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, now the National Academy of Medicine) set these targets in 2009 from pre-pregnancy BMI, and ACOG (the American College of Obstetricians and Gynecologists) still points to them. 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 |
|---|---|
| Underweight | No IOM guideline exists |
| Normal weight | 37–54 lb (16.8–24.5 kg) |
| Overweight | 31–50 lb (14.1–22.7 kg) |
| Obese | 25–42 lb (11.3–19.1 kg) |
The blank row is not an oversight. The IOM had too few twin pregnancies with an underweight starting BMI to derive a range, and said so rather than guessing. If that is your situation, the singleton figure is definitely too low and the number has to come from your provider.
The twin ranges are provisional, taken from the middle half of the gains in twin pregnancies that reached 37 weeks or more. The IOM published them in pounds, so the kilograms here are converted. Some charts print 37–55 lb, which comes from converting rounded kilograms back the other way.
Rate of gain
Gain is not meant to be linear. Most women gain only 0.5–2 kg (1–4 lb) across the entire first trimester, and almost all of it comes in the second and third. The IOM publishes a weekly rate for those two trimesters, and the calculator prints the one for your category:
| Pre-Pregnancy BMI | Weekly gain after week 13 |
|---|---|
| Underweight | 0.44–0.58 kg (1–1.3 lb) |
| Normal weight | 0.35–0.50 kg (0.8–1 lb) |
| Overweight | 0.23–0.33 kg (0.5–0.7 lb) |
| Obese | 0.17–0.27 kg (0.4–0.6 lb) |
There is no published weekly rate for twins.
Use pre-pregnancy weight
This matters enough to repeat. The IOM categories are defined on pre-pregnancy body composition, so entering your current weight partway through a pregnancy pushes you into a higher BMI band and produces a lower target than you actually need. If you do not know your pre-pregnancy weight, your first prenatal appointment record usually has it.
Individual health factors modify these guidelines, and your obstetrician or midwife may set something different for good reason.
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.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.
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