Pregnancy Weight Gain Target Calculator

How it is calculated

The tool first works out the pre-pregnancy BMI: BMI = weight (kg) / height (m)2, with height typed in centimetres and divided by 100. The BMI picks a row of recommended total gain, in kilograms. For a single baby: under 18.5 gives 12.5 to 18.0 kg; 18.5 to under 25 gives 11.5 to 16.0 kg; 25 to under 30 gives 7.0 to 11.5 kg; 30 and above gives 5.0 to 9.0 kg. A BMI exactly on a boundary falls into the higher band.

For twins the ranges are 16.8 to 24.5 kg (BMI 18.5 to under 25), 14.1 to 22.7 kg (25 to under 30) and 11.3 to 19.1 kg (30 and above). The 2009 IOM guidance gives no twin range for an underweight mother, so twins with a BMI under 18.5 show no result. Target weights are your starting weight plus the low and the high end of the range. These are totals for the whole pregnancy, not weekly targets; other countries and clinicians may use different ranges.

Common mistakes

• Entering the current pregnant weight instead of the weight before pregnancy; the BMI category, and so the range, would be wrong.
• Typing height in metres (1.65) or in feet instead of centimetres. The BMI then comes out absurdly large (227,731.86 for 1.65 and 62 kg) and the highest-BMI range is applied without any warning.
• Reading the range as a pace: it is the total for the whole pregnancy, not an amount per month or per week.
• Selecting twins with a pre-pregnancy BMI under 18.5 and expecting a range; the guidance used has none, so the tool shows nothing.
• Treating the range as personal advice; it is a general guideline, and a care provider may set a different target for your situation.