Ex-Wall Street titan Sallie Krawcheck on the No. 1 mistake women in their 30s tend to make

Professional women in their 30s have often achieved a level of career success that they dreamed of in their 20. Also, women in their 30s are often focused on their personal lives in a new way. They may be cultivating relationships or raising children.

That means the one thing professional women in their 30s often forget to do is network.

That lapse often helps men leapfrog women in their careers at that stage, says Ellevest CEO Sallie Krawcheck in her new book, "Own It."

"Our thirties, once we've all proven that we're good at our jobs, is exactly when networking becomes that much more valuable," says Krawcheck. "This is the time where the guys' network is one of the key reasons they can move past us at work. They are going to the cocktail parties."

Women aren't skipping the networking events because they don't know how important they are.

On the contrary, women say "networking is the number one unwritten rule of success in business," writes Krawcheck. This is particularly true for female entrepreneurs. For women looking to start a business, the breadth and depth of your network "can be absolutely key."

Women in their 30s just don't make it a priority. "Life is busy, after all," says Krawcheck.

In particular, if a woman has taken time away from the office to have kids, then networking is even more critical. "We're playing catch-up (and can be all the harder, because going out and meeting new people usually means missing a few bedtimes)," she writes.

"That's why God gave many kids fathers," Krawcheck tells CNBC. "Dads can take care of the kids."

Krawcheck knows what it takes to climb the career ladder. After establishing herself as a top-ranked research analyst, she served as CEO of Smith Barney, CEO of Merrill Lynch Wealth Management and CFO of Citigroup. She's now the co-founder and CEO of Ellevest, a digital investment platform for women that aims to close the gender gap in investing.

Also, your standing bestie brunch date does not count as networking. Your closest friend is not as likely to be your key to your next professional opportunity as is someone you meet outside your inner circle, Krawcheck says.

"The truth is that your next business connection is far more likely to come from someone you barely know, a loose connection, than from a friend or close connection," says Krawcheck.

"You and your friends and your work colleagues, and the people you speak to all the time, tend to traffic in the same connections and information."

It's people who are outside of your cohort who know about opportunities that you and your friends don't.

"So do yourself a favor. Find at least one hour each week to network, in person. Call the babysitter, duck out of work early, skip just this one spin class," says Krawcheck.

"Whatever you have to do to find the time, find it. Trust me, in the long run, it's worth it."

For more about Sallie Krawcheck's career, don't miss " Ex-Wall Street titan Sallie Krawcheck reveals the jaw-dropping sexual harassment she endured—and why she didn't report it ."