(CNN) -

Lyn Balfour promised her son, limp in her arms, that she would never let it happen to another parent. She vowed to her baby she would tell every mother, every father, everyone who has had the responsibility of caring for a child. That's all she could do now.

She would be the world's horrific reminder that people can and do leave their children in hot cars.

"The pain -- it's not like a normal death in your family where you lose a child ... you get to grieve and move on," Balfour said, her voice cracking. "That pain is every day. It's always there. It never goes away."

Leaving a child in a car seems unfathomable to many. Isn't a child a caregiver's priority at all times? What kind of person just forgets? If you are quick to say, 'I could never,' consider that people who devote their lives to studying these incidents say that anyone of any age or profession is liable to do it. So are people who are educated and not, rich, poor or middle class, mothers as much as fathers. It happens more than one might think: about three to four times a month in the United States. Criminal charges can vary widely from case to case.

On Thursday, a suburban Atlanta dad whose toddler died after he left him in a car for seven hours on a sweltering summer day will appear in court. Justin Ross Harris faces murder and second-degree child cruelty charges.

It has been seven years since Balfour forgot her 9-month-old Bryce in her backseat while she spent hours at work. That morning she was rushing to deal with an emergency at her Charlottesville, Virginia, job. Her routine was off. She normally dropped Bryce off at day care.

But that day she had tucked the 9-month-old in a car seat directly behind her driver's seat, rather than his usual spot behind the passenger seat. She parked, got out and went inside to work.

About 4 p.m., the sitter called her to see how Bryce was doing.

Balfour paused. She was confused. Wasn't the baby with his sitter?

"No, Lyn, you didn't drop him off this morning," the babysitter answered.

Stunned, realizing what she'd done, Balfour ran to her car. She started CPR on Bryce.

The mother's cries for help would be heard on a 911 call later played in court, but it was too late.

Overheated, Bryce died.

How often does it happen?

At least 44 children died in 2013 from heatstroke caused by being left in cars in the United States, according to national nonprofit organization KidsAndCars.org. At least 13 children have died this year for the same reason. Over the past decade, the group figures, there have been at least 388 children who have died of vehicular heatstroke.

KidsAndCars bases its data on U.S. news reports, and when it's possible, the group's volunteers confirm the information independently with law enforcement, attorneys and families, director Amber Rollins said.

Rollins said she's unaware of any group that tracks the number of children worldwide who have died after being left inside vehicles.

The group hears from parents who reach out after years of living in silent shame about the time they forgot their child in a car, she said.

Balfour works with KidsAndCars now, and recounts her story on its website.

During a recent interview with CNN's Brooke Baldwin, Balfour recalled learning on the day of her son's funeral that she would be prosecuted for his death.

Charged with second-degree murder, Balfour could have gone to prison for 40 years, according to the Washington Post, which told her story and others involving caregivers who left their children in cars.

A jury listened to the mother's 911 call, her voice full of panic and horror.

They found her not guilty.

"I never made excuses for his death," she said. "It was my lapse in responsibility ... why he's not here."

Over the years, Balfour has gone over and over in her head how she could have done it. She is a person skilled at doing many things at once. She's a former service member with one tour in Bosnia and two tours in Iraq and a professional who, she says, won a Bronze Star for managing tens of millions of dollars in projects.