Rosemeri Rodrigues Viana was 13 when she had sex for the first time. Six months later she was pregnant. Last May, she gave birth to a second child. She was 16.

Today, she lives with her mother in a cramped three-room shanty in one of Rio de Janeiro's largest and seediest slums, her startling brown eyes weary from diaper changes, midnight feedings and the burden of an uncertain future.

"It's just so much work," Rosemeri, now 17, said as she watched intently as her 6-month-old son, Alex, played on a tiny bed. Her daughter, Thayma, 2 1/2, was with Rosemeri's mother in another room. Both pregnancies, she said, were accidents: "I just didn't think I would get pregnant."

The plight of teenagers like Rosemeri has become alarmingly common in Brazil as the birthrate among adolescents has soared, a mystifying development given the unprecedented efforts of the past decade here to provide more information about, and access to, contraceptives.

Television commercials trumpet the use of condoms. Neighborhood clinics distribute birth-control pills and other contraceptives. Numerous cities have created programs to reduce the number of pregnancies and cases of sexually transmitted diseases among teens.

Yet those efforts have found only limited success, goading this vast, mostly Roman Catholic nation into a sober second-guessing of its famously laid-back notions of sexual freedom. What kind of message does Carnaval, the annual feast of Brazilian sensuality, send to teenagers? What is the impact of easy access to pornography? Does television nudity make a difference? Do teenagers notice when a TV star gets pregnant and isn't married?

Last July, Health Minister Jose Serra lashed into Xuxa Meneghel, an immensely popular host of television programs for children and teenagers, for having a child out of wedlock. "I can only imagine how many teenagers are influenced by" Meneghel's decision, he said, adding that such examples "stimulate 12- and 13-year-old girls to have children." Later, he would say that adults are free to make their own choices.

Serra's initial reaction stemmed in part from recent statistics, which show a 19 percent increase in the birthrate for girls aged 15 through 19 from 1993 to 1998. The birthrate among girls aged 10 to 14 leaped 31 percent during the same period. Overall, the number of births among girls aged 10 through 19 rose from 565,000 in 1993 to 698,000 in 1998, the latest year for which statistics are available.

These statistics, combined with the fact that about 40 percent of pregnant teenagers abandon school, have compelled the Health Ministry to make prevention of teen pregnancy a top priority. The ministry plans, among other things, to train 640,000 teachers and health professionals--including gynecologists to psychologists--to provide more and better information to teenagers.

"In a country where human capital is the main variable for development," said Lucimar Cannon, the government's top official on adolescent health, "you can't have hundreds of thousands of girls who never finish school or develop a career."

There may be no consensus on what has encouraged adolescent pregnancy, but counselors, gynecologists and Brazilian health officials agree that today's teenagers are much more sophisticated sexually. In the early 1990s, most teens said their first sexual experience came at age 15 or 16. Today, many more are having sex at 12 or 13.

"Most of the questions that children and teenagers used to ask were for basic information," said Albertina Duarte, an expert on adolescent sexuality here. "How was I born? At what age can I start a relationship? Now they ask me: 'How do I know I'm giving the girl pleasure?' "

At Luiz Delfino middle school, which participates in Rio's program to prevent AIDS and pregnancy among teenagers, 24 bright, carefree students clustered in a circle one recent morning to talk about sex and the Brazilian teenager. Students spoke only with a promise of at least partial anonymity.

The group was typical, said Sonia Cotrim, who runs sexuality workshops at the school. Five of the students--two of them 14, one 15, two 16--said that they are sexually active. A third of the group knew someone who had had a sexually transmitted disease; two-thirds knew someone who became pregnant at 14 or younger.

They said television, which some acknowledged watching for eight hours a day, has had an enormous impact on their attitudes. "I don't think there's anybody in this room who hasn't seen people, naked people, having sex on TV," said Fabio, 14.

Everyone nodded.

One phenomenon students grapple with is the ficar--usually used as a verb, meaning "to stay," in Portuguese. Years ago this time-honored tradition of dating, or staying with, someone for one or two days typically involved hugging and kissing. Today, students and counselors say, these quickie relationships often include a lot more, including sex.

A ficar can begin anywhere, the students said. "At parties, between classes, on the street, if you see someone you like at the bus stop," said Janaina, 16. Two-thirds of the students said they have participated in such relationships; five of them have had at least 10 ficars in the past year.

"We try to teach faithfulness to one partner," Cotrim whispered as she listened to the students, "but they don't seem to understand the danger of these one-day relationships."

They say they do.

"I had a friend who got pregnant, and she couldn't remember the name or the face of the boy," said a girl named Iracema, 17.

The discussion revealed that boys control the ficars. They don't want to date for more than a day or two, the girls said, so what choice do we have?

Counselors and girls say this deference extends to sexual relations. "Sometimes the girl wants the guy to wear a condom and he says no, and the girl gives in," said Patricia, 16, who said she is sexually active but doesn't take part in ficars.

Marcelo, 14, said he knows about condoms, but "I thought that when I had sex with someone, the only way I could get sick was through bad luck. So I would have sex, and I would think, 'Today's my lucky day. There's no way I'm going to get AIDS.' " He said the lessons he has learned through the workshops have helped him discard such beliefs and to apply the lessons Rosemeri said she ignored.

Today, Rosemeri is struggling to finish a government program designed to help teenage mothers earn their high school diplomas. The program pays her $75 a month, and the children's father--with whom she has broken up--helps her financially. She has no immediate job prospects.

She said she regrets leaving school after the birth of her first child but that it was too exhausting. She would creak out of bed at 6 a.m., go to classes, return home to help with housework and care for her child, and fall asleep by 6 p.m. Then she would wake up and do her homework.

"I don't regret giving birth to my children, but if I had known it would be like this, I wouldn't have become pregnant," she said. "If I could give teenagers advice, I would tell them--use precautions."

Brazil's Sexual Revolution

Over the past few years, the rate of teen pregnancy has risen dramatically and its social impact worries officials.

Some facts reported by the Brazilian Health Ministry and the United Nations:

* One in 10 Brazilian girls between 15 and 19 has at least two children.

* 50% of teenagers who become pregnant in Brazil have their first child by the time they are 16.

* Only 14% of sexually active teenagers between 15 and 19 say they use contraceptives.

* 40% of Brazilian teenagers who become pregnant leave school either to get married or work.

How Brazil compares with other nations in a study by the Alan Guttmacher Institute.

Annual number of abortions per 1,000 girls ages 15 through 19:

U.S. 36

Dominican Rep. 36

Brazil 32

Colombia 26

Peru 23

Britain 19

Japan 6

Unmarried adolescent males are likely to have several partners over one year:

Sexually Partners

Country active per year

Brazil 61% 2.6

Kenya 54 1.6

Ivory Coast 43 2.4

Tanzania 37 2.5

Thailand 29 3.8

Togo 18 2.0

Philippines 15 1.8

Data for charts refer to latest years available, ranging from 1994 to '96

SOURCE: "Into A New World" by Alan Guttmacher Institute

CAPTION: Rosemeri Rodrigues Viana, 17, plays in her home with her son Alex, while daughter Thayma peeks in from the floor below.

CAPTION: Rosemeri swings her daughter by the arms in one of Rio de Janeiro's largest slums, where she and her children live with her mother in a three-room shanty.