Pregnancy Weight Gain Calculator
Pregnancy Weight Gain Calculator
Healthy weight gain during pregnancy supports fetal growth, placenta development, and maternal reserves for breastfeeding. Gaining too little or too much can affect pregnancy outcomes. This calculator uses IOM (Institute of Medicine) guidelines and is for informational purposes only — always work with your healthcare provider to set individual targets.
Conversion Formula
Pre-pregnancy BMI is calculated from height and weight (kg/m2). The IOM guideline range is selected based on BMI category. For weekly estimates: first trimester assumes 1-4 lbs total; thereafter, remaining gain is distributed linearly across weeks 14-40.
Step-by-Step Examples
140 lbs, 65 in, Week 20, imperial = Normal weight (BMI 23.3); Recommended: 25-35 lbs; By week 20: ~11 lbs
BMI = (140 × 0.453592) / (65 × 0.0254)^2 = 63.5 / (1.651)^2 = 63.5 / 2.726 = 23.3. Normal range = 25-35 lbs. Week 20: 2.5 + ((30-2.5)/27 × 7) = 2.5 + 7.1 = ~10 lbs by week 20.
200 lbs, 66 in, Week 30, imperial = Obese (BMI 32.3); Recommended: 11-20 lbs; By week 30: ~12 lbs
BMI = (200 × 0.453592) / (66 × 0.0254)^2 = 90.7 / (1.676)^2 = 90.7 / 2.81 = 32.3. Obese range = 11-20 lbs mid = 15.5. Week 30: 2.5 + ((15.5-2.5)/27 × 17) = 2.5 + 8.2 = ~11 lbs.
Frequently Asked Questions
How much weight should I gain during pregnancy?
The Institute of Medicine (IOM) recommendations depend on pre-pregnancy BMI: underweight (BMI under 18.5) should gain 28-40 lbs, normal weight (18.5-24.9) should gain 25-35 lbs, overweight (25-29.9) should gain 15-25 lbs, and obese (30+) should gain 11-20 lbs. These are for single pregnancies; twins require more.
Does pre-pregnancy BMI affect recommended weight gain?
Yes, significantly. A normal-weight woman (BMI 22) aims for 25-35 lbs total, while an obese woman (BMI 35) is advised to gain only 11-20 lbs. This is because excess pre-existing fat stores can supply additional energy for fetal development. Gaining too much increases risks of gestational diabetes, C-section, and large-for-gestational-age babies.
How much weight gain is expected week by week?
In the first trimester (weeks 1-13), most women gain only 1-4 lbs total. In the second and third trimesters, normal-weight women typically gain about 1 lb per week. The weekly rate varies by BMI category — underweight women may gain slightly more per week, overweight and obese women slightly less.