How this calculator works
This lookup uses IOM 2009 Table S-1 for singleton total gain and second- and third-trimester mean weekly rates with their ranges. Those rates assume 0.5 to 2 kg total gain in the first trimester; they are not first-trimester weekly rates. Twin totals come from the Summary's provisional guidance for normal, overweight and obese pre-pregnancy BMI bands. The published twin pound endpoints are preserved and converted to kilograms to one decimal place; the report's Summary gives more rounded kilogram equivalents. IOM issued neither an underweight twin total range nor twin weekly rates. These population references need clinical interpretation and do not establish an individual weekly target or apply to higher multiples. The result is an estimate for information only and is not medical advice.
Formula
Select the pre-pregnancy BMI band: below 18.5, 18.5 to below 25, 25 to below 30, or at least 30. Singleton totals and mean weekly rates with ranges are lookups from Table S-1. Provisional twin totals are 37 to 54, 31 to 50 and 25 to 42 lb for the three supported bands respectively; kg = lb × 0.45359237. No total is calculated for underweight twins.
Worked example
At BMI 21.7 with a singleton, total gain is 11.5 to 16.0 kg and the second- and third-trimester mean is 0.42 kg/week, with a range of 0.35 to 0.50. At BMI 31 with twins, the provisional total is 25 to 42 lb, displayed as 11.3 to 19.1 kg. At BMI 17 with twins, the result advises asking a care provider because IOM 2009 has no range for that group.
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Frequently asked questions
Does this use my current BMI?
No. The lookup uses BMI before pregnancy, not BMI after pregnancy weight gain.
Why is there no total gain range for underweight twins?
IOM 2009 found insufficient evidence to issue even a provisional range for this group. Ask your care provider for an individual plan; this tool does not borrow a range from another BMI band.
Do the weekly rates apply throughout pregnancy or to twins?
No. Table S-1 supplies means and ranges for singleton pregnancies in the second and third trimesters only. A mean is not an amount that everyone must gain each week. The provisional twin guidance supplies term totals without a weekly rate.