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
Frequently asked questions
How much weight should I gain during pregnancy?
The IOM 2009 guidelines base the target on your pre-pregnancy BMI for a single baby: 28–40 lbs (12.5–18 kg) if underweight (BMI < 18.5), 25–35 lbs (11.5–16 kg) for normal weight (18.5–24.9), 15–25 lbs (7–11.5 kg) if overweight (25–29.9), and 11–20 lbs (5–9 kg) if obese (BMI ≥ 30). Enter your pre-pregnancy weight and height and the calculator applies the right range automatically.
How much weight should I gain per week while pregnant?
First-trimester gain is small — roughly 0.5–2 kg (1–4.4 lbs) in total by week 13. From the second trimester onward the IOM expects a steady weekly rate, typically 0.35–0.50 kg (about 0.8–1 lb) per week for normal-weight women, with lower rates for overweight and obese categories. The calculator interpolates these rates to show the expected range for your exact gestational week.
How is pre-pregnancy BMI calculated?
BMI = weight in kilograms divided by height in metres squared, using your weight before conception. The IOM categories are: underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, and obese 30 or above. Your category determines which total and weekly weight gain range applies to your pregnancy.
What should I do if I am gaining too much or too little weight in pregnancy?
Being outside the expected range for a single visit is not automatically a problem — weight gain is rarely perfectly linear. What matters is the trend across several weeks, since consistently low gain is associated with fetal growth restriction and consistently high gain with gestational diabetes and delivery complications. Always discuss your individual curve, nutrition, and activity plan with your OB-GYN or midwife rather than adjusting your diet on your own.
Are the weight gain recommendations different for twins?
Yes. For twin pregnancies the IOM 2009 provisional guidelines recommend higher totals: 37–54 lbs (17–25 kg) for normal-weight women, 31–50 lbs (14–23 kg) for overweight, and 25–42 lbs (11–19 kg) for obese women. This calculator uses the singleton ranges, so it should not be used to track a multiple pregnancy.