BMI in Pregnancy Calculator — Weight Gain Guide

Formula

How this is calculated

The Institute of Medicine (IOM, now the National Academy of Medicine) published its landmark 2009 gestational weight gain guidelines by stratifying recommended gain ranges according to pre-pregnancy BMI. The logic is that women with lower pre-pregnancy weight need more gain to support fetal growth without drawing excessively on maternal reserves, while women with obesity already have more energy stores and should gain less to minimise complications such as gestational diabetes, pre-eclampsia and caesarean delivery.

The calculator first computes pre-pregnancy BMI (kg/m²) and assigns it to one of four IOM categories. For singletons, the ranges are: underweight (BMI < 18.5) → 12.5–18 kg; normal (18.5–24.9) → 11.5–16 kg; overweight (25–29.9) → 7–11.5 kg; obese (≥ 30) → 5–9 kg. For twin pregnancies the IOM 2009 addendum specifies higher ranges. If you enter a current weight and gestational week, the tool also estimates whether your gain is on track using the IOM weekly rate targets for the second and third trimesters (after the first trimester, where about 1–2 kg total gain is typical regardless of category).

This is a screening and education tool only. Actual weight-gain recommendations must account for pre-existing conditions, fetal growth, nausea, multiple pregnancies not covered by these tables, and clinical judgement. Always discuss weight goals with your midwife or obstetrician.