The approach
This calculator applies the Institute of Medicine (IOM) guidelines to estimate how much weight you should gain during pregnancy based on your pre-pregnancy BMI category. It calculates your pre-pregnancy BMI from height and weight, assigns you to a category, and shows the total recommended gain range and your progress toward that target at your current week of pregnancy.
This is an estimate, not professional medical advice. Always consult your healthcare provider about your individual pregnancy weight gain goals.
The formula
Pre-pregnancy BMI = weight (kg) / [height (m)]² → IOM category → Total gain range → Weekly average gain
The calculator first determines your BMI category:
- Underweight: BMI < 18.5 → 28–40 lb (12.7–18.1 kg) total gain
- Normal weight: BMI 18.5–24.9 → 25–35 lb (11.3–15.9 kg) total gain
- Overweight: BMI 25–29.9 → 15–25 lb (6.8–11.3 kg) total gain
- Obese: BMI ≥ 30 → 11–20 lb (5–9.1 kg) total gain
Once your category is assigned, the calculator divides the recommended range by your current week of pregnancy to show average weekly gain and your expected weight at that gestational age.
Worked example
Suppose you are 5'6" (167.6 cm) tall, weighed 145 lb (65.8 kg) before pregnancy, and are now at 20 weeks.
Step 1: Calculate pre-pregnancy BMI
- Height in meters: 5'6" = 1.676 m
- BMI = 65.8 ÷ (1.676)² = 65.8 ÷ 2.81 = 23.4
- Category: Normal weight (18.5–24.9)
Step 2: Identify recommended range
- Normal weight = 25–35 lb total gain
- Midpoint = 30 lb
Step 3: Calculate weekly average
- 30 lb ÷ 40 weeks (full term) ≈ 0.75 lb/week
- At 20 weeks: 0.75 × 20 = 15 lb expected gain
- Expected current weight: 145 + 15 = 160 lb
Step 4: Assess progress
- If you currently weigh 158 lb, you're 1 lb below the midpoint average—right on track
- You should aim for 25–35 lb by delivery (170–180 lb)
Common mistakes
A frequent error is comparing your gain to someone else's. Pregnancy weight gain is highly individual and depends on your starting BMI, metabolism, how the baby is growing, and fluid retention. Underweight people may need 40 lb; obese people may need only 11 lb—both are healthy if they follow their category.
Another mistake is gaining all weight early. Most guidelines expect minimal gain in the first trimester (1–4 lb) and then steady gain later. Rapid early gain may signal fluid retention or other factors worth discussing with your provider.
Finally, don't assume you must hit the exact midpoint. The IOM ranges are wide because individual needs vary. Your healthcare provider may adjust targets if your baby is measuring large or small, or if you have gestational diabetes or other conditions. Regular prenatal check-ins are more important than hitting a specific number.