Where the numbers come from
The IOM 2009 report (now the National Academies of Sciences, Engineering and Medicine) updated the 1990 IOM guidelines after a decade of pooled outcomes data, including the addition of obesity-class ranges. Health Canada and SOGC formally adopted the IOM 2009 ranges and they remain the operative reference in Canadian prenatal care.
Why a range, not a target
The displayed band is what minimises pooled risk across multiple outcomes: baby size at birth, preterm delivery, gestational diabetes, preeclampsia, caesarean delivery, postpartum weight retention. There’s no single “right” number — anywhere inside the range is consistent with good outcomes data. Outside the range raises some risks; far outside it raises them substantially.
This isn’t prenatal care
A reference number from a webpage doesn’t replace your obstetrician, midwife, or registered dietitian. They factor in your specific medical history — gestational diabetes risk, prior pregnancy complications, history of disordered eating, multiples, age, and many other variables — that no calculator can capture. Use this as a baseline; let your team set your actual goal.