In other words, you're probably parked at a desk for 40-plus hours per week for the first time in your life (who has time for the gym?).
Plus, you may be hanging out at restaurants and bars with friends: Alcohol can add a significant number of calories to your daily intake, notes Joshua D. Brown, PhD, psychologist and director of clinical services at the Weight Management Center at the Medical University of South Carolina.
Then there's the fact that the average woman ties the knot around age 26, and married life isn't always kind to your waistline. Married couples tend to gain more weight than their single and dating counterparts, Gordon-Larsen says.
Some 20-something tricks to halt weight gain in its tracks: Schedule the whole week's worth of workouts with reminders on your BlackBerry at the beginning of the week, Brown suggests. Once you've made an appointment with yourself, it's harder to bail. If you're out drinking, alternate one alcoholic beverage with a glass of water or seltzer to trim calories.
And if you're married, you and your spouse should encourage each other to eat healthy and exercise (maybe even work out together). The earlier you can get into some healthy routines, Brown says, the sooner you can nip weight gain in the bud.