How Sheryl Sandberg's take on single moms shifted after her husband died.

Her husband, Dave Goldberg, died unexpectedly while the pair were on vacation. The tragedy placed Sandberg, the COO of Facebook and a mother of two, in a role she'd never been in before: single parent.

"For me, this is still a new and unfamiliar world," she wrote in a Facebook post in honor of Mother's Day on May 6, 2016. "Before, I did not quite get it. I did not really get how hard it is to succeed at work when you are overwhelmed at home. I did not understand how often I would look at my son’s or daughter’s crying face and not know how to stop the tears."

And some of those messages, she noted, should have been crafted differently.

"Some people felt that I did not spend enough time writing about the difficulties women face when they have an unsupportive partner or no partner at all," Sandberg wrote of her best-selling book, "Lean In." 

"They were right."

"Lean In," a best-seller that coaches women (and men) on how to counter gender bias at work, was no stranger to criticism. Many felt Sandberg downplayed the effects of systemic sexism in the work place, as well as overlooked the unique challenges faced by disadvantaged women and women of color. 

Judging from her post, however, it seems being a single parent is giving Sandberg some new perspective:

"I will never experience and understand all of the challenges most single moms face, but I understand a lot more than I did a year ago."

Noting she's "extremely fortunate" to have the financial means to support her family, Sandberg said she's in the minority — "the odds are stacked against" most moms doing it alone. And even though the number of single-parent households has been on the rise, our workplace policies don't reflect this modern reality.

The U.S. remains the only advanced economy in the world that doesn't require employers to guarantee paid maternity leave, as Sandberg pointed out. Far too many single moms also work in jobs that don't provide paid time off if they or their children are sick.

How are working single moms — a group that is already disproportionately poor — supposed to get ahead when they're living in these circumstances?

"Single moms have been leaning in for a long time — out of necessity and a desire to provide the best possible opportunities for their children," she said in her post.

"Being a mother is the most important — and most humbling — job I’ve ever had. As we rightly celebrate motherhood, we should give special thanks to the women who are raising children on their own. And let’s vow to do more to support them, every day."