Being too skinny can decrease fertility.
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Underweight women run the risk of weakened immune systems, disrupted hormone regulation and hair loss. Your ideal body weight is based on your height and frame size. Shorter-statured women should ideally weigh less than taller women. Whether or not you're too skinny is a matter of preference. However, women who are 5 feet tall and weigh 100 pounds are not classified as underweight, according to the University of Washington.
Ideal Body Weight
The University of Washington reports that women should weigh 100 pounds for the first 5 feet of height, plus 5 pounds for each additional inch. The same source notes that large-framed women should add 10 percent, while women with small frames should subtract 10 percent. For example, while a medium-framed, 5-foot-tall woman should weigh about 100 pounds, a small-framed woman of the same height should ideally weigh about 90 pounds and a large-framed 5-foot-tall woman should weigh about 110 pounds, based on the University of Washington guidelines.
Determining Frame Size
Your wrist circumference determines your frame size. Measure your wrist circumference by placing a tape measure around the bony part of your wrist. Women who are 5 feet tall are small framed if their wrist circumference is less than 5.5 inches, medium framed if their wrist size is 5.5 to 5.75 inches and large framed if their wrist circumference is greater than 5.75 inches, according to MedlinePlus.
Body Mass Index
Body mass index is a good indicator of whether a woman's body weight is within a healthy range. A normal BMI for women 5 feet tall is 18.5 to 24.9, according to the National Heart, Lung and Blood Institute. The NHLBI classifies BMIs less than 18.5 as underweight. You can determine your BMI by using a BMI calculator or by multiplying your body weight in pounds by 703, dividing by your height in inches, then dividing by your height in inches once again. For example, a woman who is 5 feet tall and weighs 100 pounds has a BMI of 19.5, which is within a healthy range.
A 5-foot-tall woman who weighs 100 pounds, has a large amount of muscle mass and too little body fat is at risk for reproductive failure, according to the American Society for Reproductive Medicine. According to the American Council on Exercise, women should ideally have body compositions consisting of at least 10 percent body fat, and female athletes should maintain body fat percentages of 14 to 20 percent. Your doctor or a trainer at your local gym can help you estimate your body fat percentage.