Do you think your teen is having sex? Many parents would say no. The harsh reality is that only about half of those parents would be right.

Children are engaging in sex at younger ages than in the past, experts say, and research indicates that about 40 percent to 50 percent of 15- and 16-year-olds in California are having sex.

Combine that with a steady increase in sexually transmitted diseases among teenagers and a rising teen birth rate, and you have a growing need for parents to educate their children.

"Comprehensive sex education really makes a difference," said Dr. Glennah Trochet, Sacramento County's public health officer. "People always think teens aren't influenced by their parents, but studies show that what their parents tell them, especially if they are very clear, really does influence young people."

Sacramento County has essentially had an epidemic of chlamydia and gonorrhea among 15- to 24-year-olds for the past nine years, she said. About one of every 25 females and one of every 80 males ages 15 to 19 in Sacramento County had a reported case of chlamydia or gonorrhea in 2008, according to the state Department of Public Health. And those are only the reported cases, said Trochet, who explained that people may go for years without knowing they have a disease.

"Young people are having unprotected sex, and we need to do a better job of sex education and maybe getting kids to delay sexual activity," Trochet said.

Birth rates among 15- to 19-year-olds also rose to 38.8 per 1,000 in Sacramento County last year, up from 37 per 1,000 in 2006, mirroring a national trend.

Given those statistics, what's a parent to do?

Start talking.

"Parents should know that even if their children are not sexually active, they probably are exposed to peer groups who are sexually active," said Dr. Angela Rosas, a pediatric gynecologist with the Children's Specialists Medical Group of Sacramento. "Questions will come up."

Here is some advice from the pros on having these critical conversations.

Start early: Age 10 or 11 is not too young to start the discussions, said Dr. Julius Licata, a clinical psychologist in Orefield, Pa., who runs www.teencentral.net, a Web site that offers teens anonymous access to expert counseling and advice.

"I'm seeing kids 13 and 14 saying they have had multiple sex acts with people," he said.

If questions haven't arisen by pre-adolescence, the parent needs to initiate the conversation.

You may be uncomfortable talking about sex. That's OK. Acknowledge your discomfort while telling your child that this is too important a topic to let embarrassment get in the way.

There's no such thing as the perfect time: Sitting down with your child and saying you're going to talk about the birds and the bees isn't always the best method. Look for "spontaneous" teachable moments in the car, on a walk. Perhaps you'll notice a pregnant teen at your child's school or happen upon a love scene in a movie while watching with your kids.

Let your child know that you are receptive to questions by saying something like, "I'm always here to listen."

Consider getting a book to help with starting the conversation. Grab your spouse, make it a date night and head to a bookstore or library.

Dozens of books are geared to explaining sex and sexual health for teenagers. Take time to scan the contents and make sure the book reflects your values. Read the book before giving it to your child so you're not caught off-guard by questions he or she may have.

What to say: Be clear about your expectations and your family's stance on abstinence and sex and the reasoning behind it, experts say.

Start by saying, "I don't think you're ready," then be prepared to state your reasons. Avoid being defensive, hostile or angry.