How the recommended range is determined
The Institute of Medicine (now the National Academy of Medicine) re-examined US pregnancy weight-gain guidelines in 2009 and published recommended total-gain ranges by pre-pregnancy BMI category: underweight (BMI <18.5) 12.5–18 kg (28–40 lb); normal weight (18.5–24.9) 11.5–16 kg (25–35 lb); overweight (25.0–29.9) 7–11.5 kg (15–25 lb); obese (≥30) 5–9 kg (11–20 lb). Provisional ranges for twin pregnancies are higher and apply only to normal-weight, overweight, and obese mothers — there is insufficient evidence for an underweight twin range. The 2nd- and 3rd-trimester weekly-gain ranges come from the same report. These are population-level guidelines and not individualised targets.
How to use it
Enter your weight and height as they were just before you became pregnant — these determine which IOM BMI tier applies. Choose whether you are carrying a single baby or twins. Optionally enter your current week of pregnancy to see the IOM weekly-gain range for that trimester. We display only the recommended range, never a single target number, and we never estimate your current weight gain. For your individual plan, follow your obstetrician or midwife's guidance.
Frequently asked questions
- How was this calculated?
- Your pre-pregnancy BMI is calculated as kg ÷ m², then matched to the IOM 2009 weight-gain table for singleton or twin pregnancies. We display the published range verbatim — there is no estimation or modelling on top of it.
- Should I follow this if my doctor says different?
- Follow your provider. The IOM 2009 ranges are population averages for the general US population. Your obstetrician or midwife knows your individual history, medical conditions, and pregnancy course, and may recommend a different target. This tool is educational only.
- What about twins?
- IOM 2009 publishes provisional ranges for twin pregnancies in normal-weight, overweight, and obese mothers. For underweight mothers carrying twins, IOM explicitly states there is insufficient data — we show no range in that case, only a prompt to consult your provider.
- Why are the recommended ranges different by BMI?
- The IOM committee reviewed evidence linking gestational weight gain to outcomes including birth weight, postpartum weight retention, gestational diabetes, and pre-eclampsia. The recommended ranges narrow as pre-pregnancy BMI rises because larger gains in higher-BMI mothers are associated with worse outcomes for both mother and baby. The ranges are a balance across these outcomes — not an ideal weight.
- Does this work for triplets or higher-order multiples?
- No. IOM 2009 does not publish ranges for triplets or higher. Care plans for higher-order multiples are highly individual and must be set by your obstetrician.
References
- Institute of Medicine (US) and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington (DC): National Academies Press; 2009.
- ACOG Committee Opinion No. 548: Weight Gain During Pregnancy (2013, reaffirmed). American College of Obstetricians and Gynecologists.
- CDC — Weight Gain During Pregnancy (based on IOM 2009).