Baby Size & Growth Calculator
Estimate your baby's weight at any point in pregnancy by entering gestational age and parental size. Useful at prenatal appointments to compare against ultrasound measurements.
About this calculator
Fetal weight estimation relies on gestational age because growth is non-linear — slower in early weeks and exponential in the third trimester. For pregnancies under 20 weeks the formula uses baseWeight = (gestationalWeeks − 8)^2.5 × 0.5, and at 20 weeks or later it switches to baseWeight = (gestationalWeeks − 6.5)^2.8 × 0.16, reflecting the accelerated growth phase. A parental size factor then scales the result to account for genetic variation in fetal size. Finally, if imperial units are selected, the gram estimate is converted to ounces by dividing by 28.35. This model is an approximation; clinical ultrasound biometry remains the gold standard for fetal weight estimation.
How to use
Suppose you are 28 weeks and 3 days pregnant, use metric, and have a parental size factor of 1.0. Total weeks = 28 + 3/7 ≈ 28.43. Because 28.43 ≥ 20, baseWeight = (28.43 − 6.5)^2.8 × 0.16 = (21.93)^2.8 × 0.16 ≈ 4,874 × 0.16 ≈ 780 g. With parentalSize = 1.0 the adjusted weight stays ≈ 780 g. Enter your weeks, days, measurement system, and parental size factor, and the calculator returns the estimated fetal weight instantly.
Frequently asked questions
How accurate is a fetal weight calculator compared to an ultrasound?
Online fetal weight calculators use population-based growth curves and are best used as a general reference, not a clinical diagnosis. Ultrasound biometry — which measures head circumference, abdominal circumference, and femur length — is significantly more accurate, typically within 10–15% of actual birth weight. This calculator adds a parental size factor to personalise the estimate slightly, but individual variation is large. Always discuss your baby's growth trajectory with your midwife or obstetrician.
What does gestational age mean and how do I calculate it?
Gestational age is the number of weeks and days since the first day of your last menstrual period (LMP), not the date of conception. It is the universal standard clinicians use to track fetal development. If you know your LMP, simply count forward; if not, your dating ultrasound will provide a gestational age. Enter both the full weeks and any remaining days for the most precise weight estimate.
Why does fetal weight increase so rapidly in the third trimester?
During the third trimester (weeks 28–40), the fetus lays down fat stores, develops muscle mass, and undergoes rapid organ maturation — all of which add significant weight. The growth formula reflects this by using a higher exponent (2.8) for pregnancies at or beyond 20 weeks compared with the earlier phase. On average, a fetus gains roughly 200–250 g per week in the final trimester. This is why third-trimester nutrition and monitoring are especially important.