Fitness & Health · Live
Pregnancy Weight Gain Calculator,
personalised to your BMI and week.
Enter your pre-pregnancy weight, current weight, and gestational week to see your IOM-recommended weight gain range, track whether you’re on target, and view a full week-by-week trajectory curve — for both singleton and twin pregnancies.
Inputs
Your pregnancy
- Pre-pregnancy BMI
- 23.8 — Normal weight
- Recommended total
- 24.9–35.1 lb
- Gained so far
- 15 lb
- Status
- Above target
IOM recommended total gain
Week 20 · Singleton
Full-term goal · 20 weeks remaining · Trimester 2
Above target
+15 lb
gained so far
Weight gain curve
IOM recommended range — weeks 1 to 40
Milestone reference
Expected weight gain range by week
| Week | Trimester | Low (lb) | High (lb) |
|---|---|---|---|
4 | 1st | 0.4 | 1.5 |
8 | 1st | 0.7 | 2.9 |
12 | 1st | 1.1 | 4.4 |
16 | 2nd | 4.2 | 8.8 |
20now | 2nd | 7.3 | 13.2 |
24 | 2nd | 10.4 | 17.6 |
28 | 3rd | 13.4 | 22 |
32 | 3rd | 16.5 | 26.5 |
36 | 3rd | 19.6 | 30.9 |
40 | 3rd | 22.7 | 35.3 |
| Your gain · week 20 | 15 lb | ||
Clinical guide
How much weight should you gain during pregnancy?
The answer depends on your pre-pregnancy body mass index (BMI). Since 2009, guidelines from the Institute of Medicine (IOM): now the National Academy of Medicine — have been the clinical standard used by obstetricians, midwives, and physicians worldwide. The IOM links the recommended weight gain range directly to pre-pregnancy BMI, reflecting decades of research on pregnancy outcomes at different gain levels.
IOM 2009 guidelines: singleton pregnancies
| Pre-pregnancy BMI | Category | Total gain | 2nd & 3rd tri. rate |
|---|---|---|---|
| < 18.5 | Underweight | 28–40 lb (12.7–18.1 kg) | ~1 lb/wk (0.44–0.58 kg/wk) |
| 18.5–24.9 | Normal | 25–35 lb (11.3–15.9 kg) | ~1 lb/wk (0.35–0.50 kg/wk) |
| 25.0–29.9 | Overweight | 15–25 lb (6.8–11.3 kg) | ~0.6 lb/wk (0.23–0.33 kg/wk) |
| ≥ 30 | Obese | 11–20 lb (5.0–9.1 kg) | ~0.5 lb/wk (0.17–0.27 kg/wk) |
IOM 2009 guidelines: twin pregnancies
| Pre-pregnancy BMI | Category | Recommended total gain |
|---|---|---|
| < 18.5 | Underweight | 50–62 lb (22.7–28.1 kg) |
| 18.5–24.9 | Normal | 37–54 lb (16.8–24.5 kg) |
| 25.0–29.9 | Overweight | 31–50 lb (14.1–22.7 kg) |
| ≥ 30 | Obese | 25–42 lb (11.3–19.1 kg) |
How the week-by-week model works
The IOM specifies totals at term and a first-trimester reference. This calculator models:
- Weeks 1–12: Linear ramp from 0 to the first-trimester reference (0.5 kg lower, 2.0 kg upper, the same for all BMI categories).
- Weeks 13–40: The BMI-specific weekly rate is added on top of the first-trimester baseline, producing a curve that reaches the at-term range at week 40.
What gestational weight gain is made of
For a normal-weight person gaining ~12 kg total, the approximate distribution is:
- ~3.4 kg: baby
- ~0.6 kg: placenta
- ~0.8 kg: amniotic fluid
- ~1.5 kg: uterine muscle growth
- ~1.5 kg: breast tissue
- ~1.3 kg: increased blood volume (~50% more)
- ~1.0 kg: extravascular fluid
- ~2.0–3.0 kg: maternal fat stores (breastfeeding reserve)
If you're above or below the range
Being slightly outside the range at any point doesn’t mean something is wrong — the IOM range is a guideline, not a rigid limit. Many women lose weight in the first trimester from morning sickness, then gain more rapidly in the second. What matters is the overall trajectory, discussed with your care provider — not hitting an exact weekly number.
Why gestational weight gain matters for both mother and baby
Weight gain outside the IOM range, either above or below — is associated with measurable pregnancy outcomes. Gaining above the upper bound is linked to gestational diabetes, hypertensive disorders, caesarean delivery, postpartum weight retention, and higher birth weight (macrosomia). Gaining below the lower bound is associated with preterm birth, small-for-gestational-age infants, and reduced breastfeeding duration. Neither extreme is safe — the range exists because pregnancy outcomes deteriorate on both sides.
How early weight loss affects your tracker
Some women lose weight in the first trimester due to nausea and vomiting. This calculator computes gain relative to your pre-pregnancy weight, so a negative early reading is normal and expected. The IOM model allows for essentially zero gain in the first 12 weeks; catch-up in the second trimester typically brings the trajectory back into range without any intervention.
Disclaimer
This calculator is for educational purposes only. Gestational weight gain goals should be set with your obstetrician, midwife, or family physician, who can account for your specific health history and any pregnancy complications. Never adjust nutrition or activity based solely on this or any online tool.