In putting together Fortune’s 2009 list of the 40 Under 40, here’s the trend that stood out: Men outnumber women in our rankings by a ratio of 7 to 1.

Yes, there are only five women on the list. We can express indignation at this because it wasn’t our intention or our choice. In fact, we shook the trees pretty hard to make sure we weren’t missing anyone.

Make no mistake: The women who made our list–financial analyst Meredith Whitney, Google VP Marissa Mayer, Ning CEO Gina Bianchini, CNBC anchor Erin Burnett and Coca-Cola marketing’s Wendy Clark–have all zoomed past men in their respective fields. But there are no women listed who, say, founded a Google or a Facebook or a Twitter. None who are self-made hedge fund billionaires. None who made it to the top 10. So, while the accomplishments of those five women are tremendously impressive, they still, for the most part, trail those of most of the men on our list.

What gives? I asked Sheryl Sandberg, COO of Facebook and one of the most prominent leaders in Silicon Valley, to weigh in. She cited some interesting numbers: Just 15 of Fortune 500 CEOs are women. Yet at the college level, more women are getting degrees than men–and even some professional schools are graduating more women than men. “So the question is,” Sandberg says, “what happens between leaving school and age 40 to make this list predominantly male?”

Sandberg (who, by the way, would have made the 40 Under 40 list if she hadn’t turned 40 this year) says she suspects that “disequilibrium of household responsibilities,” as she calls it, is a key reason women slip behind. Even when both men and women in a household work fulltime, she notes, studies have shown that women do the majority of the childcare and housework. Perhaps that’s not so surprising, but Sandberg’s point is that advancements on the domestic front seriously lag the gains women have made in the workforce.

“Over the past 40 years, we have made more progress in the workplace than in the home,” Sandberg says. “I believe the No. 1 thing we could do to change the numbers in the professional world is to find a way to balance responsibilities in the home.”

McKinsey director Joanna Barsh, who pioneered the firm’s Centered Leadership Project to help develop women leaders, thinks along the same lines. She points to the practical limitations that child rearing can put on women’s careers. “Not everybody slows down,” Barsh says. “But an awful lot of women choose to slow down and enjoy those years.” And for many women, she notes, the slowdown comes at the precise time when career opportunities tend to surge. “Your hormones are causing you to want to take care of everyone in your family at the very moment when you’re building your goal lines.”

Many women “zig and zag” rather than pursue one goal in a straight line, as most men are programmed to do. Women can still accomplish a great deal career-wise, but it might happen when they’re well past 40. Barsh’s idea for fixing the gender imbalance on the 40 Under 40 list? “Give women a ten-year handicap!”

Well, we can’t do that. But we can hope to see more women in the 40 Under 40 rankings in coming years. To be sure, half the people on our “Ones to Watch” list of fast risers are women. I asked Sandberg how long she thinks it will be until a woman co-founds a $100 billion company. Her answer: “Who says she hasn’t already?” She makes a good point. And we’ll be watching.

P.S. Tune into Postcards tomorrow for further discussion. And for more of Sandberg’s insights about navigating a successful career, read her first-personer, “Don’t Leave Before You Leave.”