Why the guideline depends on pre-pregnancy BMI
The Institute of Medicine's (IOM) 2009 weight-gain guidelines — still the reference most U.S. obstetric providers use — set a different total weight-gain range for each pre-pregnancy BMI category, because starting body composition affects both the risks of gaining too little and the risks of gaining too much. A woman who starts pregnancy underweight is guided toward a higher total gain than a woman who starts obese, because the priorities on each end of that spectrum differ: adequate gain to support fetal growth in one case, avoiding excess gain in the other. Neither end of the range is a 'better' or 'more disciplined' outcome — the categories exist because the same pound of gain carries different implications depending on where someone starts.