Why the target depends on your starting BMI
The U.S. Institute of Medicine's widely used guidelines set a different total gestational weight gain range for each pre-pregnancy BMI category, for a singleton (single-baby) pregnancy: Underweight (BMI under 18.5) — 12.5 to 18 kg. Normal weight (BMI 18.5-24.9) — 11.5 to 16 kg. Overweight (BMI 25-29.9) — 7 to 11.5 kg. Obese (BMI 30+) — 5 to 9 kg. The pattern is deliberate: someone starting with less body reserve is guided toward a larger gain, and someone starting with more is guided toward a smaller one.
A worked example across three starting points
For someone 160 cm tall weighing 55 kg pre-pregnancy (BMI 21.5, Normal category): the recommended total gain is 11.5 to 16 kg. The same height at 75 kg pre-pregnancy (BMI 29.3, Overweight category): the recommended range drops to 7 to 11.5 kg. The same height again at 45 kg pre-pregnancy (BMI 17.6, Underweight category): the recommended range rises to 12.5 to 18 kg — three different targets from three different starting weights, at the exact same height.
Why these numbers aren't identical to non-pregnancy weight guidance
Pregnancy weight gain includes far more than fat — the baby itself, the placenta, amniotic fluid, increased blood volume, and breast tissue growth all contribute meaningfully to total weight gain, alongside any maternal fat stores. This is why even the "Obese" category's range (5-9 kg) is still a positive number rather than zero or negative — some weight gain is a normal and necessary part of a healthy pregnancy regardless of starting BMI category, not something to be minimized across the board.
These are ranges for singleton pregnancies specifically
These IOM ranges assume a single baby — a twin or higher-order multiple pregnancy has meaningfully higher recommended total weight gain in every BMI category, since more than one baby, placenta, and associated tissue growth are involved. Anyone expecting multiples should use guidance specific to that situation rather than these singleton-pregnancy ranges.
Using this as a guide, not a strict target
These ranges are population-level guidelines intended to flag when weight gain is trending unusually high or low for a healthy pregnancy — they're a reference point for a conversation with a healthcare provider, not a strict pass/fail target to hit exactly. Individual health conditions, pregnancy complications, and provider-specific guidance can all reasonably shift what's appropriate for a specific pregnancy away from these general population ranges.