Underweight, malnourished teenagers (such as those with an eating disorder) have an increased risk of osteoporosis and may not have menstrual periods.
A woman who is considered obese may have her doctor recommend she gain only 10 pounds, while a woman who is underweight when she conceives may be encouraged to aim for a weight gain of 45 to 50 pounds.
The human body, when taxed by extra weight (or too little weight; many severely underweight women do not even menstruate), does not get pregnant as easily.
Because nutrients are only poorly digested and absorbed, the person with CF is often ravenously hungry, underweight, and shorter than expected for his age.
Knowing your BMI is important as it can tell you if you're overweight or underweight and if you're potentially at risk for certain health conditions.