If a woman weighs 150 lbs, and has a BMI of 20, she is considered:

A body mass index (BMI) of 20 falls within the range considered healthy. The BMI categories define healthy weight as being between 18.5 and 24.9. Therefore, a woman weighing 150 lbs with a BMI of 20 is within this healthy range. This classification reflects an appropriate balance between height and weight, suggesting that the individual is not underweight, overweight, or obese.

The other classifications—underweight, overweight, and obese—are determined by BMI ranges outside the healthy weight category. Underweight is defined as a BMI below 18.5, while overweight falls between 25 and 29.9, and obesity begins at a BMI of 30 or higher. Hence, since a BMI of 20 clearly fits the healthy weight category, it signifies that she maintains a weight that is generally considered beneficial for health.