On the surface, maternity leave is simple: You have a baby, you take time off work to take care of yourself and your baby, and then you go back to work.

But now a new breed of working mom is quickly redefining the concept.

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From Marissa Mayer, who assumed the helm as CEO of Yahoo while six months pregnant, and announced she’d only be taking “a few weeks of leave,” she’d “work through,” to moms who use the “downtime” to launch new businesses, they’ve certainly changed the new mommy landscape.

The question is: By working through mat leave—or using that time to alternate between 4 a.m. breast-feeding sessions and hatching a business plan—are they helping or hurting women? And have we officially entered the age of the maternity leave overachiever?

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Dr. Jennifer Gardner, a pediatrician, used her maternity leave after the birth of her first child, William, to jumpstart an idea she had been sitting on for years: Healthy Kids Company, which educates families on the importance of nutrition.

“Maternity leave was actually a great time to start working on it. I knew I was eventually going to return to full-time work as a pediatrician, but while my son was sleeping, I had the free hours necessary to build my website,” she says. ”Starting a company to educate families had always been a passion of mine, but prior to maternity leave, I didn’t really have the time to devote to my idea.”

“The day I gave birth to my second child two weeks ago, I was also doing PR for an event in New Jersey with Bravo and the Real Housewives.”

“My maternity leave was my security blanket,” agrees Bridget O’Brien, a former New York City school teacher who used her time away two years ago to found her own PR and event planning firm. “For years, I had been doing event planning free of charge for friends, nonprofits and charities. Working as a public school teacher in New York City is a noble job, but I hated the political aspect behind the scenes.”

O’Brien talked the idea through with her husband—she knew she would be happier and that it would create a better environment for their daughter.  ”I would be able to work from home and set my own schedule,” she explains, so she used her mat leave to start amassing clients. After giving birth in January 2010, she officially launched her company in May of that year.

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In fact, O’Brien says that while running her own business can be stressful, she doesn’t resent her even busier schedule. “The day I gave birth to my second child two weeks ago, I was also doing PR for an event in New Jersey with Bravo and the Real Housewives,” she says. “Though I was in labor and handling work on my cell phone, it’s easier because it’s my own company. If I were doing this for someone else, I wouldn’t have been as passionate.”