How this calculator works
The lookup uses pre-pregnancy BMI and the IOM 2009 provisional guidance specifically for twins. The comparison table includes all four BMI bands and makes the missing underweight recommendation visible. Scalar results have a kg/lb selector; the table keeps the published kilogram ranges. The report based these provisional totals on a selected group of twin births at 37 to 42 weeks with average birth weight of at least 2,500 g. They are not individual predictions, weekly gain rules or targets for an earlier delivery. No weekly curve is invented from a term total. The result is an estimate for information only and is not medical advice.
Formula
BMI = pre-pregnancy kg / height in metres², or use a known pre-pregnancy BMI. Twin total gain: BMI 18.5 to below 25 -> 17 to 25 kg; 25 to below 30 -> 14 to 23 kg; 30 or higher -> 11 to 19 kg. Add each endpoint to pre-pregnancy weight for the corresponding end weight. BMI below 18.5 has no IOM 2009 range.
Worked example
At 200 cm and 100 kg before pregnancy, BMI is 25. The provisional twin total gain reference is 14 to 23 kg. Adding those values to 100 kg gives 114 to 123 kg. These are end-of-pregnancy reference weights, not a target at the current week.
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Frequently asked questions
Why does an underweight result show no numerical range?
IOM 2009 did not have enough evidence to recommend a twin gain range below BMI 18.5. Discuss a personalised plan with your maternity care team.
Can I divide the total by the number of pregnancy weeks?
That would create a constant weekly rule that the cited twin guidance does not provide. Weight gain changes through pregnancy and a term total does not establish a weekly target.
Are these ranges suitable for triplets?
No. This tool implements twin guidance only. Higher-order multiple pregnancies require individual assessment.