Just when the tantrums have subsided and you think it’s safe to take your child on an extended shopping trip again, don’t be surprised if you encounter another land mine in the checkout line.

“Mommy, how did the baby get into that lady’s tummy?” your 5-year-old asks in a loud voice, pointing at the very pregnant woman in front of you.

As unnerving as such questions often are for parents, they’re completely normal. “In preschool, kids start noticing and asking questions about how mom and dad have different body parts,” says Jenna Saul, MD, a child and adolescent psychiatrist in Auburndale, WI. “Then, by the time they turn 5, the curiosity about body parts turns into a preoccupation with where babies come from.”

At my own house, the conversation began even earlier. At 2, my daughter spotted a scar on my stomach, and I fumbled my way through a TMI explanation of a C-section: my first sex talk fail.

That first (uncomfortable) sex question

Whether the first sex question happens in private or very much in public, it catches almost every parent off guard.

Katrina Alcorn, an Oakland, CA, blogger, says she never worried about the “sex talk.”

“I didn’t think it would be a big deal,” says Alcorn, who has three children. “I’m progressive. I’m body positive. I’ll make sure my kids know what they need to know.”

Then, in the car one day, Alcorn’s second grade daughter announced that she wanted to marry a girl because she didn’t want to die in childbirth.

“I was just floored,” Alcorn says. “But I tried to gather my thoughts and address her concerns one by one. I said, first of all, it’s really rare that people die in childbirth, and I don’t think that would happen to you. Second of all, it’s fine if you want to marry a girl, and you don’t have to decide now. Finally, you can adopt a baby whether you’re with a boy or a girl.”

Alcorn was proud of herself for dealing with her daughter’s questions with such aplomb — but in the end her child got the last word. “She said, ‘I still want to marry a girl because I think kissing boys is gross and anyway, I don’t want to have sex.’ I couldn’t believe the sex talk snuck up on me without me being prepared for it!”

Why you should talk sooner rather than later

Although teenagers today are waiting longer to have sex, research shows that 13 percent have had sex by age 15, and by their 19th birthday, seven in 10 teens have had intercourse. And because young adults are not marrying until their mid-20s, on average, this means they’re at increased risk for unintended pregnancy and sexually transmitted infections.

So even though talking to your young child — preschool to fifth grade — about sex may seem premature, it’s actually the ideal time to do it. As your child enters her tweens and teens and becomes self-conscious about her body and about personal matters in general, it will become increasingly difficult to raise the issue. Take advantage of this window of opportunity to create a foundation of openness and honesty with your child.

“This stuff is very hard and complicated to talk about, but for me it is a health issue,” says Robie H. Harris, a former teacher and now celebrated author of a series of children’s books about sex and the body, including It’s Perfectly Normal, and Who Has What. “I write these books because I feel that this is part of life, and it’s okay to wonder about it. It’s important not just to kids’ physical health, but also to their emotional health.”

Not one talk, but many

Most experts agree that “the talk” really isn’t a talk anymore, but an ongoing conversation, one that starts much earlier than it did even a few decades ago.

“Limiting your child’s education about sex to a single talk produces an atmosphere of shame,” says Wendla A. Schwartz, M.D., a child and adolescent psychiatrist and medical director of Solutions Psychiatric Associates in Los Gatos, CA. “Children will definitely ask, and if a parent has in his mind that a 5-year-old isn’t ready for ‘the talk,’ then he gets flustered and says, ‘Go ask your mother,’ and then the mother gets flustered. Kids are great at detecting discomfort, so by the time ‘the talk’ comes around at puberty, they’ve got the idea that sex is shameful and bad, and that’s going to stick with them forever.”

Instead, make it an ongoing, low-key dialogue between you and your child that begins when they are very young and goes on throughout their tween and teenage years.

What to say, how to say it

When it comes to sex, the best strategy is to let your child lead the discussion, rather than giving her a full-blown, lengthy presentation.

“In the very early ages, parents need to focus their efforts on really listening to their children and answering their questions truthfully,” says Saul. “At first, using the child’s own language to describe body parts is a good way to make kids comfortable; then you can teach them the actual names — penis, vagina, womb — as it becomes appropriate.”

Schwartz agrees that parents should let kids take the lead. “The best approach with all kids is to only answer the question they ask,” she says. “One of the really beautiful things about young children is that they’re incredibly inquisitive. They have such a tremendous level of curiosity that you really don’t have to worry that they’re going to forget to ask. As they’re ready for the information, they will probe for it.”

So when the questions start coming, give as brief and as honest an answer as you can and know that when they’ve learned enough, they’ll tune out — and that’s fine. Be prepared by having some age-appropriate books on hand before your child starts asking questions. Robie Harris recommends reading through the books by yourself first, to make sure you agree with the information and the way it’s presented. Books can help neutralize a charged topic; they also give your child the opportunity to do additional research on her own.

Kids are resilient

Don’t worry if you flub the sex conversation the first time — or even the second.

“We all make mistakes,” says Schwartz, who has stumbled on the topic of sex with her own kids. “Don’t freak out if you don’t get things right. Remember: over the years you’ll get plenty of chances to ‘practice’ giving good information. Besides, lucky for us, kids are amazingly resilient.”