By Zencalcy Editorial TeamLast reviewed About us
Recommended weight gain in pregnancy depends on pre-pregnancy body mass index, because the range linked to better outcomes differs for underweight, healthy-weight, overweight and obese women. This calculator uses the Institute of Medicine ranges and the average second- and third-trimester rates to give both a total target and an approximate gain at your current week. It is a planning estimate, not a prescription.
IOM gestational weight-gain ranges by pre-pregnancy BMI.
BMI = weight(kg) ÷ height(m)² Gain by week ≈ weekly rate × max(0, weeks − 12)
The IOM total ranges are 12.5–18 kg for underweight, 11.5–16 kg for healthy weight, 7–11.5 kg for overweight and 5–9 kg for obese. Average second- and third-trimester rates are about 0.51, 0.42, 0.28 and 0.22 kg per week respectively. First-trimester gain is small and is treated as zero in the linear estimate from week 12. Twin pregnancies use different guidance.
| Symbol | Meaning |
|---|---|
| BMI | Pre-pregnancy body mass index in kg/m². |
| Total gain | Recommended total gestational weight gain in kilograms. |
| Weekly rate | Average gain per week in the second and third trimesters. |
BMI = 60 ÷ 1.65² = 22.0, which is healthy weight. The recommended total gain is 11.5–16 kg at about 0.42 kg per week after week 12. By week 28 the linear estimate is 0.42 × (28 − 12) = 6.7 kg. A woman at 160 cm and 80 kg has a BMI of 31.3, an obese range, with a 5–9 kg total target and about 4.0 kg estimated by week 30.
Weight gained in pregnancy is not simply fat. It includes the baby, placenta, amniotic fluid, increased blood volume, uterine and breast growth, and maternal energy stores. That is why the recommended ranges are broad and why a single weekly reading can mislead. Fluid retention, constipation and the timing of meals can move the scale by a kilogram or more. The IOM ranges are population guidance, and your maternity team may individualise them for twins, medical conditions or a very high or low starting BMI. The useful question is not whether one week matches a chart, but whether the overall trajectory supports the pregnancy. If your appetite, nausea or weight change worries you, raise it early rather than trying to correct it alone.
Continue with the Gestational Age Calculator, Conception Date Calculator, or the hCG Calculator — or browse more gynecology & pregnancy tools and all health calculators.
Also see the Ovulation Calculator and the Pregnancy Due Date Calculator.
Large studies link different gestational weight-gain ranges to better maternal and infant outcomes at different starting BMIs. Using pre-pregnancy weight avoids the circular problem of measuring BMI after weight has already changed in pregnancy.
Pregnancy dating and fertility estimates assume regular cycles and typical ovulation. They cannot confirm pregnancy, viability or gestational age, and they do not replace ultrasound or prenatal care. Seek urgent care for pain, bleeding or reduced fetal movement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026