Pregnancy Weight Gain Calculator
Healthy weight gain during pregnancy is essential for fetal development and maternal well-being. This calculator uses the Institute of Medicine (IOM) 2009 guidelines, which set recommended total weight gain ranges based on your pre-pregnancy Body Mass Index (BMI): 28–40 lbs for underweight women, 25–35 lbs for normal weight, 15–25 lbs for overweight, and 11–20 lbs for obese women carrying a single baby.
Beyond the total target, knowing whether your week-by-week gain is on track matters just as much. The IOM provides expected weekly rates for the second and third trimesters—typically 0.35–0.50 kg/week for normal-weight women—and a smaller first-trimester gain of roughly 0.5–2 kg. This calculator combines both dimensions to tell you if your current gain is within, below, or above the expected range for your gestational week.
How it works
Pre-pregnancy BMI = weight (kg) ÷ height² (m). IOM category thresholds: Underweight < 18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥ 30. Expected gain at week w: for w ≤ 13, linear interpolation of first-trimester target (0.5–2 kg by category); for w > 13, first-trimester gain + (w − 13) × weekly rate, capped at total recommended range.
Use cases
- Tracking gestational weight gain against IOM benchmarks at every prenatal visit
- Understanding how pre-pregnancy BMI changes the recommended total weight gain target
- Identifying whether current weight gain is below, on track, or above the expected range for the gestational week
- Planning nutrition and activity goals in consultation with an OB-GYN or midwife
- Evaluating weight gain progress after dietary or lifestyle changes during pregnancy