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.
Underweight, malnourished teenagers (such as those with an eating disorder) have an increased risk of osteoporosis and may not have menstrual periods.
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.
Obesity is associated with a plethora of very serious health conditions, and being underweight increases your risk of infertility and osteoporosis.
Some celebrities such as Nicole Ritchie and Lindsey Lohan seem to have taken this interest too far, and to some, appear frighteningly underweight.
It's as unhealthy to be severely underweight as it is to be very overweight, but many plus size women and men fall into a category of good health.
Since these infants are underweight and undersized at birth, these charts make allowances for the amount of catch-up growth that may be involved.
Most women need to gain about 25 pounds during pregnancy, although more gain may be necessary if you were underweight before becoming pregnant.