Pregnancy Weight Gain Calculator
See your expected weight gain by week of pregnancy based on IOM guidelines.
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📘 What is the Pregnancy Weight Gain Calculator?
Healthy pregnancy weight gain is not a single universal number — it depends substantially on your pre-pregnancy BMI, following guidelines from the Institute of Medicine (IOM). This calculator shows both the recommended total range for the full pregnancy and a projected gain so far based on current week of gestation.
⚙️ How Pregnancy Weight Gain is calculated
Why pre-pregnancy BMI changes the target range
Someone starting pregnancy underweight is recommended to gain more (12.5–18kg) than someone starting at a higher BMI (5–9kg for an obese starting BMI), since the goal is a healthy outcome for both parent and baby, not a single fixed target.
Why gain is not linear throughout pregnancy
The first trimester typically involves comparatively little weight gain (around 1–2kg total), with the majority of the recommended gain occurring more steadily through the second and third trimesters — this calculator models that uneven distribution rather than assuming a flat weekly rate.
These are guidelines, not strict individual targets
IOM ranges are population-level guidelines. Multiples (twins or more), pre-existing conditions, and individual circumstances can reasonably shift the appropriate target — always discuss your specific situation with your doctor rather than treating this calculator as a strict requirement.
IOM total gain ranges by pre-pregnancy BMI
Underweight: 12.5–18kg · Normal: 11.5–16kg · Overweight: 7–11.5kg · Obese: 5–9kg
🧮 Worked examples
Example — normal BMI, week 20
Pre-pregnancy BMI of 23 (normal range), currently at 20 weeks gestation.
→ Total recommended range for full pregnancy: 11.5–16kg. Projected gain so far at week 20: approximately 4.1–5.3kg
Comparing to a different starting BMI
Same 20-week point, but pre-pregnancy BMI of 32 (obese range) instead.
→ Total recommended range drops to 5–9kg — illustrating how much pre-pregnancy BMI shifts the appropriate target
💡 Original insights & how to use this calculator
Tracking progress against a personalised range rather than a generic number
Comparing your actual weekly weigh-ins against the projected range for your specific BMI category gives more useful context than a single flat target shared across all pregnancies.
Discussing concerns with your doctor using concrete numbers
If your actual gain is tracking meaningfully outside the projected range, having the specific numbers in hand makes for a more productive conversation with your healthcare provider.
Understanding why "eating for two" oversimplifies real needs
Caloric needs increase only modestly during pregnancy, particularly in the first trimester — the recommended weight gain ranges reflect this more nuanced reality rather than supporting a dramatic increase in intake from day one.
💡 Expert Tips
These ranges are guidelines, not strict targets — always discuss your specific situation with your doctor, especially with multiples or other complications.
How to read your result
The IOM ranges are population guidelines based on pre-pregnancy BMI category — they're a healthy range to discuss with your doctor, not a strict target to hit exactly. Individual factors (multiples, pre-existing conditions, activity level) can reasonably shift what's healthy for you specifically.
⚠️ Common Mistakes
✕ Using current weight instead of pre-pregnancy weight to determine your BMI category.
✓ The IOM ranges are based on your BMI before pregnancy started — using your current (already-increased) weight to look up the category will point you at the wrong range.
✕ Panicking about being slightly outside the range in early weeks.
✓ Weight gain is naturally uneven — often minimal in the first trimester with more gain in the second and third. A single week's reading rarely matters as much as the overall trend by trimester.
✕ Applying general population ranges to a twin or multiple pregnancy without adjustment.
✓ These IOM ranges are calibrated for a single baby. Multiple pregnancies have different, generally higher, recommended ranges — discuss this specifically with your doctor.
Frequently Asked
How much weight should I gain during pregnancy?▾
It depends on your pre-pregnancy BMI — IOM guidelines suggest roughly 11.5-16kg for a normal starting BMI, with different ranges for underweight, overweight, or obese starting points.
🔎 Explore related calculators
BMI Calculator
Example 1 — Metric
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9 — Normal weight