Being a mother is "the most important - and most humbling - job I've ever had", Facebook's number one woman has said.
Sheryl Sandberg shared an emotional post as Mother's Day was celebrated in the United States, urging people to give "special thanks" to the women raising children on their own.
The social media giant's number two executive said she understood single mothers' plight "a lot more" since losing her husband Dave a year ago in an accident involving a treadmill.
Life since her husband died had become "a new and unfamiliar world", the mother-of-two said.
"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. How often situations would come up that Dave and I had never talked about and that I did not know how to handle on my own."
-- Adam Grant (@AdamMGrant) May 7, 2016
Sandberg said the odds were stacked against single mums, many of whom live in poverty, work two jobs or don't get paid leave to care for themselves or children if they get sick.
Same-sex couples were particularly vulnerable because they often do not have the same rights as married heterosexual couples, she added.
She said: "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.
"Our widespread cultural assumption that every child lives with a two-parent heterosexual married couple is out of date."
Sandberg admitted her 2013 book Lean In should have given more attention to the difficulties women face when they have an unsupportive partner or no partner.
It was important to "recognise that the world does not make it easy" for single mothers, she added.
"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."