Average Birth Weight
Shows the approximate average birth weight by gestational age and sex. Also check the difference from the average and the low birth weight threshold (under 2500 g).
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Input
Shows the approximate average birth weight based on gestational age and sex. Enter a birth weight (optional) to also see the difference from the average and its weight category.
Gestational age (weeks)
40 wk (around the due date)
wkSex
g
Result
Average weight guide at 40 weeks · Boy
About 3,230g
This is within the full-term range (37–41 weeks).
Sex
Boy
Gestational age
40 wk
Low birth weight threshold
under 2500 g
Average birth weight by gestational age (g)
| Gestational age | Boy | Girl |
|---|---|---|
| 28 wk | 1,200 | 1,130 |
| 29 wk | 1,380 | 1,310 |
| 30 wk | 1,570 | 1,490 |
| 31 wk | 1,760 | 1,680 |
| 32 wk | 1,960 | 1,870 |
| 33 wk | 2,160 | 2,060 |
| 34 wk | 2,370 | 2,260 |
| 35 wk | 2,560 | 2,440 |
| 36 wk | 2,740 | 2,610 |
| 37 wk | 2,900 | 2,770 |
| 38 wk | 3,030 | 2,900 |
| 39 wk | 3,140 | 3,010 |
| 40 wk (selected) | 3,230 | 3,110 |
| 41 wk | 3,300 | 3,180 |
| 42 wk | 3,340 | 3,220 |
How it works
- Choose a gestational age (weeks) and sex to see the approximate average birth weight (g) for a baby born at that point. Enter a birth weight (optional) to also check the difference from the average and its category.
- The average figures are approximate values based on Japanese standard data for birth size by gestational age (following the trend of Itabashi et al., 2014, widely used in Japan). Boys tend to be slightly heavier than girls, and a full-term baby at 40 weeks (around the due date) is roughly 3000 g on average.
- Low birth weight means a birth weight under 2500 g, very low birth weight under 1500 g, and extremely low birth weight under 1000 g. Conversely, 4000 g or more is called high birth weight (macrosomia). These are common definitions of weight categories.
- Births from 37 weeks 0 days to 41 weeks 6 days are called full-term, and births before 37 weeks are called preterm. The earlier the week, the lower the average weight.
- Even at the same gestational age, birth weight varies widely between individuals and is affected by many factors such as the mother's condition, multiple births, and genetics. The values in this tool are only average guides for a population and do not indicate the appropriate weight for a specific baby.
- This tool provides estimates based on general figures and is not a diagnosis or medical service. Always consult a doctor for health-related decisions.
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