How this calculator works
First we work out your pre-pregnancy body mass index: BMI = weight (kg) / height (m)². Then we look up the total weight gain recommended by the US Institute of Medicine (now the National Academy of Medicine) in its 2009 report Weight Gain During Pregnancy:
| Pre-pregnancy BMI | One baby | Twins (provisional) |
|---|---|---|
| Under 18.5 | 12.5-18 kg (28-40 lb) | Not enough evidence |
| 18.5-24.9 | 11.5-16 kg (25-35 lb) | 17-25 kg (37-54 lb) |
| 25-29.9 | 7-11.5 kg (15-25 lb) | 14-23 kg (31-50 lb) |
| 30 or more | 5-9 kg (11-20 lb) | 11-19 kg (25-42 lb) |
For your week-by-week range, we follow the IOM assumption of 0.5-2 kg gained by the end of the first trimester (week 13), rising evenly from zero. From week 13 to week 40 the range rises evenly to the total above - for one baby this closely matches the IOM's suggested weekly rates for the second and third trimesters.
BMI does not tell the whole story of your health, and these ranges are a starting point for a conversation with your midwife or doctor, not a target to worry over. Looking for your dates instead? Try the due date calculator.
