Pregnancy Weight Gain Calculator
The recommended weight-gain range for your stage of pregnancy, based on your weight before it began — with the weekly rate and where your current weight sits.
The ranges come from the 2009 Institute of Medicine (now the National Academy of Medicine) guidelines, which are used internationally and set the total gain by pre-pregnancy BMI: less for those starting heavier, more for those starting lighter.
How the pregnancy weight gain calculator works
The ranges come from the 2009 Institute of Medicine (now the National Academy of Medicine) guidelines, which are used internationally and set the total gain by pre-pregnancy BMI: less for those starting heavier, more for those starting lighter. Gain is slow in the first trimester — about half a kilo to two kilos in total — then steadier through the second and third at a weekly rate that also depends on starting BMI. Twins have their own, much higher ranges.
Formula: expected gain = first-trimester allowance + weekly rate × weeks since week 13
Worked examples
| Inputs | Recommended gain by now | Note |
|---|---|---|
| BMI 21 at 20 weeks | 2.9–5.5 kg | healthy-weight range, about 3–5.5 kg by now |
| Starting overweight, 30 weeks | 3.4–6.6 kg | a smaller recommended gain |
| Twins at 28 weeks | 10.1–14.8 kg | a much higher range |
FAQFrequently asked questions
Where do these ranges come from?
The 2009 Institute of Medicine (now National Academy of Medicine) guidelines, which set total gain by pre-pregnancy BMI and are used internationally.
Why does my starting BMI change the target?
Because the evidence links both very low and very high gain to poorer outcomes, and the healthiest amount differs depending on where you started.
Is gaining outside the range a problem?
Not automatically. It is a signal to discuss with your midwife or doctor, who will look at your whole history rather than a single number.
Why is so little gain expected early on?
The baby is tiny for the first three months; most of the gain is later, and much of it is blood volume, fluid, placenta and the baby itself rather than fat.
Do these apply to twins?
The twin ranges are included and are considerably higher. Triplets have no formal guideline and need individual advice.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Centers for Disease Control and Prevention — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.