| | Conditions & Treatments | Obesity If you are a five foot six ninety pound female underweight? A person who is 5'6 and 90 pounds has a body mass index (BMI) of 15.5, which is considered underweight. Obesity - Related Articles How to Prevent Childhood Obesity How accurate is body mass index? Foods that Endomorphs Should Avoid What Is Sibutramina? How does obesity affect the taxpayer? What is the Relationship between menstruation and obesity? What is the average bmi for female 5 foot weighing 7 stone-?