Pregnancy Weight Gain Calculator
Compare your weight gain so far against the Institute of Medicine recommended range for your pre-pregnancy BMI and pregnancy week.
Recommended total gain — Normal weight
11.5–16 kg
- Pre-pregnancy BMI
- 23.9
- Weekly gain (2nd & 3rd trimester)
- 0.35–0.5 kg/week
- Expected gain by this week
- 4.6 kg
- Your gain so far
- 5 kg
- Status
- Within range
Recommended ranges follow the Institute of Medicine (IOM, 2009) guidelines for gestational weight gain, based on pre-pregnancy BMI. The expected-by-week trajectory uses ~1.6 kg of gain over the first trimester (weeks 1–13), then the midpoint of the IOM weekly rate for the 2nd and 3rd trimesters. Information only — not a substitute for antenatal care.
How to use this calculator
Pick metric or imperial units and choose whether you are carrying a single baby or twins. Enter the weight you were just before becoming pregnant, your height, the current week of pregnancy (1–42) and your weight today. The calculator returns the total weight-gain range recommended by the Institute of Medicine for your pre-pregnancy BMI category, the weekly rate of gain expected in the 2nd and 3rd trimesters, an expected gain by the current week, and whether your actual gain sits within that trajectory.
How the calculation works
The calculator first converts your inputs to kilograms and metres and computes your pre-pregnancy body mass index using the standard WHO formula: BMI = weight (kg) ÷ height (m)². It maps that BMI to one of four IOM categories (underweight, normal, overweight, obese) and looks up the recommended total gain and weekly rate. The expected trajectory assumes a small first-trimester gain of about 1.6 kg over weeks 1–13, then the midpoint of the IOM weekly range each week thereafter. Twin recommendations use the IOM 2009 provisional ranges and are not provided for underweight BMI because the evidence base is insufficient.
Worked example
A woman who was 65 kg and 165 cm tall before becoming pregnant has a pre-pregnancy BMI of 65 ÷ 1.65² = 23.9 — within the WHO normal range. The IOM recommended total gain for normal-BMI singleton pregnancy is 11.5–16 kg, with a 2nd & 3rd trimester rate of 0.35–0.50 kg per week. At week 20 the expected gain is roughly 1.6 + (20 − 13) × 0.425 ≈ 4.6 kg. If she now weighs 70 kg, her actual gain is 5.0 kg — within the on-track band around the expected trajectory.
Frequently asked questions
Where do these recommended ranges come from?
They are the 2009 Institute of Medicine and National Research Council guidelines — "Weight Gain During Pregnancy: Reexamining the Guidelines" — which set total-gain and weekly-rate targets by pre-pregnancy BMI. ACOG, the NHS, the CDC and the WHO Maternal Health programme all reference these ranges. They have not been revised at the time of writing.
Why is gestational weight gain banded by pre-pregnancy BMI?
Outcomes for both mother and baby — pre-term birth, gestational diabetes, hypertensive disorders, large- or small-for-gestational-age babies — depend on starting BMI. A woman who began pregnancy underweight benefits from more total gain; one who began obese benefits from less. The IOM bands match each BMI category to the range of total gain associated with the lowest combined risk in observational cohorts.
My doctor told me a different target. Which is right?
Always follow your clinician. The IOM ranges are a population starting point; an individual antenatal team will adjust for medical history (gestational diabetes, prior pregnancy outcomes, multiple gestations beyond twins, eating disorders, very tall or very short stature). Use this calculator for orientation, not as a substitute for personalised antenatal advice.
What if I am carrying triplets or higher-order multiples?
IOM 2009 only provides recommendations for singleton and twin pregnancies. Triplet and higher-order pregnancies are managed individually because of the much wider clinical variation, and there is no validated population range to compute against. The calculator does not produce a result for triplets — your maternal-fetal medicine team will set a target.
How is weekly weight gain measured in practice?
Antenatal clinics typically weigh at scheduled visits rather than weekly, so the rate is averaged across the interval. Small week-to-week swings caused by fluid, time of day, or recent meals are normal — the trajectory over four to six weeks is what matters. If your gain is well outside the IOM band over a sustained period, raise it at your next appointment.
Does it matter what I gain in the first trimester?
IOM gives no first-trimester rate, only a typical total of about 0.5–2.0 kg. Many women lose a little weight in the first trimester from nausea — that is not in itself a concern. The bulk of healthy weight gain happens in the 2nd and 3rd trimesters, which is why the calculator uses a small linear ramp for weeks 1–13 and the per-week IOM rate from week 14 onwards.