If you're in a relationship, beware of the month of January. Along with unwanted pounds, bad habits and gifts that don't fit, people often mark the beginning of a new year by jettisoning less-than-ideal romantic partners. After the winter holidays and before the big lovefest of Valentine's Day, January presents an opportune, if cold and dreary, window for a fresh start. Among some therapists, sociologists and advisers to the lovelorn, it's known as breakup month. "You would not believe the huge influx of letters I get in January," said Lisa Daily, a syndicated online dating and relationships columnist based in Sarasota, Fla. The people writing in, she says, are both recently dumped and completely surprised. "They say everything was going great over the holidays. This came out of the blue."

The season of heartbreak affects the ordinary and the famous. Brad and Jen dropped the bombshell of their breakup last January. Ben Affleck saw two very public relationships go south after the holidays - with Gwyneth Paltrow in January 1999 and Jennifer Lopez at the dawn of 2004.Tom Cruise got out of his relationship with Nicole Kidman a week before Valentine's Day in 2001, then broke up with Penelope Cruz in January 2004. Alec Baldwin and Kim Basinger? Chris Evert Lloyd and her fellow tennis star, John Lloyd? Splitsville in January. It happens in fictional worlds, too. On one episode of Homicide: Life on the Street, the long-running television series filmed in Baltimore, the female medical examiner breaks up with a detective she's been dating right after New Year's. Asked for an explanation, she tells him that the holidays are over.

Daily blames the phenomenon on "relationship freeze" she says takes place between Thanksgiving and New Year's. By the time the holidays start, she points out, you may have long-laid plans to travel with your now-not-so sweetie. You've put down money you'd rather not lose. And if you broke up before the end of the year you - and your ex - would be suddenly alone while everyone else made merry. "I think what it speaks to is that romance has its practical and even Machiavellian and manipulative nature," said Pepper Schwartz, a professor of sociology at the University of Washington who studies relationships. "People look out for their own welfare, and they'll do things that make it easier for them."

Jamie Braman, executive director of Maryland's Upscale Singles, a group for single professionals over 30, said her membership always spikes at the beginning of the year. Men in particular, she said, have told her they're loath to break up over the holidays, "because this way they have a guarantee of a date and things to do over the holidays. They break up after New Year's and before Valentine's Day, because there's no way they're going to sit there at Valentine's Day and buy a gift and pretend they're into this person."

But women often break it off in January, too. In their dating days, Daily said, "my girlfriends and I never dumped a guy before Christmas. You don't want to be that jerk." January breakups can still be devastating. Ask Baltimore technical writer Alisa Hoffman. Three years ago, she spent her first holiday season with her boyfriend of about nine months. They picked out the perfect tree and decorated it together. They had Christmas dinner with her mother, then coffee later with his sister. They spent a quiet evening together on New Year's Eve. Soon after that, though, a girlfriend showed Hoffman a profile on match.com. There, next to his advertisement for new companions, was a picture of Hoffman's guy. "And there was our tree in the background," Hoffman recalled. The January breakup that followed might have come after the holidays, but in retrospect, it still ruined them. Hoffman, 41, particularly regretted the money she'd spent on gifts for her man. "It was a waste," she said.

Even relationships that may have seemed healthy before the holidays may be shaky by January because of the extra expectations and intimacy of the season. "They show their boyfriend to Mom and Dad, and Mom and Dad go, 'Are you kidding?' " said sociologist Schwartz. "Or they go to a party and their partner gets plastered, and they see a side of their partner that scares them. The holidays create a finer focus on the relationship and they do put people into situations that they're not in every day."

Then there are those relationships that start during the holidays and don't turn out to be as, well, festive as they might have seemed when the mistletoe was hanging and the alcohol flowed. That's what happened a couple of years ago to Singin Parks, a 31-year-old tennis and golf instructor, who started dating someone he ran into at several holiday parties. She seemed flirtatious and fun. The relationship fizzled before January was over. "She put out a persona that wasn't really her," Parks said. "Once I found out who she really was, I found out she was a very normal girl. I found out she's very rigid."

On college campuses, January breakups are a natural consequence of winter breaks, said Jess Beaton, a Johns Hopkins University senior who writes a sex column for the Johns Hopkins News-Letter. "We're all going on a six-week vacation," she said. "If it's too new, you don't want to stay together. If it's not new, you have to have that conversation of, 'Are we staying together?' "Beaton, 21, has been through a beginning-of-the-year split herself, but she said there is comfort in the fact that it's a time when many others are single, too. "At least then when the breakup happens it's in groups, so it's a little easier to deal with," she said. "You're sort of on course with everyone else's social schedule."

For married couples, separation may be postponed until January to give children one last chance for a traditional family holiday, said Sharyn Sooho, co-founder of divorcenet.com and a family lawyer outside Boston. Often, it's also one last chance for the marriage."If there was discontent before and the holidays are another opportunity to face one another across the table and be disappointed again, that will only reinforce the idea about divorce," Sooho said.

A less-than-happy couple might be able to make it through the winter holidays because so many events revolve around family and friends, said Mark Epstein, an attorney in Pikesville. But by Valentine's Day, going through the motions no longer works. "I just remember that there were a couple of times I was dating somebody for a relatively long period of time, and by the time New Year's came around and made its way to Valentine's Day, I remember thinking I was really letting myself settle for feelings I really didn't feel satisfied with," said Epstein, 53, who is divorced. "Valentine's Day is really a time where you take a look where you are." But there's a bright spot. Epstein said he's in a relationship now that he expects will pass the test of January.

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Ringing out the old Think you might be starting 2006 with a breakup? Here are some things to consider:

- If you got a bad holiday gift from your sweetie, a split might be in the offing. "It makes us aware the person is not very thoughtful or doesn't know us very well," said dating advice columnist Lisa Daily.

- Think about waiting until February before breaking up a marriage, said Sharyn Sooho, co-founder of divorcenet.com. After the inflated expectations and extra stress of the holidays, your relationship may need to return to normal patterns for a few weeks to show whether it's really beyond repair. (But if abuse is involved, she says, don't wait.)

- If you do split, try not to overanalyze the altered meaning of the holidays you just spent with your ex, says University of Washington sociologist Pepper Schwartz. Instead, look at your new solo status as an opportunity for new experiences in the coming year.