Pregnancy Weight Gain Calculator
Estimate a recommended pregnancy weight-gain reference range from your pre-pregnancy BMI, gestational week, and singleton or twin pregnancy, based on the IOM/National Academies 2009 guidelines.
How It's Calculated
Formula
\text{Singleton, week} \le 13: \text{cumulative} = \dfrac{\text{week}}{13} \times \text{first-trimester range (0.5–2 kg)}
\text{Singleton, week} > 13: \text{cumulative} = \text{first-trimester range} + (\text{week} - 13) \times \text{mean weekly rate}This calculator reports a reference range for pregnancy weight gain, based on the Institute of Medicine's (National Academies) 2009 guidelines — the same source used by the CDC and ACOG. Your pre-pregnancy BMI is classified into one of four categories using the CDC adult BMI cutoffs (Underweight <18.5, Normal 18.5–24.9, Overweight 25.0–29.9, Obese ≥30.0) — the same cutoffs this site's BMI Calculator uses. For SINGLETON pregnancies, the calculator reports a low/high cumulative reference band for the gestational week you entered: for the first 13 weeks, it's a proportional share of the IOM's stated 0.5–2 kg first-trimester range; after week 13, it adds your BMI category's mean second/third-trimester weekly rate for each additional week. For TWIN pregnancies, the IOM report only publishes a TOTAL recommended gain range by delivery — it does not publish a twin-specific first-trimester allowance or weekly rate, so this calculator shows only that total range rather than inventing a week-by-week curve for twins. Twin pregnancies with an underweight pre-pregnancy BMI are not covered: the IOM report states there wasn't enough data to make a recommendation for that combination. This is an estimate for general informational and educational purposes, based on a published formula — it is not a medical diagnosis, and it is not a substitute for individualized advice from a qualified healthcare provider. These are population-level reference ranges, not an individualized prenatal care plan — follow your own healthcare provider's guidance, especially if you have a multiple-gestation pregnancy beyond twins or a pre-existing condition.
Worked Examples
Normal weight (BMI 22), week 20, singleton
- BMI 22 falls in the Normal weight category (18.5-24.9): total range 11.5-16 kg, mean rate 0.42 kg/week
- Week 20 is 7 weeks past the first-trimester boundary (week 13)
- Low: 0.5 + 7 × 0.42 = 0.5 + 2.94 = 3.44 kg
- High: 2.0 + 7 × 0.42 = 2.0 + 2.94 = 4.94 kg
Normal weight (BMI 22), week 20, twin
- BMI 22 falls in the Normal weight category
- IOM twin table: 17-25 kg total recommended gain by delivery
- No week-by-week curve is shown — only this total range
Frequently Asked Questions
Why don't twin pregnancies get a week-by-week curve?
The IOM 2009 report only publishes a total recommended weight-gain range for twin pregnancies by delivery — it doesn't publish a twin-specific first-trimester allowance or weekly rate the way it does for singleton pregnancies. Rather than invent an unverified twin weekly rate, this calculator shows only the verified total range for twins.
Why is there no result for an underweight twin pregnancy?
The IOM report states there wasn't enough data to make a weight-gain recommendation for underweight pre-pregnancy BMI with twins, so this calculator does not show a fabricated figure for that combination.
Is the cumulative figure a 'required' amount I must gain?
No. It's a reference range derived from published population-level guidelines, not an individualized prescription. Your own healthcare provider's guidance should take priority, especially if you have any pregnancy complication.
Where do the BMI category cutoffs come from?
The same CDC adult BMI cutoffs (18.5 / 25.0 / 30.0) used by this site's BMI Calculator — verified directly against the IOM 2009 report, which uses the identical cutoffs for its pregnancy weight-gain categories.