A baby's typical size depends on gestational age. Using the Hadlock 1991 ultrasound weight standard, the median fetus weighs about 331 g at 20 weeks, 1,210 g at 28 weeks and 3,619 g at 40 weeks. Enter weeks and days of pregnancy to see the median weight in grams and pounds, the trimester and an everyday size comparison.
Expecting parents often wonder how big the baby is right now, and the weekly pregnancy apps rarely say where their figures come from. This calculator takes your gestational age in weeks and days and returns the median estimated fetal weight from the Hadlock sonographic standard, a growth table widely used by ultrasound software. Between whole weeks it interpolates linearly, so 20 weeks 3 days falls between the 20- and 21-week values. You also get the weight in pounds and ounces, the trimester, a fruit or vegetable of comparable size, and how far along the baby is toward the average weight at 40 weeks. The figure describes a typical baby, not yours: real fetuses spread widely around the median, and only an ultrasound scan with measured head, abdomen and femur gives an estimate for an individual pregnancy.
Use the − and + buttons to choose completed weeks of pregnancy, from 10 to 40, dated from the last menstrual period.
Pick 0 to 6 days beyond the whole week; the weight is interpolated between the two neighbouring weeks.
The headline is the median weight in grams; below it are pounds and ounces, the trimester and a size comparison.
The progress bar shows the current weight as a share of the 3,619 g median at 40 weeks.
Result Estimated baby weight (50th percentile): 1,210 g
In pounds and ounces: 2 lb 10.7 oz
Size comparison: About the size of an aubergine
Trimester: Third trimester
Share of average 40-week weight: 33%
At the start of the third trimester the median estimated weight is 1,210 g, about a third of the 40-week average.
Result Estimated baby weight (50th percentile): 360 g
In pounds and ounces: 0 lb 12.7 oz
Size comparison: About the size of a banana
Trimester: Second trimester
Share of average 40-week weight: 10%
Three days past the anatomy-scan week, the weight sits three sevenths of the way from 331 g to 399 g.
Result Estimated baby weight (50th percentile): 3,619 g
In pounds and ounces: 7 lb 15.7 oz
Size comparison: About the size of a watermelon
Trimester: Third trimester
Share of average 40-week weight: 100%
The Hadlock median at 40 weeks is 3,619 g, roughly 8 lb at birth.
EFW = W(w) + (W(w+1) − W(w)) × d ÷ 7
EFWW(w)dHadlock and colleagues built their in utero weight standard from serial ultrasound measurements of a large group of pregnancies, publishing the 10th, 50th and 90th percentile weight for each week from 10 to 40. This tool reads the 50th percentile for your completed week and, when you add days, moves linearly toward the next week's value. Pounds and ounces use 1 oz = 28.3495 g. Size comparisons are informal anchors chosen to match the weight order, not measured objects.
The Hadlock median at 20 weeks 0 days is 331 g, a little under 12 ounces. By 21 weeks it rises to 399 g, so weight gain is already about 70 g a week around the time of the anatomy scan.
The Hadlock weight standard begins at 10 weeks. Before that, embryos are usually described by crown-rump length rather than weight, and a gram figure would be too small and variable to be meaningful.
Quite different. In the Hadlock data the 10th and 90th percentiles at 28 weeks lie roughly 20 percent below and above the median. Parental height, ethnicity, placental function and maternal health all shift growth, so a baby away from the median can be perfectly healthy.
The sonographer measures the head circumference, abdominal circumference and femur length, and software combines them in a regression formula, often also by Hadlock. That estimate has an error of about 10 to 15 percent either way.
In absolute grams, during the third trimester. Between 32 and 36 weeks the median rises from 1,953 g to 2,813 g, more than 200 g a week, while early in pregnancy the weekly gain is only tens of grams.
No. They are familiar anchors arranged so that each week's item is larger than the last. Produce varies a lot in size, so use them for a sense of scale and rely on the gram figure for numbers.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 19). Baby Size Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/health/pregnancy-and-fertility/baby-size-calculator/
Tools Directory Editorial Team. “Baby Size Calculator.” Tools Directory, 19 Oct. 2026, https://tools.milunir.com/health/pregnancy-and-fertility/baby-size-calculator/. Accessed 28 Oct. 2026.
Tools Directory Editorial Team. “Baby Size Calculator.” Tools Directory. Last modified October 19, 2026. Accessed October 28, 2026. https://tools.milunir.com/health/pregnancy-and-fertility/baby-size-calculator/.
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