Average Fetal Weight at 31 Weeks Gestation
At 31 weeks gestation, the average estimated fetal weight is approximately 1500 grams (ranging from 1400-1700g depending on the reference standard used), though significant physiologic variation exists based on fetal sex, maternal factors, and population characteristics. 1
Reference Values from Clinical Guidelines
The Society for Maternal-Fetal Medicine provides specific weight estimates at 31 weeks gestation in their clinical guidelines for fetal anemia management, which serve as standardized reference values 1:
- Mean estimated fetal weight: 1500g (approximately 3.3 pounds)
- Range across percentiles: 1400-1700g depending on growth trajectory
These values are derived from population-based fetal growth references that incorporate multiple biometric measurements including biparietal diameter, head circumference, abdominal circumference, and femur length 2.
Factors Influencing Fetal Weight at 31 Weeks
Fetal sex significantly affects weight, with female fetuses weighing approximately 6.6% less than male fetuses at comparable gestational ages 3.
Maternal characteristics independently influence fetal weight by 1-4.5% each, including maternal height, weight, age, and parity 4. The effect of these factors is graded across percentiles, with maternal height having the strongest positive effect on lower weight percentiles 4.
Ethnic and geographic variation plays a measurable role even after adjusting for maternal covariates, indicating that population-specific factors contribute to fetal weight differences 4, 5.
Clinical Context and Normal Variation
The physiologic range for fetal weight at 31 weeks is remarkably wide 4. When using the 5th to 95th percentile range, normal variation can span several hundred grams, reflecting the biologic diversity of human fetal growth under optimal maternal conditions 4.
For clinical assessment of fetal growth restriction, the American College of Obstetricians and Gynecologists defines abnormal growth as an estimated fetal weight below the 10th percentile for gestational age 2. At 31 weeks, this would correspond to weights below approximately 1300g, though the exact cutoff depends on the specific growth reference used 1.
Severe fetal growth restriction is defined as estimated fetal weight below the 3rd percentile, which carries significantly increased risk of adverse perinatal outcomes regardless of Doppler findings 1, 2.
Important Clinical Considerations
Accurate pregnancy dating using first-trimester crown-rump length is essential for interpreting fetal weight at any gestational age, as misdating can lead to misclassification of growth abnormalities 1, 2.
Population-based references should be used rather than customized standards when determining fetal weight percentiles, according to current American College of Obstetricians and Gynecologists recommendations 2.
The World Health Organization fetal growth charts, based on low-risk pregnancies from 10 countries across multiple continents, demonstrate that at 31 weeks gestation, fetal weight shows asymmetric distribution with wider variation above the 50th percentile in the third trimester 4.