A normal pre-pregnancy BMI, at 20 weeks
A 165 cm, 62 kg pre-pregnancy weight (BMI 22.8, normal) targets a total gain of 11.5–16 kg, with roughly 4.1–5.1 kg expected by week 20.
Pregnancy weight gain calculator
This is an estimate for information only. It does not replace the personalized guidance of a healthcare professional monitoring your pregnancy.
Result
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How it works
The Institute of Medicine sets a total recommended gain range by pre-pregnancy BMI category, plus a suggested weekly rate for the second and third trimesters (first-trimester gain is typically much smaller and less linear). This calculator applies both: the overall target range, and where you should roughly be by your current week.
A twin pregnancy is given a higher target range, since the guidelines differ meaningfully from a single pregnancy.
Formulas used
IOM total gain range (singleton)
Underweight 12.5–18 kg · Normal 11.5–16 kg · Overweight 7–11.5 kg · Obese 5–9 kg
Worked examples
Checking an actual gain against the range
The same profile now weighing 68 kg at week 20 has gained 6 kg — above the expected range for that week, worth discussing at a prenatal visit.
Assumptions and limits
- BMI category is based on pre-pregnancy height and weight.
- The weekly rate applies only from the second trimester onward; first-trimester gain uses a smaller, simplified estimate.
- Guidelines are population averages — a healthy individual pregnancy can reasonably fall outside them.
Frequently asked questions
Why does a lower starting BMI mean a higher target range?
The guidelines aim for a similar healthy total outcome for mother and baby; someone starting with less reserve is guided to build more during pregnancy, while someone starting with more reserve needs to add less.
Why is first-trimester gain treated differently?
Weight gain in the first trimester is typically small (often just 1–2 kg total) and not very linear, since morning sickness and appetite changes vary a lot — the weekly-rate guideline applies mainly to the second and third trimesters.
What if my gain is outside the expected range?
Being outside the range for one week is not necessarily a concern on its own — bring it up at your next prenatal visit, since your care provider can assess it alongside the rest of your health picture.
Does this apply the same way to a twin pregnancy?
The total range is higher for twins, reflecting the different, less-studied guidelines for multiples — the weekly expected-gain figure here is a simplified scaling, not a separate twin-specific curve.
Updated
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