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Marissa Mayer, please take your full maternity leave

Congratulations to Yahoo CEO Marissa Mayer, the mother of a three-year-old, who's expecting twin girls this December. In an announcement on the company's Tumblr account, Ms. Mayer expressed the "great happiness" and excitement she and her husband, Silicon Valley investor Zachary Bogue, feel about the news. She shared that her health is good and that she expects "to approach the pregnancy and delivery as I did with my son three years ago, taking limited time away and working throughout."

Marissa Mayer
Eric Piermont | AFP | Getty Images
Marissa Mayer

Mayer made news both when she was hired while pregnant — and when she took only two weeks leave after giving birth. Those of us fighting for paid leave for all workers hope she'll take longer this time. Even with nannies and an equal partner, twins are a huge amount of work — and an older sibling likely will want some special attention as well.

I say this as a working mother, a twin and the great aunt of twins. When my nephew and his wife had their twins, I went to Seattle for a week to help out. The couple also had a four-year-old. Two parents, one grandma and one great aunt – and every one of us was busy every minute. I came back exhausted.

Read MoreYahoo! chief Mayer says she's pregnant with twins

Mayer expressed her commitment to her family as well as her career, both of which she acknowledged will require "hard work and thoughtful prioritization."

Even as I wish Mayer and her family the best, my heart is with all the other mothers in this country who also went back to work within two or three weeks — most of them not by choice. According to data analyzed from a Department of Labor survey by Abt Associates, an astounding 1 in 4 women in the U.S. return to work in that period of time — with disastrous consequences for their own health, breastfeeding and the well-being of their infants.

Unlike Marissa Mayer, these mothers will not be able to bring their babies to a nursery next to their office; they will not have a retinue of professional helpers; and they do not have paid leave. Half of the nation's pregnant workers, in fact, receive no pay whatsoever after giving birth. Many who do are using saved-up sick days and vacation.

Having a baby is a great joy, but it's no vacation — especially if it throws a family into financial chaos.

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Many people are rooting for Marissa Mayer to take a longer leave in order to be a better role model for women in professional and executive positions. Yahoo gives 16 weeks paid to biological mothers, eight to fathers and adoptive or foster parents. How many in senior positions will feel comfortable taking the full time if the top company mom takes so little? There's a real danger that those who take the leave allowed on paper will be looked at as less committed and dedicated — and less competent at time management.

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Most male CEOs behave similarly to Mayer. Let's be clear: Our goal in urging companies toward gender diversity at the highest levels isn't to get women to act like male CEOs, but to change the workplace culture altogether. We need fathers to take the whole time allotted for leave as well — and for company culture to encourage, rather than punish, those who do so.

Many of us are also rooting for Marissa Mayer to take one more step. She's already lengthened the amount of paid leave available for Yahoo employees. We hope she will recognize the disparity between her own situation and that of so many Americans, and join other business leaders in calling for a national family and medical leave insurance fund, where everyone makes small contributions to a pool that enables workers to draw a significant portion of their pay while on leave.

Now, that would be a cause to say Yahoo!

Commentary by Ellen Bravo, the executive director of Family Values @ Work, a national network of coalitions in 21 states working to pass policies like paid sick days and family and medical leave insurance. Follow her on Twitter @Ellen_Bravo.