(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.
Hostin's daughter wasn't hurt.
Had Hostin never experienced what it was like to forget her own child, she would have said that all parents must be prosecuted for an act like that.
"I punish myself all the time for leaving my girl in that car for a few minutes," she wrote.
What charges have caregivers faced?
When parents go to court after leaving their children in hot cars, the outcomes can vary greatly.
Daniel Gray was recently sentenced to four years in prison for the death of his infant, who was left in a car for three hours during a hot Arizona summer, according to the Arizona Republic.
Gray pleaded guilty to manslaughter.
In April, authorities arrested Eric Fred Carlin in Florida for allegedly leaving his 9-month-old son in a hot car for 20 minutes.
The baby survived, but Carlin was charged with child neglect without great bodily harm and unattended child in a motor vehicle in excess of 15 minutes, CNN affiliate WPTV said.
In March, investigators charged Shanesha Taylor of Arizona with felony child abuse after she allegedly left her children in her vehicle while she went on a job interview.
Recently, Taylor spoke publicly for the first time about what happened, giving details of her life to a New York Times reporter.
Without childcare options, Taylor said she chose to leave her kids in her car so she could go to an interview for a job that she needed to pull herself out of poverty.
Taylor parked in a Scottsdale lot. There was no shade, and it was 71 degrees, the newspaper reported. Attributing to a police report, the Times said Taylor left her sons in their car seats, cracked her car's tinted windows and left the keys in the ignition with the fan on but the engine off.
In 10 minutes, a car can heat up 20 degrees Fahrenheit, and cracking a window does little to keep a car cool; a child's body temperature can rise up to five times faster than an adult's, according to the National Highway Traffic Safety Administration.
Many who felt passionately about Taylor's case spoke out on social media. While some viewed her as a criminal who should be prosecuted, others saw her as a desperate single mother and a symbol of Americans suffering in a terrible economy.
She is out of jail on $9,000 bond, according to Jerry Cobb, a spokesman for Maricopa County Attorney's Office.
Taylor has pleaded not guilty.
How can accidents be prevented?
KidsAndCars has tips on how to keep the unthinkable from happening. Place something you'll need, such as a cell phone, handbag or briefcase, near the child in the back seat. Keep a large stuffed animal in the child's car seat when it's not occupied.
When the child is in the car seat, put the stuffed animal in the front passenger seat to remind you that the child is in the back. Tell a child's day care center or babysitter that they will always be called if your child isn't coming in as scheduled. If the child is absent without an explanation, the day care center or babysitter is expected to contact a parent or another designated caregiver.
Those human tricks may be the best available options for caregivers. Technology, by and large, doesn't seem to have caught up to the problem yet.
Pam D'Angio with Texas-based Baby Alert International is trying to change that. The company has designed the ChildMinder Infant-Toddler Elite Pad System. It's essentially a sensor under an infant or child safety seat that is activated by a child's weight. If someone tries to get out of the car without removing the infant, an alarm sounds on a key fob, she said.
CNN's Gary Tuchman tested the product with a Sugar Land, Texas, family. The ChildMinder appeared to work.
But Tuchman noted that products like the ChildMinder are not foolproof. Batteries go low. Someone might not hear the alarm.
And two years ago, the National Highway Traffic Safety Administration issued a study that deemed devices intended to prevent infant car heatstroke deaths "inconsistent and unreliable."
Researchers noted numerous concerns, CNN reported in 2012. Some devices required caregivers to situate a child a certain way within a restraint. Others stop functioning when they get wet and cell phone interference hinders some, the report said. In its analysis -- the first of its kind -- the government stressed that the devices should not be the sole option for caregivers trying to avoid accidentally forgetting their child in a car.
Memory expert Diamond argues that products like the ChildMinder could at least be a good start.
A parent himself, he turned his attention to the topic of parents forgetting their children in cars when he was horrified by a news story about a Florida dentist whose son died in a hot car.
"The bottom line is that there hasn't been sufficient government funding to address this," he said. "You put a man on the moon but you can't detect a child in a car? If there were funding, there would be technology."
Diamond said in his car a sensor on the passenger side door rings if he puts a bag of groceries or something relatively heavy there.
"If we can figure that out," he said. "We can come up with something to tell us if our child is still in the car."
CNN's Holly Yan contributed to this report.