Pregnancy Weight Gain Calculator
Calculate recommended pregnancy weight gain from pre-pregnancy BMI and current week.
Returns total range and weekly targets per ACOG guidelines.
Pregnancy weight gain guidelines are based on pre-pregnancy Body Mass Index (BMI), as established by the Institute of Medicine (IOM) in their landmark 2009 guidelines. Weight gain during pregnancy supports the growing baby, placenta, amniotic fluid, and necessary changes in the mother’s body.
Pre-pregnancy BMI determines the recommended range:
BMI = Weight (kg) / Height (m)²
Or in imperial: BMI = (Weight lbs / Height inches²) × 703
IOM recommended total weight gain by pre-pregnancy BMI:
| BMI Category | BMI Range | Single Baby | Twins |
|---|---|---|---|
| Underweight | < 18.5 | 28–40 lbs (12.5–18 kg) | Consult doctor |
| Normal weight | 18.5–24.9 | 25–35 lbs (11.5–16 kg) | 37–54 lbs (17–25 kg) |
| Overweight | 25.0–29.9 | 15–25 lbs (7–11.5 kg) | 31–50 lbs (14–23 kg) |
| Obese (Class I+) | ≥ 30.0 | 11–20 lbs (5–9 kg) | 25–42 lbs (11–19 kg) |
Typical weight gain pattern by trimester:
| Trimester | Weeks | Expected Gain |
|---|---|---|
| First | 1–12 | 1–4 lbs total (0.5–2 kg) |
| Second | 13–26 | ~1 lb/week (0.45 kg/week) |
| Third | 27–40 | ~1 lb/week (0.45 kg/week) |
Where the pregnancy weight goes (at term, for a normal-weight woman gaining 30 lbs):
| Component | Amount |
|---|---|
| Baby | 7–8 lbs (3.2–3.6 kg) |
| Placenta | 1.5 lbs (0.7 kg) |
| Amniotic fluid | 2 lbs (0.9 kg) |
| Uterus (growth) | 2 lbs (0.9 kg) |
| Breast tissue (growth) | 2 lbs (0.9 kg) |
| Blood volume increase | 4 lbs (1.8 kg) |
| Body fluid increase | 4 lbs (1.8 kg) |
| Maternal fat stores | 6–8 lbs (2.7–3.6 kg) |
Worked example: Pre-pregnancy weight: 145 lbs, Height: 5'5" BMI = (145 / 65²) × 703 = (145 / 4225) × 703 = 24.1 → Normal weight category Recommended gain: 25–35 lbs At 20 weeks with 8 lbs gained: on track (approximately 1 lb/week from week 13 onward is expected).
When weight gain is outside the range:
- Too little weight gain is associated with premature birth and low birth weight
- Too much weight gain is associated with gestational diabetes, C-section delivery, and heavier birth weight
- Both extremes increase risks — which is why monitoring matters throughout all three trimesters
Important: These are population-level guidelines. Every pregnancy is individual. Always follow your OB, midwife, or healthcare provider’s personalized recommendations for your specific situation.