I didn't have to figure out how to break the news of Jamie Lynn Spears's pregnancy to my kids. Disconcertingly attuned to all celebrity news at ages 10 and 12, the girls broke it to me at the dinner table -- along with an explanation of who, exactly, Jamie Lynn Spears is.

It turns out, in case you are a fellow inhabitant of Planet Clueless, that: (1) this is a different person from Jamie Lee Curtis; (2) Britney Spears has a younger sister; (3) she is the "good one."

It also turns out -- and this is about the closest I can get to a music joke -- that, oops, Jamie Lynn did it, at least once. The supposedly virginal star of Nickelodeon's "Zoey 101," is, at age 16, three months pregnant.

Okay, Teachable Moment Alert. But what, exactly, to teach?

Mom: So, what do you think the lesson is here?

Ten-year-old Julia, brightly: Don't have sex until you get married!

Uh, um, is that the lesson? Did I hear Daddy's car in the driveway? Anybody want more peas?

This is the conundrum that modern parents, boomers and beyond, confront when matters of sex arise. The bright-line rules that our parents laid down, with varying degrees of conviction and rather low rates of success, aren't -- for most of us, anyway -- either relevant or plausible. When mommy and daddy didn't get married until they were 35, abstinence until marriage isn't an especially tenable claim.

Nor is it one I'd care to make. Would I prefer -- as if my preference much matters -- that my daughters abstain until marriage? No; in fact, I think that would be a mistake. But I'm not especially comfortable saying that, quite so directly, to my children, partly because that conversation gets so complicated, so quickly.

A few weeks ago, the girls and I were watching "Gossip Girl," the odious television series about overprivileged Upper East Side teenagers. (In a bad parental bet, I OK'd this show at the start of the season, thinking it might offer some cautionary tales about wretched excess. Turns out the kids consider it more of a roadmap. But that's another column.)

In this episode, one high school girl was about to have sex with her boyfriend.

Insert maternal throat-clearing:

Mom: I'd like to point out, for the record, that I don't approve of this behavior.

Emma, 12, with an air of worldly sophistication: Oh, Mom, don't be ridiculous! How old were you when you had sex?

OMG, as the kids say. Is there a parental equivalent of the Fifth Amendment?

I am saved by Julia, who announces that I am so irredeemably dweeby -- Emma, she was in the chess club, for goodness sake -- it is inconceivable that I had sex with anyone before Daddy.

Problem dodged -- for now, anyway.

And this is where, I think, the Spears news was actually a welcome development. It's generated a lot of hand-wringing, heartfelt but ultimately misplaced, over what message the fallen role model sends to tween fans. Sorry, but I don't imagine a lot of 16-year-olds, in the grip of hormonal urges, thinking: Gee, Jamie Lynn did, why not go for it?

But they might think: Whoa, birth control.

The facts are sobering. More than 60 percent of high school seniors report having had sex at least once. One in three girls in the United States gets pregnant by age 20.

And so the message I choose from Spears's pregnancy -- and the one, once I recovered my composure, I ultimately delivered, is this: It could happen to you -- even if you're the kind of "conscientious" girl who, as Jamie Lynn's mother described her, is never late for curfew. And so, whenever you choose to have sex, unless you are ready to have a baby, don't do it without contraception.

I saw the movie "Juno" the other day, and I'm considering taking Emma, despite the edgy content, because it reinforces that point, this time stripped of the distracting veneer of celebrity. Juno MacGuff is a cheeky 16-year-old who finds herself confronting the implacable plus-sign of a positive pregnancy test after a single encounter with her sweetly clueless high school boyfriend.

Juno chooses to continue her pregnancy, arranging to give the baby up for adoption, and the movie evokes the difficult reality of being a lumberingly pregnant high schooler. Crowded hallways part, classmates stare as Juno waddles to her next class. Juno pretends to be unconcerned when her Tic Tac guzzling boyfriend asks another girl to the prom.

As the convenience store clerk announces when Juno shakes the stick of a home pregnancy test like a balky Etch A Sketch: "This is one doodle that can't be undid."

That's the lesson Jamie and Juno didn't learn in time. But perhaps their pregnancies -- one fictional, the other all too real -- will teach others that one, unassailable truth in a world of otherwise muddled messages.

Marcus is a member of The Washington Post's editorial page staff.

(Dec. 27, 2007)