Fetal Weight Estimator
Estimate your baby's approximate weight based on gestational age in weeks. Useful during prenatal checkups to track fetal growth milestones trimester by trimester.
About this calculator
Fetal weight varies dramatically across the three trimesters. This calculator uses a piecewise linear model to approximate average fetal weight (in grams) based on gestational age in weeks. The formula applied is: weight = gestationalWeeks × 15 (before 20 weeks), weight = gestationalWeeks × 50 (weeks 20–29), and weight = gestationalWeeks × 150 (weeks 30 and beyond). These multipliers reflect the accelerating pace of fetal growth, especially in the third trimester when fat accumulation and organ maturation drive rapid weight gain. While this provides a useful ballpark, clinical ultrasound biometry remains the gold standard for accurate fetal weight estimation.
How to use
Suppose you are 32 weeks pregnant. Because 32 ≥ 30, the third-tier formula applies: weight = 32 × 150 = 4,800 g (approximately 4.8 kg). For a 25-week fetus, use the middle tier: 25 × 50 = 1,250 g. For an early pregnancy at 16 weeks, use the first tier: 16 × 15 = 240 g. Simply enter your gestational age in weeks and the calculator selects the correct formula automatically.
Frequently asked questions
How accurate is a gestational-age-based fetal weight estimate compared to ultrasound?
Gestational-age formulas give a population-average approximation and are far less precise than ultrasound biometry, which measures head circumference, abdominal circumference, and femur length. Ultrasound estimates carry an error margin of roughly ±15%, while simple age-based formulas can deviate more. They are best used for educational reference rather than clinical decision-making. Always discuss growth concerns with your OB or midwife.
What is considered normal fetal weight at each trimester?
In the first trimester (up to 13 weeks) fetuses typically weigh under 30 g. By the end of the second trimester (around 27 weeks) average weight is roughly 900 g–1,000 g. At full term (40 weeks) the average is about 3,400 g (7.5 lbs). Individual variation is wide and influenced by genetics, nutrition, and placental function.
When should I be concerned about fetal weight being too low or too high?
A fetus consistently below the 10th percentile for gestational age may be diagnosed with intrauterine growth restriction (IUGR), while one above the 90th percentile may be macrosomic. Both conditions carry potential risks including preterm birth, delivery complications, and metabolic issues. Serial ultrasounds every 2–4 weeks are typically ordered to monitor growth trends. Consult your healthcare provider if scans suggest growth is outside the normal range.