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Pregnancy weight gain calculator: IOM guidelines by trimester
Find your recommended weight gain range based on your pre-pregnancy BMI and current week, following the Institute of Medicine 2009 guidelines for singleton and twin pregnancies.
Pregnancy weight gain calculator
Your result
About this tool
Why pregnancy weight gain guidelines exist
The Institute of Medicine (IOM) revised its pregnancy weight gain guidelines in 2009 to account for pre-pregnancy BMI. The core finding: women who gain weight within the recommended range have better outcomes for both themselves and their baby — lower rates of gestational diabetes, pre-eclampsia, caesarean birth, and infant macrosomia (large birth weight).
Too little weight gain is linked to preterm birth and low birth weight. These recommendations are starting points; your midwife or obstetrician will tailor them to your specific situation.
Step by step
How to use the pregnancy weight gain calculator
- 1
Enter your height and pre-pregnancy weight
Use your weight from before pregnancy, or your weight at your first antenatal appointment.
- 2
Enter your current week of pregnancy
Weeks run from 1 to 42. Your midwife or scan report will confirm your exact week.
- 3
Choose singleton or twins
IOM guidelines differ for twin pregnancies — select twins if you are expecting more than one baby.
- 4
Your recommended total gain and current-week target appear instantly
Use the weekly rate as a guide, not a precise daily target.
Methodology
IOM 2009 guidelines — singleton pregnancies
| Pre-pregnancy BMI | BMI range | Recommended total gain | Weekly rate (2nd–3rd trimester) |
|---|---|---|---|
| Underweight | Below 18.5 | 12.5–18 kg | 0.44–0.58 kg |
| Normal weight | 18.5–24.9 | 11.5–16 kg | 0.35–0.50 kg |
| Overweight | 25–29.9 | 7–11.5 kg | 0.23–0.33 kg |
| Obese | 30 and above | 5–9 kg | 0.17–0.27 kg |
For twins (normal weight): 17–25 kg total. First trimester: approximately 0.5–2 kg total gain regardless of BMI category.
Worked example
Example calculation
A woman with a pre-pregnancy BMI of 22 (Normal weight), now at week 20. First trimester ≈ 1–2 kg. Weeks 14–20 = 7 weeks × 0.35–0.50 kg/week = 2.45–3.5 kg extra. Expected gain by week 20: approximately 3.5–5.5 kg.
Be aware
Assumptions and limitations
- These are population-level guidelines, not individual prescriptions.
- Does not account for fluid retention, pregnancies beyond twins, or pre-existing conditions.
- Weight gain in pregnancy is non-linear — do not worry about hitting a precise weekly number.
- Always consult your healthcare provider before changing diet or exercise during pregnancy.
Questions
Frequently asked questions
What if I've already gained more than recommended?
Discuss it with your midwife or doctor. Excessive gain is linked to complications, but the response should be maintaining — not losing — weight. Dieting during pregnancy is not recommended and could affect your baby's development.
Should I track weight daily?
Weekly tracking in the same conditions (morning, before eating, same clothing) is more useful than daily weigh-ins, which are dominated by fluid fluctuation. A gradual upward trend over weeks is what matters, not a precise day-to-day number.
Is weight gain different for twins?
Yes. Twin pregnancies require more weight gain because you are supporting two babies, two placentas, and more amniotic fluid. The Institute of Medicine provides separate recommendations for twins, which this calculator applies automatically when you select that option.
Important health disclaimer
These guidelines are for general educational purposes only. Pregnancy weight management requires individual assessment by a qualified healthcare professional. Do not make dietary changes or restrict weight gain during pregnancy without medical guidance.