Pre-Pregnancy BMI & Weight Gain Calculator
Calculate pre-pregnancy BMI and IOM-recommended gestational weight gain range by category: underweight, normal, overweight, and obese in lbs and kg.
Pregnancy BMI and weight gain guidance uses pre-pregnancy BMI to set individualized total gestational weight gain targets. Weight gain recommendations differ significantly based on starting BMI — underweight women need to gain more; obese women should gain less. These targets are established by the Institute of Medicine (IOM) and endorsed by major obstetric organizations.
Pre-pregnancy BMI formula: BMI = Weight (kg) ÷ Height (m)²
Recommended total gestational weight gain (IOM guidelines):
| Pre-pregnancy BMI | Weight Category | Recommended Gain (singleton) |
|---|---|---|
| < 18.5 | Underweight | 28–40 lbs (12.5–18 kg) |
| 18.5–24.9 | Normal weight | 25–35 lbs (11.5–16 kg) |
| 25.0–29.9 | Overweight | 15–25 lbs (7–11.5 kg) |
| ≥ 30.0 | Obese | 11–20 lbs (5–9 kg) |
For twin pregnancies, add approximately 10 lbs to each category.
Trimester weight gain distribution:
- First trimester (weeks 1–13): 1–4.5 lbs total (0.5–1 kg) — minimal gain expected
- Second trimester (weeks 14–27): 1 lb per week average
- Third trimester (weeks 28–40): 1 lb per week average
Where pregnancy weight goes (typical breakdown for 30 lb gain):
- Baby: 7–8 lbs
- Placenta: 1.5 lbs
- Amniotic fluid: 2 lbs
- Uterine enlargement: 2 lbs
- Breast tissue: 2 lbs
- Blood volume increase: 4 lbs
- Fluid retention: 4 lbs
- Maternal fat stores: 7–8 lbs
Worked example: Pre-pregnancy weight: 145 lbs (65.9 kg). Height: 5'5" (165 cm = 1.65 m).
- Pre-pregnancy BMI: 65.9 ÷ (1.65)² = 65.9 ÷ 2.7225 = 24.2 — Normal weight
- Recommended gain: 25–35 lbs
- Target weight at delivery: 145 + 25 = 170 lbs (minimum) to 145 + 35 = 180 lbs (maximum)
- Weekly gain target (2nd and 3rd trimester): approximately 1 lb/week is on track for the middle of the recommended range.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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