BOSTON, June 19 (Reuters) - A Massachusetts city is investigating an apparent teenage "pregnancy pact" that has at least 17 high-school girls expecting babies, four times more than last year, including many aged 16 or younger.

A high school health clinic in the city of Gloucester became suspicious after seeing a surge in girls seeking pregnancy tests. Local officials said on Thursday nearly half of those who became pregnant appear to have entered into a pact to have their babies together over the year.

"Some girls seemed more upset when they weren't pregnant than when they were," Gloucester High School principal Joseph Sullivan told Time magazine, which broke news of the pact on its Web site.

Sullivan was not immediately available to comment. But local officials said at least some of the men involved in the pregnancies were in their mid-20s, including one man who appeared to be homeless. Others were boys in the school.

Carolyn Kirk, mayor of the port city 30 miles (48 km) northeast of Boston, said authorities are looking at whether to pursue statutory rape charges. "We're at the very early stages of wrestling with the complexities of this problem," she said.

"But we also have to think about the boys. Some of these boys could have their lives changed. They could be in serious, serious trouble even if it was consensual because of their age ? not from what the city could do but from what the girls' families could do," she told Reuters.

Under Massachusetts law, it is a crime to have sex with anyone under the age of 16.

NATIONAL TREND

"At the very least these men should be held responsible for financial support, if not put in jail for statutory rape as the mayor has suggested," Greg Verga, chairman of the Gloucester School Committee, told Reuters in a telephone interview.

Nationwide, teen pregnancies are showing signs of rising after steadily declining from 1991 to 2005. This trend was highlighted on Thursday when Britney Spears' 17-year-old sister Jamie Lynn, star of Nickelodeon's popular TV show "Zoey 101," gave birth to a baby girl, according to People magazine.

"The data seem to be indicating that the declines that we had seen through the 1990s are coming to a close," said David Landry, a researcher at the Guttmacher Institute, a New York-based nonprofit group focusing on reproductive issues.

Birth rates for teenagers aged 15 to 17 rose by 3 percent in 2006, the first increase since 1991, according to preliminary data released in December by the National Center for Health Statistics.

Landry cautioned against attributing the trend to Hollywood following the recent hit movie "Juno," in which a teenager gets pregnant and decides to have the baby, and "Knocked Up," a comedy about a one-night stand.

"The trend emerged before those movies," he said.

In Gloucester, the 1,200-student school administered 150 pregnancy tests to students in the past academic year. The school forbids the distribution of condoms and other contraception without parental consent - a rule that prompted the school's doctor and nurse to resign in protest in May.

"But even if we had contraceptives, that pact shows that if they wanted to get pregnant, they will get pregnant. Whether we distribute contraceptives is irrelevant," said Verga.