When author Sabrina Weill was going over a national survey on teens and sex for her new book, one figure in particular jumped out at her.

When asked whether sex should be romantic, nearly one-fifth of 1,059 12- to 17-year-olds answered: "Don't know."

Coupled with the news from a government study from the Centers for Disease Control that teenagers who have not yet had intercourse are having oral sex, the information tells Weill that today's young people have no idea what intimacy is. And that it's up to their often-squeamish parents to tell them.

In "The Real Truth About Teens & Sex" (Perigee, 2005, $23.95), Weill says teenagers' sexual behavior has gotten increasingly public — and casual.

She reports that nearly one in four 14-year-olds claim they knew peers who had had sex at home — while their parents were home. One in 10 teens agreed that it is "normal for someone my age to have sex with someone they met at a party."

As a teen magazine columnist and editor — she was founding editor of CosmoGirl! and is a former editor of Seventeen — Weill spent years talking and corresponding with teenagers about sex. The topic needs to be discussed much earlier than most parents think it does, Weill says.

Her survey, produced in conjunction with the National Campaign to Prevent Teen Pregnancy, shows that by the time they are in ninth grade, a third of teens are having sex.

"I think parents don't want to seem uncool," Weill said. "There's really a generation gap and a communication gap."

As a result, many teenagers aren't sure just what the rules are. "If you knew for sure your parents didn't want you having sex, you probably wouldn't be having it when they were home," Weill said.

She says that while teenagers appear to have become more sophisticated and more casual about sex, their judgment and emotions are still as fragile as they ever were. And that's an explosive combination.

"I rarely hear from girls that 'I'm into friends with benefits, it's good for my self-esteem,' " Weill said. "I think boys do have feelings, too, and they can feel hurt after and they can feel used. They don't always want what they say they want to do."

But the good news is that parents who do send clear messages to their kids can have a lot more effect than they may think.

In a separate 2004 National Campaign to Prevent Teen Pregnancy poll, 87 percent of teenagers said they believed it would be easier for teenagers to postpone sexual activity if they could have more open, honest conversations about the topic with their parents.

Robert Blum, a professor at Johns Hopkins' Bloomberg School of Public Health who studies sexual behavior of adolescents, said research shows that teenagers can process messages about sex from their parents that may seem contradictory. "They can understand messages of 'not now,' and messages of contraception," he said.

The messages shouldn't come in the form of one big talk, Weill and Blum said, but in a running conversation and commentary. Parents should start by watching television with their teenagers and listening to their music. When they see sexual situations, they can start talking about what's on screen in an open-ended way. What are the potential ramifications of what's happening? How might the characters feel the next day?