Lindsey Mathias was confused and disappointed at first when she opened the thick, much-awaited envelope from the University of Southern California this spring. The letter inside told her she had been accepted to the Los Angeles university but also said she could not enroll until January.

Mathias, who lives in Winter Park, Fla., said she quickly got over fears that a midyear start to college would be socially and academically awkward. "In the long run, I don't think it matters when you start," the 18-year-old said. "I waited four years to go to USC, I think I can wait four more months."

She is part of an increasingly common trend in U.S. higher education: freshmen who are told to arrive on campus in January or February, even though most of their friends started college in August or September. Colleges and universities face a demographic bulge of high school graduates who are qualified applicants. Yet schools still need tuition-paying students to fill classroom and dormitory slots left empty each spring by those who study abroad, take off-campus internships, drop out or graduate in December.

So a bargain of sorts is struck: Colleges try to guarantee a fully populated campus, and some students accept an offer to arrive midyear at a top choice school rather than enroll elsewhere in the fall.

"We have such an incredibly strong pool of applicants, and we know we have limited space in the fall. We wanted to provide an opportunity for a limited number of students in the winter," explained Mae Brown, assistant vice chancellor for admissions at the University of California, San Diego.

UC San Diego expects about 4,600 freshmen to start in the fall and about 200 in winter, she said. USC projects about 2,750 fall freshmen and 250 in spring 2009. UC Berkeley expects about 4,450 fall freshmen and 988 in the spring semester.

National organizations for college admissions say they do not have a registry of how many schools offer midyear enrollment. But experts say it has been increasing in recent years. Other schools with the policy include Pepperdine University, the University of Maryland, American University, Middlebury College, Colby College and Brandeis University.

In most cases, students are offered midyear enrollment without having known it was a possibility. The offers are usually kept apart from fall waiting lists, although some schools allow students to start in fall if vacancies occur.

Beyond initial surprise and for some, sadness, the big issues for midyear students are what to do with the extra time and whether the delay has any repercussions. They and their parents want assurance that the admittance is not a backdoor entry that leads to second-class citizenship on campus, carrying a stigma that the students are in a group with weaker credentials.

Colleges contend there is little difference between the groups and that some students are put in the spring pool because of their majors or for geographic diversity, not grades. They insist that graduation rates are similar and that many midyear students manage to finish with the fall group by taking extra classes or summer courses.

Peter Van Buskirk, former admissions dean at Franklin & Marshall College in Pennsylvania and author of the 2007 book "Winning the College Admission Game," said the practice can be a "win-win" for schools and applicants. But he warned of potential problems if midyear students have weaker high school resumes and colleges don't adequately orient them.

"Some of the students may be at more risk academically, and they might be more at risk socially since they are entering school at a time friendships and relationships already have been established," he said.

Some students resent the delay, while others describe it as an unexpected blessing that allowed them to get a better sense of what they want to study and why.

Mathias, who plans to major in journalism, expects to spend the fall taking transferable general education classes, such as English and psychology, at a community college in Florida. Then she plans to jump wholeheartedly into USC campus life.

"It's kind of bittersweet," she said. "I really want to go to USC. I don't want to wait. But if this is what it takes, that's what I'll do."