(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.

On KidsAndCars, Balfour wrote of that day: "I am in shock and overcome in disbelief that this cannot be happening to me, I cannot be the type of mother who would accidentally forget her child. ..."

Who would do such a thing?

A mother is just as likely as a father to leave her child in a vehicle, the Post reported.

The people who have done it range in age and financial and educational background, experts say. A veterinarian, a doctor, a dentist, a professor, a school principal and a rocket scientist are among the parents who have accidentally forgotten about their children and are now among KidsAndCars' members.

It doesn't matter if a person is highly organized or often absentminded, though people with intense demands in their lives can have more stress and be more sleep-deprived -- two factors that can increase the likelihood their minds could be sidetracked away from a baby in the backseat, said David Diamond, a psychology and molecular physiology professor at University of South Florida.

He studies memory, and for a decade has focused his attention on a phenomena that's come to be known as Forgotten Baby Syndrome. He, too, said he wasn't aware of any international figures on how many children have died after being left in vehicles.

Diamond wrote recently on HLNTV.com that Forgotten Baby Syndrome, is a "failure of prospective memory, which refers to the planning and execution of an action in the future."

Two brain structures process prospective memory: the hippocampus, which stores new information, and the prefrontal cortex, which enables planning, he wrote. The hippocampus processes that a child is in a car. The prefrontal cortex enables a person to create a route, factoring in a change in plans like going to day care rather than going directly to work.

Forgotten Baby Syndrome seems to involve a "clash between prospective memory and another form of memory, referred to as habit memory," he wrote. "Habit memory is formed subconsciously through repeated activities, such as learning how to ride a bike or, in the case of FBS, repeatedly driving to and from home and work."

Lately, Rollins has noticed a greater awareness of how diligent, loving caregivers forget their children in cars. More people who have gone through the experience seem willing to discuss it, she said. They're opening up online, particularly in the comments section of news stories.

"More people seem to be saying, 'Hey, this happened to me, this can happen to wonderful parents,'" Rollins said.

After the hot car death of the Georgia toddler in June, CNN legal analyst Sunny Hostin wrote about how she and her husband accidentally left their infant daughter in a hot car during a shopping trip.

They realized the mistake minutes into a shopping trip, and ran to the car, she said in a CNN.com Opinion piece.

"Walking into the garden center, my husband turned to me and said: 'My God. We left Paloma in the car.' I screamed, dropped my purse, ran to the car and opened the door. The car was already warm. Her face already flushed. But she was fine and still sleeping. I was ashamed, embarrassed and horrified at what I had done.

"It dawned on me immediately -- I could have killed my girl," she wrote.