The 44-year-old Mom star gave birth to the boy, her and the Jurassic World actor's first child, in August 2012, nine weeks before her due date.
"Please join me in supporting GAPPS (the Global Alliance to Prevent Prematurity and Stillbirth) on Thursday, May 6th at 6:30pm (PDT)," Anna wrote on Instagram on Wednesday, April 28, alongside a black and white photo of herself smiling at baby Jack, who stares at her while wearing a feeding tube. "I will be sharing the story of my son's preterm birth 8 years ago as part of @healthybirths 60 minute virtual fundraiser."
Another photo of Anna with baby Jack was posted on GAPPS' Instagram page.
She and Chris, 41, have spoken about their son's birth before. The infant weighed just 3 pounds and 12 ounces when he was born. He spent a month in the NICU before Anna and Chris brought him home.
She continued, "I was at the hospital and the goal was to be in bed for four weeks...but on the seventh day, I went into labor again. I was in denial. For hours I was like, 'I'm just having indigestion.' Chris and I were so scared, but when Jack came out, even though he was so tiny, he looked so good to us."
Both Anna and Chris, who also shares 10-month-old daughter Lyla with wife Katherine Schwarzenegger, have also used their platforms to raise awareness about the health needs of premature babies and other infants requiring specialized care. In 2014, the Guardians of the Galaxy actor gave an emotional speech at a March of Dimes luncheon.
"I made promises in that moment [in the NICU] about what kind of dad I wanted to be," Chris said, "and I just prayed that he'd live long enough that I could keep them."