Look up a fetal weight chart by week in lbs and grams, with the typical length for each gestational week. It’s a general guide to averages, since healthy babies vary widely.
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.
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.
Fetal Weight Chart by Week in lbs and Grams
The average baby weighs about 0 lb 11 oz at 20 weeks, roughly doubles by 24 weeks, and reaches about 3 lb 12 oz (1,702 g) at 32 weeks and 7 lb 10 oz (3,462 g) at 40 weeks. These are population averages for each week, not a measurement of your baby, and a wide range either side is normal.
| Week | Average Weight | Average Length | Trimester |
|---|---|---|---|
| 20 | 300 g (0 lb 11 oz) | 25.6 cm (10.1 in) | Second |
| 24 | 600 g (1 lb 5 oz) | 30.0 cm (11.8 in) | Second |
| 28 | 1005 g / 1.01 kg (2 lb 3 oz) | 37.6 cm (14.8 in) | Third |
| 32 | 1702 g / 1.70 kg (3 lb 12 oz) | 42.4 cm (16.7 in) | Third |
| 36 | 2622 g / 2.62 kg (5 lb 12 oz) | 47.4 cm (18.7 in) | Third |
| 40 | 3462 g / 3.46 kg (7 lb 10 oz) | 51.2 cm (20.2 in) | Third |
So how much does a baby weigh at 20 weeks in pounds? On average, under a pound: about 11 oz. Baby weight at 28 weeks in kg is about 1.01 kg, and by 36 weeks the average is about 2.62 kg.
How the Calculator Works
It looks up the typical average weight and length for the week you enter, drawn from standard growth-chart averages, then converts the units. It shows what’s average for the week, not a measurement of any specific baby.
Formula: Weight and length are read directly from an average-by-week reference table (weeks 8-42), then converted for display: pounds/ounces = weight in grams ÷ 28.3495, split into whole pounds and remaining ounces; inches = length in cm ÷ 2.54. Trimester is set as First for weeks 1-13, Second for weeks 14-27, and Third for weeks 28 and beyond. Length is measured crown-to-rump in early weeks and head-to-heel from around week 20 onward, which is why length increases more sharply around that point.
- Take the average weight for the week: 1,702 g at 32 weeks.
- Divide by 28.3495 to get ounces: about 60 oz.
- Split into pounds and ounces: 60 oz = 3 lb 12 oz.
How to Compare a Scan With These Averages
Expect a gap. The chart shows average figures for each week, while an ultrasound gives an estimated weight for your own baby, and the two measure different things.
- Scan weights are calculated from head, abdomen and thigh-bone measurements, and they can be noticeably off in either direction, especially late in pregnancy.
- Use the gestational week your maternity team confirmed, because a week’s difference changes the average a lot in the third trimester.
- Healthy babies vary with parents’ size, twins and the baby’s sex, so a figure above or below the average is not a diagnosis.
Clinicians plot growth on centile charts and watch the trend across scans. That trend matters far more than how closely any single scan matches the average for the week.
If your baby is measuring small or large, ask your midwife what it means. Contact your maternity unit straight away about reduced movements, and never use a fetal weight calculator to reassure yourself. The obstetric ultrasonography article on Wikipedia explains how scans estimate a baby’s size.
Common questions about baby weight by week
How much does a baby weigh at 32 weeks?
The average at 32 weeks is about 1,702 g (3 lb 12 oz), with a length of about 42.4 cm. Healthy babies vary widely around this figure, and only your scan estimates your own baby’s size.How much should a baby weigh at 36 weeks?
There isn’t one weight a baby should be. The average at 36 weeks is about 2,622 g (5 lb 12 oz), and a wide spread above and below that is considered healthy. Your midwife or doctor can tell you what your scan means.What is the average baby weight at 28 weeks in kg?
About 1.01 kg (1,005 g, or 2 lb 3 oz), with a length of about 37.6 cm. Week 28 is also the start of the third trimester.Why does my ultrasound estimate differ from the average shown here?
Scan weights are calculated from measurements with a built-in margin of error, and real babies fall above or below the population average. A difference from this general reference is usually not meaningful on its own; your provider can put the scan in context.Why does length switch from crown-to-rump to head-to-heel?
Early on the baby is curled up, so length is measured crown-to-rump. From around week 20 the legs can be measured more reliably, so length is taken head-to-heel, which is why it seems to jump around that week.Is a baby measuring small or large for its week necessarily a problem?
Not necessarily. Many healthy babies measure smaller or larger than average throughout pregnancy. Only your doctor or midwife, using your actual scans and the growth pattern over time, can say whether a measurement needs follow-up.Related Calculators
To see what’s typically developing alongside this week’s average size, try the baby development week calculator. You can also confirm your current gestational week with the pregnancy week calculator or check your estimated delivery date with the due date calculator.