You might know that certain foods and drinks are loaded with hidden calories, but other unexpected factors such as your friends or your zip code could cause you to gain weight. Here are a few surprising things that might contribute to an expanding waistline.

  • Skipping meals.

    Not eating breakfast or lunch can make you feel hungrier later, which leads to overeating or binge eating. Eating breakfast boosts your metabolism, which enables you to burn more calories throughout the day. Nutritionists recommend that you eat five or six small meals. This doesn't mean that you eat more food, but spreading out it over several meals helps your body burn the calories more efficiently.

  • Your friends.

    A person with a close friend who is obese is 57 percent more likely to become obese, according to a study published in the New England Journal of Medicine. The effect is not as profound with spouses (37 percent more likely) or siblings (40 percent). The reason goes beyond eating meals and going for drinks together, the researchers say. Having overweight or obese friends changes your perception of what an acceptable body type is.

  • Your birth control.

    Some women say they gain weight when taking the pill. Experts note that the benefits of birth control far outweigh this downside. Some people say the hormones in birth control may cause weight gain by allowing your body to retain more water or by increasing your appetite. Other researchers say birth control doesn't cause weight gain.

  • Where you live.

    The rate of obesity increases as neighborhood home value decreases, the Centers for Disease Control and Prevention found. Lack of healthy, fresh food options and an increased reliance on driving could cause increased obesity in certain parts of the country.

Watching your portion size and exercising regularly are two ways to help you maintain your weighteven if some of these other factors sneak up on you!