My doctor tells me i'm at least 20 pounds underweight.. i'm a white female 15 year old i'm 4 ft 11 inches & i weigh about 85 pounds. am i underweight?

Yes & no: Usually we calculate body mass index (BMI) to determine if "normal" height for weight eg 18.5-24.9. You're underweight at 17.2 (http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm) if you're an adult. However, in adolescents, we use BMI percentile (http://nccd.cdc.gov/dnpabmi/Calculator.aspx) w/underweight as less than 5th percentile, in which case you're healthy (5th-85th) at 12th

Definitions matter: Your weight places you at the lowest 5% of people your age and stature with a Body mass index of 17.3. The average desired weight at your age and height is 98.5lb and a BMI of 19.8.You can certainly be healthy at a lower than average weight and you might look at this as a chalange to put on some muscle mass (muscles are denser) while you are at an age where this is easier.