The formula
the range depends on the BMI before pregnancy, not on absolute weight
Where it comes from
The ranges are the Institute of Medicine's, the most widely used internationally. A woman starting underweight should gain between 12.5 and 18 kg; one starting obese, between 5 and 9. Total weight and the pace of it are different questions: in the first trimester barely half a kilo to two is gained.
How to work it out by hand
- Work out your BMI from your pre-pregnancy weight and your height
- Find your category: underweight, normal, overweight or obese
- Look up the recommended total range for that category
- Spread that gain out: little in the first trimester and steadily afterwards
What is worth knowing
Of the weight gained, only part is the baby: at the end of pregnancy it weighs 3 to 3.5 kg. The rest is spread between the placenta, amniotic fluid, the growth of the uterus and breasts, extra blood volume and fat stores. That is why the scales rise a good deal more than the baby weighs, and why it does not all come off straight after the birth.