How the weight gain calculator works
The IOM framework
The Institute of Medicine (2009) ranges — still the standard in most guidelines — key off pre-pregnancy BMI:
| Pre-pregnancy BMI | Category | Singleton total gain |
|---|---|---|
| < 18.5 | Underweight | 12.5–18 kg (28–40 lb) |
| 18.5–24.9 | Normal | 11.5–16 kg (25–35 lb) |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lb) |
| ≥ 30.0 | Obese | 5–9 kg (11–20 lb) |
Twins raise the ranges substantially (e.g., normal BMI: 17–25 kg). The ranges are population guidelines balancing newborn birthweight against maternal retention — your clinician may individualize above or below them.
The shape matters as much as the total
First trimester: ~0.5–2 kg total regardless of BMI category (appetite often has other plans; that is fine). Second and third: a steady rate — roughly 0.4 kg/week at normal BMI, 0.3 overweight, 0.22 obese, 0.5 underweight. The calculator shows where you sit against the week-appropriate trajectory, not just the final range.
What the gain consists of
At the normal-BMI midpoint (~13 kg): baby ~3.4 kg, placenta and amniotic fluid ~1.8, uterus and breast tissue ~1.9, blood ~1.9, body fluid ~1.9, fat stores ~2.7 (the lactation reserve). That "fat" slice is the only part the body doesn't structurally need — and it is the part retained after birth. Gain above range is mostly that slice; gain below range risks fetal growth. The range is a balance, not a vanity number.