You may have heard of the freshman 15-those inevitable pounds college students are likely to gain in their first year. But what about the newlywed 30? In a recent University of North Carolina study, which tracked 1,200 couples over a five-year period, married people gained six to nine pounds more than their single counterparts. Why does marriage lead to weight gain? Read on as we reveal the top four reasons.

1. The Honeymoon Period. In the time surrounding a wedding, many couples are engaged in an endless eating-and-drinking bacchanal: the bridal shower; the bachelor and bachelorette parties; the rehearsal dinner; the wedding itself with its appetizers, carving stations, three-course meals, four-tier cake, and open bar; the honeymoon, which is all about decadent overindulgence. And when you're madly in love, who has time to cook? But if you keep up this behavior, pounds will soon follow. A San Diego State University study found that dining at a restaurant more than once a week can increase the risk of obesity

2. The Company You Keep. When researchers at Harvard University reviewed the Framingham Heart Study, a data-intensive long-term investigation of the health habits of 12,000 residents living in Framingham, Massachusetts, they found an interesting correlation: If your friends and family members are obese, the likelihood that you will become overweight as well is rather high. What does that mean for couples? In a July 2007 article published in the New England Journal of Medicine, the researchers claimed that if your spouse is overweight, your obesity risk increases by 37 percent.

3. Pregnancy. According to the American Pregnancy Association, a woman can expect to gain an average of 25 to 40 pounds before giving birth. This weight is important, since much of it is needed to provide nourishment for the baby. Approximately 10 pounds are lost in the birthing process, and additional weight due to fluid retention is shed in the week following the birth. The rest can be lost by good, old-fashioned diet and exercise. The jury is still out on whether breastfeeding helps mothers lose weight; in fact, a 2004 study found that there was no significant difference in weight loss between women who breastfed and those who didn't. And women aren't the only ones to gain weight during pregnancy: Researchers at Duke University Medical Center found that men up their risk of obesity by 4 percent with the birth of each child.

4. Complacency . It's the biological imperative in action: When you're single, you'll do everything you can to attract the opposite sex. Once you're married, the need to remain slim and attractive can be easily sidelined by other concerns—work, kids, household chores. Public health experts at the Yale University School of Medicine call it the "I've got him/her now, so I don't have to work so hard" mentality, and when coupled with the other factors mentioned, it can be a surefire ticket to weight gain.