You're pregnant. Even if you were overweight to start with, you'll still need to gain 10 to 15 pounds -- but slowly. Focus on eating healthy foods, not skimping on meals.
You're newly nursing. You need about 500 more calories per day to breastfeed, and dieting in the first 12 weeks of nursing can compromise milk supply. So wait that long before trying to drop the baby weight.
You're too thin. If your doctor tells you that your body mass index (BMI) is 18 or less -- a 5 foot 5 woman who's under 108 pounds, for example -- you should focus on gaining, not losing.
You're ill. Your body needs protein and calories to fight off an infection.
You've had surgery. Healing burns calories, so dieting now may slow down the process. Your doctor can tell you when it's safe to start losing weight.
You're really stressed. It takes a lot of effort to eat less and exercise more. Wait until things get better (we didn't say perfect!) before you go on a weight-loss diet.