Cathy Donnelly, 58, will welcome her seventh grandchild when she gives birth to a baby girl in three months.
Her daughter Shannon Fischer discovered that she was unable to conceive due to scarring in her uterus.
But Donnelly gave them a chance to become parents by volunteering to act as a surrogate for the couple.
“I felt bad for them - I just thought what’s nine months of my life? They are going to have a child for the rest of theirs,” the Mirror quoted her as saying.
“It was long process and involved a heavy dose of fertility drugs for both of us but in the end it all paid off. I was standing in line for a coffee when I got a call from my doctor confirming I was pregnant. I was just bawling - people must have thought someone close to me died.
“But it has been challenging. Now I’m showing I make sure people aren’t staring or looking at me because they’re probably thinking ‘look at that old lady’,” she said.
Michael Murray, specialist at the Northern California Fertility Medical Center, said that though there have been cases of mothers acting as surrogate for their daughters before, it is typically something the medical community does not advise.
But Donnelly was willing to accept the risks, and while she admits her pregnancy hasn’t been easy, she has had no complications so far.
Shannon plans to name her daughter Zoey Hope Catherine after her mother, and said that the experience has made her cherish their mother-daughter relationship.
“It just changes your relationship 360 degrees,” she said.
“We were close before but it’s just on a whole new level. I want to be like she is to my own daughter, that caring and willing to do stuff for her,” she added.