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.
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.
How the Fetal Weight Calculator Works
This tool looks up the typical average fetal weight and length for a given gestational week, drawn from standard growth-chart averages. Individual babies vary widely in size, so this 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.
- Week of pregnancy — the input, entered or dragged on the slider, from 8 to 42 weeks.
- Average weight — the typical fetal weight for that week, in grams/kg and pounds/ounces.
- Average length — the typical length for that week, in centimeters and inches.
- Trimester — First, Second, or Third, based on the entered week.
Example Scenarios
| 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 |
How to Compare a Scan With the Fetal Weight Calculator
A fetal weight calculator shows average figures for each week, while an ultrasound gives an estimated weight for your own baby. The two measure different things, so a gap between them is common.
- 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 fetal weight calculator 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 a fetal weight calculator.
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 the size a fetal weight calculator averages.
Fetal Weight Calculator FAQ
How accurate are these average fetal weight numbers for my baby specifically?
These are population averages for each week, not a measurement of your baby. Healthy babies vary enormously in size at every gestational week. Only an ultrasound scan performed by your provider can estimate your baby’s actual size, and that estimate carries its own margin of error.Why does my ultrasound estimate differ from the average shown here?
Ultrasound-based weight estimates are calculated from measurements with an inherent margin of error, and real babies naturally fall above or below the population average. A difference from this general reference is usually not meaningful on its own — your provider can put your scan result in context.What’s a normal weight range versus this “average” figure?
This calculator shows one average value per week, not a full normal range. A wide spread of weights is considered healthy at any given week, so being above or below the number shown here is not, by itself, cause for concern.Why does length switch from crown-to-rump to head-to-heel measurement?
Early in pregnancy the baby is curled up, so length is measured crown-to-rump. From around week 20, as legs extend and can be measured more reliably, length is measured head-to-heel instead, which is part of why length appears 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 ultrasound measurements and growth pattern over time, can assess whether a measurement needs follow-up.Should I compare this to my growth scan measurements?
You can use it for general context, but your growth scan report from your provider is the clinically meaningful figure. This calculator is for general reference only and isn’t a substitute for an ultrasound reading or your provider’s assessment.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.