Why does weight gain occur during pregnancy?

Remember that all weight gain in pregnancy is not directly due to the developing baby (foetus). In fact, a fair amount of the weight gain may be due to fatty deposits. A total weight gain of 12.5 kg during pregnancy may be broken up as follows: foetus 3.3 kg, placenta 0.6 kg, uterus 0.9 kg, breasts 0.4 kg, blood 1.2 kg, fat 3.5 kg, body fluids 2.6 kg.