When you visit your doctor, she will usually obtain your height and weight measurements. She can then use these numbers to help calculate your body mass index, or BMI. Your BMI can help your doctor compare your height and weight to that of your peers to determine whether you are the right weight, underweight, overweight or obese. A BMI is a fairly accurate measure of body fatness for most women, according to the Centers for Disease Control, but it might not be accurate for very short people -- under 5 feet tall -- and older women that lose muscle mass as they age.
History
Belgian polymath Adolphe Quetelet stumbled upon the BMI equation in 1832 as a way to describe the proportions of height and weight among regular adults. He collected data from several hundred Belgian citizens and found that many people's weight varied in proportion to the square of height -- people 10 percent taller than average tended to be 21 percent heavier. Insurance companies conducted the first large-scale studies of weight and height in the 1900s to show that obesity increased the risk of death and other ailments in policyholders. Physiology professor and obesity researcher Ancel Keys compiled this data into his 1972 work "Indices of Relative Weight and Obesity." This inexpensive and easy measure became a standard measure of obesity for both men and women in the 1980s.
Considerations
Height and weight calculations cannot tell doctors everything they need to know about a person's weight. The CDC recommends using the BMI as a screening tool to identify women that might be at risk for obesity and obesity-related illnesses. It then suggests that the practitioner follow up with a detailed personal assessment that includes an evaluation of her diet, physical activity, family history and health history.
How to Calculate Your Body Mass Index
Adults 20 years and older calculate their BMI using standard weight categories that are the same for both men and women. You can calculate your BMI by obtaining your weight and height measurements. Then, divide your weight in pounds by your height in inches squared and multiply that number by a conversion factor of 703. If a woman's height is 5 foot 5 inches and her weight 140 lbs., her BMI is 23.3, which puts her in a normal weight category. The BMI categories are: less than 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 overweight and 30 or greater obese.
Supplements to Height and Weight Calculations
Even if your height and weight data results in a high BMI, it doesn't necessarily mean that your health is at risk. BMI calculations don't differentiate between lean and fatty mass, which means some muscular, athletic women mistakenly end up classified as overweight. The American Society for Nutrition and the American Diabetes Association recommend that doctors supplement BMI readings with waist circumference measurements. Women with fat around their waist instead of their hips face a higher risk of heart disease and diabetes, as do women with a waist circumference of greater than 35 inches.
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