See average baby weight by week. This free fetal weight calculator shows the typical fetal weight (grams and lb/oz) and length for each gestational week — a general guide, since healthy babies vary widely.
These are average fetal weights and lengths for each week, for general interest. Healthy babies vary enormously in size, and being bigger or smaller than average is usually completely normal. Only an ultrasound scan and your provider can estimate your baby’s size.
Gestational week
weeks
Enter or slide to a week from 8 to 42 to see the typical average fetal weight and length for that gestational age. Lengths are crown-to-rump in early weeks and head-to-heel from about week 20.
Average weight
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Choose a week
In pounds & ounces
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Average length
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Trimester
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Length (inches)
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General averages for information only, not a measurement of your baby and not medical advice. Your baby’s size is estimated at an ultrasound scan; talk to your midwife or doctor about growth.
Use this free fetal weight calculator to see the average baby weight and length at each week of pregnancy. Pick a week to view the typical size for that gestational age.
What this fetal weight calculator shows
This fetal weight calculator is an average-by-week reference. For any gestational week it shows the typical fetal weight (in grams and pounds/ounces) and length. It’s a general guide — not a measurement of your own baby.
Average fetal weight by week
Week
Avg weight
Avg length
12
14 g
5.4 cm
20
300 g
25.6 cm
28
1,005 g (2 lb 3 oz)
37.6 cm
36
2,622 g (5 lb 12 oz)
47.4 cm
40
3,462 g (7 lb 10 oz)
51.2 cm
Lengths are measured crown-to-rump in early pregnancy and head-to-heel from about week 20, which is why the figure jumps around then.
How much do babies really vary?
A lot — and that’s normal. Two perfectly healthy babies at the same week can differ by a pound or more. Being above or below average isn’t a problem in itself; what matters is steady growth, which your provider tracks.
How is my own baby’s weight estimated?
Through an ultrasound scan, where a sonographer measures the head, abdomen and thigh bone and a formula estimates the weight. This page gives population averages, not a measurement of your baby — only a scan can do that.
How to use the fetal weight calculator
Enter a gestational week (8–42) or use the slider.
See the average weight in grams and pounds/ounces.
Check the average length and trimester.
Fetal Weight FAQ
How much does a baby weigh at each week?
Averages rise from a few grams early on to about 3,400 g (7.5 lb) by week 40. For example, the average is around 300 g at week 20 and 1 kg at week 28. Enter a week to see the typical figure.
How is my baby's actual weight estimated?
At an ultrasound scan, where measurements of the head, abdomen and thigh bone feed a formula. This page shows population averages, not a measurement of your own baby.
Is it bad if my baby is bigger or smaller than average?
Usually not — healthy babies vary a great deal, and being above or below average is common. Your provider looks at the growth trend over time, which matters more than a single number.
Why does the length jump around week 20?
Before about week 20, length is measured crown-to-rump; from then it's measured head-to-heel, so the figure increases at that point.
What's the average birth weight?
Around 3.4 kg (about 7.5 lb) at 40 weeks, though anywhere from roughly 2.5 to 4.5 kg is common and healthy.
Is the fetal weight calculator free?
Yes, this fetal weight calculator is completely free, needs no sign-up, and works instantly in your browser.
