Gaining weight during pregnancy is quite natural. However, there should be no large weight gain during the first trimester. Around 45% of weight gain happens between weeks 26-32 with another 20% occurring in weeks 32-40.
The weight gain associated with the best pregnancy outcomes depends on the mother’s weight at the beginning of the pregnancy. The Body Mass Index (BMI) is a useful tool in measuring this and a BMI of 24 is associated with the optimum pregnancy outcome.
BMI is calculated by dividing weight in kilograms by height in metres squared.
Pre-pregnancy BMI Optimum pregnancy weight gain
<19.8 12.5 – 18.0kg (27 – 40lb)
19.8 – 26.0 11.5 – 16.0kg (25 – 35lb)
26 – 29 7.0 – 11.5kg (15 – 25lb)
>29 6.0kg (13lb)
Reference. Institute of Medicine; Nutrition During Pregnancy (1990) Part 1: Weight Gain, p5
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Slimming diets during pregnancy are not recommended as weight loss is associated with a poor pregnancy outcome and a restriction on calories can mean intake is lacking essential nutrients.
A diet that substitutes low fat for high fat foods and is rich in fruit, vegetables and wholegrain cereals should be recommended. Women with problems managing weight gain during pregnancy may benefit from one-to-one dietary counselling.