorized into maternal, placental, and fetal factors.
Maternal factors involve conditions affecting blood flow and the environment, such as chronic hypertension, preeclampsia, diabetes with vascular disease, or chronic kidney disease. Lifestyle choices, including chronic malnutrition, severe anemia, or substance use like smoking and alcohol, also impair nutrient delivery.
Placental factors commonly cause FGR in later pregnancy by limiting the transfer of oxygen and nutrients. These include placental insufficiency, infarction, and decidual vasculopathy. Poor remodeling of the uterine spiral arteries is a mechanism leading to decreased placental function and impaired growth.
Fetal factors include intrinsic issues like genetic or chromosomal abnormalities. Infections acquired during pregnancy can also directly impede growth. While some small fetuses are constitutionally small due to parental genetics, most diagnosed with FGR have a pathology related to one of these three categories.
Lifestyle Adjustments to Support Fetal Development
Lifestyle modifications serve as supportive measures to optimize the maternal environment and maximize blood flow and nutrient delivery. Once a growth concern is identified, increased rest is recommended to conserve maternal energy and improve uterine blood flow. Providers often suggest resting in the left lateral position, as this posture maximizes circulation through major vessels, enhancing blood flow to the uterus and placenta.
Adequate hydration supports the overall maternal circulatory volume. A high-protein, nutrient-dense diet is emphasized, focusing on quality calories. Targeted dietary interventions, such as a Mediterranean-style diet rich in plant-based foods and healthy fats, may reduce the risk of delivering a small-for-gestational-age infant.
Good nutrition ensures the mother receives sufficient micronutrients and macronutrients. Immediate cessation of harmful substances like smoking, alcohol, and illicit drugs is non-negotiable, as these compromise placental function and fetal oxygen supply. Managing psychological stress through relaxation techniques is also beneficial, as high stress levels have been linked to poor fetal growth outcomes.
Clinical Monitoring and Interventions
Once a reduced abdominal circumference is detected, management shifts to heightened surveillance of fetal well-being and growth velocity. Serial ultrasounds are performed every two to four weeks to track the AC and Estimated Fetal Weight, establishing a growth curve. This repeated measurement determines if the fetus is simply small or if its growth rate is slowing, confirming FGR.
Doppler velocimetry is a non-invasive ultrasound technique used to assess blood flow resistance in vessels like the umbilical artery. Abnormal Doppler results, such as reduced or absent end-diastolic flow, indicate impaired placental function and increased risk. The Biophysical Profile (BPP) or Nonstress Test (NST) evaluates fetal health, assessing movement, breathing, muscle tone, and heart rate patterns.
Monitoring results determine the timing of delivery, balancing the risks of prematurity against the risks of remaining in a suboptimal intrauterine environment. If fetal compromise is significant, or the estimated fetal weight drops below the 3rd percentile, an earlier delivery may be recommended. Corticosteroids may be administered before an early delivery to accelerate lung development.