How to use this calculator
- Enter or choose units.
- Enter or choose week of pregnancy.
- Enter or choose pregnancy.
- The recommended total gain updates as you type. Use Copy result or Share to keep it.
Method
BMI = pre-pregnancy kg ÷ m²; by week w ≤ 13: first-trimester gain × w ÷ 13; after 13 weeks: first-trimester gain + weekly rate × (w − 13)
- first-trimester gain
- 0.5–2 kg (1.1–4.4 lb)
- weekly rate
- IOM range for your BMI group, e.g. 0.35–0.50 kg a week for normal weight
Worked example
- Pre-pregnancy BMI = 60 ÷ 1.65² = 22.0: normal weight, so the IOM total is 11.5–16 kg.
- By week 24 (11 weeks after week 13): 0.5 + 0.35 × 11 = 4.4 kg to 2 + 0.50 × 11 = 7.5 kg.
- Gained so far: 66 − 60 = 6 kg, which is within that range.
With a pre-pregnancy BMI of 22.0 the recommended total gain is 11.5–16 kg; 6 kg at 24 weeks is within the typical range.
What the result means
The total range is the IOM recommendation. The week-by-week figure is a guide built from the IOM’s assumed first-trimester gain and weekly rates, so small differences from it are not a concern. Your midwife or doctor will interpret your gain alongside your baby’s growth and your health.
Assumptions and limits
- IOM (2009) totals: underweight 12.5–18 kg (28–40 lb), normal 11.5–16 kg (25–35 lb), overweight 7–11.5 kg (15–25 lb), obese 5–9 kg (11–20 lb).
- Second- and third-trimester weekly ranges: 0.44–0.58, 0.35–0.50, 0.23–0.33 and 0.17–0.27 kg (1.0–1.3, 0.8–1.0, 0.5–0.7, 0.4–0.6 lb).
- Provisional twin totals: normal 16.8–24.5 kg (37–54 lb), overweight 14.1–22.7 kg (31–50 lb), obese 11.5–19.1 kg (25–42 lb).
- No IOM twin guideline exists for a pre-pregnancy BMI under 18.5, and no week-by-week twin ranges are given.
- Not for pregnancies with three or more babies.
- Does not replace advice from your maternity care team.
Questions
Why is less gain recommended with a higher BMI?
The guidelines balance risks to mother and baby; with more stored energy before pregnancy, a smaller gain is associated with better outcomes.
I am losing weight in the first trimester because of sickness. Is that a problem?
Small losses with nausea are common early on, but tell your midwife or doctor, especially if you cannot keep fluids down.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.