Pregnancy Weight Gain by BMI Calculator
See your recommended total pregnancy weight-gain range by pre-pregnancy BMI, plus how much you should have gained by your current week (IOM guidelines).
How it works
BMI = weight ÷ height² · IOM total gain: underweight 12.5–18, normal 11.5–16, overweight 7–11.5, obese 5–9 kg · By-week target = first-trimester 0.5–2 kg + weekly rate × (week − 13)
The Institute of Medicine ties recommended pregnancy weight gain to pre-pregnancy BMI: a woman starting underweight should gain more, one starting with obesity less, to balance the baby's growth against maternal and delivery risks. Most gain happens in the second and third trimesters at a steady weekly rate, after only about 0.5–2 kg in the first trimester. Individual guidance from a maternity provider takes precedence, especially with twins or medical conditions.
Worked example
At 165 cm and 60 kg pre-pregnancy, BMI is 22.0 (normal), so the target is 11.5–16 kg total. By week 20, the recommended range is about 2 + 0.35×7 to 2 + 0.50×7 ≈ 4.5–5.5 kg; a 5 kg gain (65 kg now) is on track.
Frequently asked questions
What if I am carrying twins?
Ranges are higher for multiples (for example about 17–25 kg at a normal BMI) and this single-baby tool does not apply. Follow your maternity provider's guidance.
Is gaining too little safer?
Not necessarily — too little gain raises the risk of a low-birth-weight baby, while too much raises risks of a large baby, gestational diabetes and delivery complications. The range aims for balance.
Why so little gain in the first trimester?
Early pregnancy adds little fetal mass, and nausea often limits intake. Most healthy gain occurs from the second trimester onward at a steady weekly pace.
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Educational estimate only, not medical advice. Cycles, nutrient needs and risk vary between people — discuss decisions with a qualified clinician.