Guidelines issued by the Institute of Medicine (IOM) provide guidance on recommended weight gain, based on your pre-pregnancy BMI. The pre-pregnancy BMI is calculated by taking your weight (in kilograms) and dividing by the square of your height (in metres).

If your BMI falls in the normal weight range (between 18.5 and 24.9), then the IOM recommends a total gestational weight gain of 11.5-16.0 kg (25-35 lbs). This corresponds to an average weight gain of 0.42kg (1.0 lb) per week. The recommended weight gain is 7.0-11.5 kg (15-25 lbs), for a BMI between 25.0 and 29.9, 5.0-9.9 kg (11-20 lbs) for a BMI greater than 30 and 12.5-18.0 kg (28-40 lbs) for a BMI less than 18.5.

For twin pregnancy, the IOM recommends a gestational weight gain of 16.8–24.5 kg (37–54 lb) for women of normal weight, 14.1–22.7 kg (31–50 lb) for overweight women, and 11.3–19.1 kg (25–42 lb) for obese women. For triplets and above, the IOM guidelines recognize there is insufficient data to determine a recommended weight gain.

Where does this extra weight go? On average, about 3.6 kg (8 lbs) can be attributed to your growing baby. The rest falls on the placenta (0.9-1.4 kg or 2-3 lbs), amniotic fluid (0.9-1.4 kg or 2-3 lbs), breast tissue (0.9-1.4 kg or 2-3 lbs), a general increase in circulating blood (1.8 kg or 4 lbs) and an enlarged uterus (0.9-2.3 kg or 2-5 lbs). 

It is important to control weight gain during pregnancy, as excessive weight gain can increase the risk of developing obesity and related cardiovascular diseases, or pregnancy complications such as gestational diabetes, hypertension, gestosis or preeclampsia.