Pregnancy BMI Calculator
Calculate pre-pregnancy BMI and get the IOM-recommended gestational weight gain range for underweight, normal, overweight, and obese starting BMI categories.
Gaining the right amount of weight during pregnancy is important for both mother and baby. Too little weight gain is linked to low birth weight and preterm birth. Too much weight gain increases the risk of gestational diabetes, cesarean delivery, and long-term weight retention.
How BMI Determines Weight Gain Guidelines
The Institute of Medicine (IOM) and most global health authorities use pre-pregnancy BMI to set personalized weight gain targets. BMI is calculated as:
BMI = weight (kg) ÷ height (m)²
Or in imperial: BMI = (weight in lbs × 703) ÷ height in inches²
IOM Recommended Gestational Weight Gain (Singleton Pregnancy)
| Pre-Pregnancy BMI | Category | Recommended Total Gain |
|---|---|---|
| Below 18.5 | Underweight | 12.5–18 kg (28–40 lbs) |
| 18.5–24.9 | Normal weight | 11.5–16 kg (25–35 lbs) |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lbs) |
| 30.0 and above | Obese | 5–9 kg (11–20 lbs) |
For Twin Pregnancies:
| Pre-Pregnancy BMI | Recommended Total Gain |
|---|---|
| Normal weight | 17–25 kg (37–54 lbs) |
| Overweight | 14–23 kg (31–50 lbs) |
| Obese | 11–19 kg (25–42 lbs) |
Rate of Gain
Weight gain should not be linear throughout pregnancy. In the first trimester, most women gain only 0.5–2 kg (1–4 lbs) total. In the second and third trimesters, the recommended rate is roughly 0.5 kg (1 lb) per week for normal-weight women, less for overweight or obese women.
Important Note
This calculator uses pre-pregnancy weight, not current pregnancy weight, because BMI categories are based on your pre-pregnancy body composition. Always discuss your specific goals with your obstetrician or midwife, as individual health factors may modify these guidelines.
How we build and check this calculator
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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