Before I had children, I assumed I'd be the kind of mother who'd never be embarrassed by my child's sexual nature. There'd be no question or behavior that would render me speechless, and I'd be just too evolved to blurt out the ridiculous warnings issued back in my grandparents' time ("Don't play with that thing; it'll fall off!").

After all, my parents were open about sex with me, so why would it be difficult to be open about sex with my own child?

I'm humbled to admit that the first time I noticed my newborn's sexual nature, those good intentions were suddenly on a time machine heading back to the 1950s.

As I changed Oliver's diaper one morning when he was about 6 months old, I noticed that his penis was red and swollen, up to maybe four times its normal size. It was enormous  -- for a baby penis, that is. My sleep-deprived brain burst into a frenzy of maternal paranoia: Could it be an infection? A blockage? I shrieked for my husband, who came in, took one look, and laughed. "He's got an erection," he said. As if on cue, Oliver grabbed hold of his penis and grinned at me.

It was a classic "duh" moment. I knew baby boys got erections. But it was one thing to read about them and another to see my infant son actually sporting one. As a new mom, I was anxious about every physical change in my baby's body, but my first encounter with a sexual one threw me for a loop. I wondered: Did I cause the erection during the diaper change? Should I have avoided touching his penis when it was hard? Did I hurt him by diapering over it? Does this mean I'm going to be utterly clueless when I'm confronted by his evolving sexuality as he grows?

Daunted by the thought of the next 16 years (plus) of sex education, I spoke with Justin Richardson, M.D, an assistant professor of psychiatry at Columbia and Cornell universities. "Even though we're so hip and cool," he says of today's crop of moms and dads, "we're as anxious as previous generations were about discussing sex." Our parents and grandparents handled a child's sexuality by pretending it didn't exist. Today's parents think we ought to be oh-so-matter-of-fact. So why do we get all squirrelly when we undiaper morning erections or see our 4-year-old mimicking Britney Spears's pelvic thrusts?

We are evolutionarily conditioned to avoid our children's sexuality, says Dr. Richardson. (Sigh of relief. I'm not a prude.) The feelings of discomfort and dread that surface when we have to acknowledge their sexual nature can be attributed to the "incest taboo," a psychological fire wall that keeps us from having sex with close relatives. The "ick" feeling we get when we picture our parents having sex, for instance, is part of that taboo, which works so well that it often prevents us from talking to our children about sex.

"You don't have to wait till you feel comfortable," Dr. Richardson says. "It may never feel comfortable. Acknowledge to yourself that you may be anxious, but set that aside and go ahead anyway." In their book, Everything You Never Wanted Your Kids to Know About Sex (But Were Afraid They'd Ask), Dr. Richardson and coauthor Mark Schuster, M.D., stress that it takes practice, which is actually a good thing because sex shouldn't be a one-time topic of conversation. Bringing it up over time is the best way to make sure your child knows it's not shameful. Plus, his own sexuality will grow and change. It isn't something that will pop up during puberty  -- it's right here, right now, staring up at you from his changing table. Here's what's going on down there:

Diana Burrell lives in Westford, Massachusetts. Her son, Oliver, is now 2.

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