Pregnancy Weight Gain Calculator — IOM Guidelines

Weight gain is considered excessive when it exceeds the Institute of Medicine (IOM 2009) total upper bound for your pre-pregnancy BMI — for example more than 16 kg (35.3 lb) for a normal-BMI singleton pregnancy — or when your trajectory is running so far above the expected-for-age band that the projected 40-week total overshoots it. Example: a normal-BMI woman who has gained 11 kg at 28 weeks is above the expected 5.75–9.5 kg band; her observed pace of 0.65 kg/week projects to about 18.8 kg at 40 weeks, which is excessive.

It depends on your pre-pregnancy BMI. For a normal BMI (18.5–24.9), the IOM-based expected range at 28 weeks is 5.75–9.5 kg (midpoint about 7.6 kg), built from the assumed 0.5–2 kg first-trimester gain plus the recommended 0.35–0.50 kg/week rate. For instance, 8 kg at 28 weeks is within the expected range. The calculator shows the exact band for your BMI and week.

BMI = weight (kg) ÷ height (m)². Your category sets your targets: underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30. Example: 165 cm and 60 kg gives BMI = 60 ÷ 1.65² = 22.0, which is the normal-weight category with an IOM total target of 11.5–16 kg.

Per the IOM 2009 guidelines, mean (range) rates for the 2nd and 3rd trimesters are: underweight 0.51 (0.44–0.58) kg/week, normal weight 0.42 (0.35–0.50) kg/week, overweight 0.28 (0.23–0.33) kg/week, and obese 0.22 (0.17–0.27) kg/week. These assume a 0.5–2 kg gain in the first trimester.

IOM 2009 singleton totals: underweight 12.5–18 kg (27.6–39.7 lb), normal weight 11.5–16 kg (25.4–35.3 lb), overweight 7–11.5 kg (15.4–25.4 lb), obese 5–9 kg (11.0–19.8 lb).

The IOM gives only provisional total ranges for twins: normal BMI 16.8–24.5 kg, overweight 14.1–22.7 kg, obese 11.3–19.1 kg. There was insufficient data to set a range for underweight women carrying twins. Because the IOM publishes no weekly twin rates, this tool shows a clearly labeled linear on-track band — e.g. at 28 weeks for normal BMI the band is about 11.8–17.2 kg, so 14 kg is within it.

A result of below or above the expected band means your gain is outside the trajectory associated with the best outcomes in the IOM's evidence review — below-range gain is linked with higher risk of low birth weight, above-range gain with higher risk of a large baby, cesarean delivery, and postpartum weight retention. Either way, discuss it with your midwife or doctor; this tool is an estimate, not a diagnosis.

The IOM calculations assume a total first-trimester gain of 0.5–2 kg (1.1–4.4 lb). Most of the recommended gain happens in the 2nd and 3rd trimesters, when the weekly rates above apply.

Give your current week and gain and the planner projects your 40-week total under two scenarios: continuing at the IOM mean weekly rate for your BMI, and continuing at your own observed 2nd/3rd-trimester rate. Example: normal BMI, 8 kg at 28 weeks — the IOM-rate projection is about 13.0 kg, inside the 11.5–16 kg target.

No. It is an educational estimate based on the IOM 2009 guidelines, not professional medical advice, diagnosis, or a treatment recommendation. Individual needs vary — always follow your own healthcare provider's guidance for your pregnancy.