How much weight gain does the IOM publish for pregnancy?
Quick answer
The 2009 IOM report publishes total gain by pre-pregnancy BMI: 12.5 to 18 kg below BMI 18.5, 11.5 to 16 kg for 18.5 to 24.9, 7 to 11.5 kg for 25.0 to 29.9, and 5 to 9 kg at 30 and above. In pounds: 28-40, 25-35, 15-25 and 11-20.
Those ranges come from Table S-1 of Weight Gain During Pregnancy: Reexamining the Guidelines, published by the Institute of Medicine and the National Research Council in 2009. On this site they live in a versioned data pack alongside the report's URL and the date the row was last checked, rather than being typed into the code - so if the guidance changes, the change is dated and visible in the changelog.
The report also publishes weekly rates for the second and third trimesters, and states that its rate calculations assume a first-trimester gain of 0.5 to 2 kg (1.1 to 4.4 lb). That is how the calculator builds a cumulative range for a given gestational week: the stated first-trimester allowance, plus the weekly rate for each week after 13, capped at the published total.
For twin pregnancies the 2009 report gives provisional total ranges - 16.8 to 24.5 kg (37 to 54 lb) for a healthy pre-pregnancy BMI, 14.1 to 22.7 kg for overweight and 11.3 to 19.1 kg for obese - and no range at all for a twin pregnancy beginning underweight, because the evidence was judged insufficient. This site reports that gap as a gap. The report publishes no weekly rate for twins either, so the tool returns the total and no weekly trajectory rather than inventing one.
What the tool does with your number is show where it sits relative to the range for the week you entered, and stop. The range belongs to the IOM and the reading belongs to a clinician. There is no scoring and no alarm colour anywhere near a body metric on this site, which is a deliberate design rule.
Run your own numbers with the Pregnancy weight gain calculator
Free, instant, no sign-up required.