The uncut version of Yahoo CEO Carol Bartz’s first-person “Just Deal With It,” which we published on Postcards last Monday, drew lots of traffic. So we’re giving you an unedited version of another first-person piece that appeared in Fortune‘s Most Powerful Women issue (September 28). This one is by Facebook COO Sheryl Sandberg. The most senior woman at Google before she joined Facebook, Sandberg is one of the fastest rising stars in business–leaping to No. 22, from No. 34, on this year’s MPWomen list–and one of the youngest too. Her resume includes two degrees from Harvard, stints at the World Bank and the U.S. Treasury, board memberships at Starbucks and the Brookings Institution…and she’s only 40.

In fact, it was Sandberg’s out-of-the-blue phone call from Mexico, where she was celebrating her 40th birthday with old girlfriends, that led to this piece. “I want to write something called “Don’t Leave Before You Leave,” about young women cutting back their career ambition, and would you consider running it your Most Powerful Women issue?” she asked me over a static-y cell connection. I immediately said “Yes” because I knew Sandberg’s commitment to encouraging the next generation of women leaders. (Her home dinner gatherings of established and up-and-coming women are sought-after invites in Silicon Valley.) I also knew Sandberg to be an adept juggler of family and career. What I didn’t know: She can write. So here is Sheryl Sandberg’s “Don’t Leave Before You Leave,” the unedited version:

Last week at work I had a conversation with a woman I will call Jamie. We have a new project, and I offered her the opportunity to be its leader.  She seemed flattered to be asked but then quickly became very hesitant. She told me she wasn’t sure she should take on more right now. Just before she got up to leave, I looked at her and quietly asked, “Are you worried about taking this on because you are considering getting pregnant sometime soon?”

A few years ago I would have been afraid to ask such a direct and personal question. Nothing is more private than the decision to have a child. Bringing up that topic in the workplace feels like a dangerous thing to do.  We are not supposed to show any bias or take childbearing plans into account as we manage people. But after watching talented woman after talented woman let her career go before she actually leaves it, I now ask this question and I ask it directly.

I always give people the option of not answering, but so far, everyone has appeared grateful for a chance to talk.  There is just one reason why I ask–to make sure people aren’t leaving before they leave.

Here is what happens.  An ambitious and successful woman starts considering having children, typically once she finds a domestic partner. She thinks hard about how busy she is and realizes that finding time for a child means something will have to give. As soon as that thinking process starts, she is already looking for ways to scale back. She no longer searches for new opportunities; if any are presented to her, she is likely to decline or offer the kind of hesitant “yes” that gets the project assigned to someone else, just like “Jamie” did last week in my office.

The problem is that even if she gets pregnant immediately, she still has nine months of pregnancy ahead of her, months of maternity leave and then another lengthy period after returning to work to even catch her breath. And since women usually start the thinking process before even trying to conceive, often several years actually pass. By the time she is back to focusing on her career, she is in a radically different place than she was before.

She was always a top performer–always on par with her peers in responsibility, opportunity, and pay. But now she is not. By not finding ways to stretch herself during the years before she has a child, she has fallen behind.

While I don’t believe that the choice to work fulltime and be a parent is the right choice for everyone, it is a wonderful–and often necessary–choice for many people. I also believe that once you have a child, it becomes necessary to make real changes, including potentially deemphasizing your career. But slowing down too early is a mistake that too many women make today, often without even realizing it. Because they sincerely want to stay in the workforce, they try to make room for everything and they slow down–or unconsciously pull back–well before their circumstances actually change. By the time they fully return, they are in jobs that no longer challenge or reward them enough to hold their attention.

I don’t know any women–or men for that matter–who do not have days when they wonder if leaving their children in someone else’s care for their careers is the right thing to do. I know I do. If your job feels less fulfilling because you have been in the same role for too long or are no longer paid comparably to your peers, that choice becomes a hard one to make day after day. One of the tragic ironies for working women today is that the very desire to stay in the workforce leads to decisions that eventually cause them to leave.

No one can know in advance the choices they will make after going through a life change as profound as becoming a parent. But if you want to preserve the option of staying in the workforce and building a career, my advice is simple.  Stay fully engaged, take on new and interesting challenges, and do so until you have a child. Keep your foot on the gas pedal until your life actually changes. Then you can make the decision to keep driving quickly, slow down, or step out of the car.

I joined Facebook as its COO when I had just returned to work from having my second child. The timing was far from ideal. As many people had told me–but I had not believed–having two children was more than double the work of having one. At the time I was not looking for a new opportunity but rather trying to get through each day. But both my husband and I recognized that if I waited until the time was exactly right, the opportunity would be gone. So I jumped in.

I can’t say it was easy. The first six months were a struggle both at work and at home. But now I am settled in, finding just enough balance to make it work, and learning and growing with new responsibilities and challenges.  Looking back, if I hadn’t taken on something new, I might easily have left the workforce by now, because it would not have been worth making the daily tradeoffs to continue in the job I’d held for the previous six years.

There is a broader lesson here that applies not just to women contemplating starting a family, but to anyone trying to plan for the future. Making decisions too early, trying to plan life too carefully, can close doors rather than keep them open. Any time you make a plan, you do it with imperfect information; the further in advance you make that plan, the less information you have. You never know how you will feel or what choices you might face.  Take life one step at a time and don’t make decisions before you have to.

A few months ago we were interviewing a fantastic woman to join Facebook’s Business Development team. After we extended an offer, she came in to ask some follow-up questions about the role. She did not mention lifestyle or hours. But she was the typical age of the people who leave before they leave.  So I shocked her by asking the question no one asks. “Priti,” I said, “I’m sorry for bringing up something so personal, and feel free to tell me you don’t want to discuss it. But just in case you are thinking that you might want to have a child sometime soon and need to stay where you are to have room to slow down, I’d love a chance to tell you why that makes it even more important that you change jobs now.”

Priti accepted our offer. And just a few weeks later, she found out she was pregnant. Her timing could not have been better.