Working moms may not want to have it all, but they certainly have to do it all.

Mornings are all about getting out the door to make it to school and work on time, while evenings are basically the same, but in reverse—with night meetings and school events thrown in for good measure.

If only home and work could co-exist peacefully. It's something that rarely happens—and something that can seem like an impossibility for new moms to master as they re-enter the workforce.

Still, there are ways to make life a little easier, says Lauren Smith Brody, author of the new book . Writing in Glamour, Smith Brody delved into her best strategies for surviving what she calls the "fifth trimester":

"It takes three trimesters to gestate a baby, another 12 weeks, the fourth trimester, for this tiny person to wake up and get acquainted with the world. But when you go back to work, whenever that might be, that’s when things really get complicated. Welcome to the fifth trimester. I desperately craved a script to help me navigate this transition."

Why is this transition so hard? Because the workplace is not set up to support working parents. Indeed, every study from the Working Mother Research Institute finds that the No. 1 benefit working moms (and dads) say they want from their employers to make their complex lives work is workplace flexibility.

For some working parents, flex may mean starting your Thursdays later so you can make it to a Mommy and Me class, while for others it may be the ability to work from home some days to save time on your commute—and everything in between.

Smith Brody agrees, adding that her survey of 732 working mothers from around the country uncovered a simple powerful thought about workplace flexibility: You won't get it unless you ask for it.

So how to ask your manager for flex? "The basic tenets of the 'ask' are the same across all types of jobs: honesty, transparency and a commitment to complete the work you were hired to do," Smith Brody explained.

Indeed, it's important to your manager's point of view as well when making the ask. In Working Mother, for example, Penny Locey, a consultant with Keystone Associates, a Boston career management firm, suggests asking for a one-month pilot test of your new schedule. After a month, you and your boss can assess how it worked. “You have to establish trust,” she said.

Because above all else, you need to remember that you are being paid to do a job that still needs to get done. Lay out your needs and your solution and your manager is more likely to make flex happen for you.