Pregnancy
Pregnancy weight gain
The recommended range depends entirely on where your BMI started — it runs from 5 kg to 18 kg across the categories. Gaining under the range is linked to restricted fetal growth; over it, to a larger baby and a higher chance of caesarean.
Result
Breakdown
Worth knowing
- Gain of 5 kg is within the expected 3-5.5 kg for week 20.
- Weekly rates apply to the second and third trimesters. First-trimester gain is typically 0.5-2 kg in total.
- These are population guidelines — your care provider's target for you takes precedence.
How this is calculated
Ranges are the Institute of Medicine's 2009 guidelines, endorsed by ACOG, keyed to pre-pregnancy BMI: 12.5–18 kg underweight, 11.5–16 kg normal, 7–11.5 kg overweight, 5–9 kg obese. First-trimester gain is treated as 0.5–2 kg in total, with the weekly rate applying after week 13.
Common questions
Why does the range depend on my starting BMI?
The IOM's 2009 guidelines are stratified by pre-pregnancy BMI because the evidence on outcomes differs by starting point. The recommended total runs from 12.5-18 kg for underweight down to 5-9 kg for obese.
I'm gaining faster than the range. Should I worry?
It is worth raising at your next appointment rather than acting on alone. These are population guidelines, and week-to-week weight moves with fluid and timing. Your care provider has context — how you started, how the baby is measuring — that no calculator does.
Why is first-trimester gain so small?
Most of the recommended gain happens after week 13. The first trimester typically accounts for only 0.5-2 kg in total, and many people gain nothing at all if nausea is significant.
Are the twin ranges as reliable?
No. The IOM issued them as provisional guidance on weaker evidence, and no range was issued for underweight BMI at all.