Lyn Balfour and Stephanie Salvilla have quite a bit in common.

They are both intelligent women. They are devoted mothers. They both made the fatal mistake of accidentally leaving their baby boys in their cars while they were at work.

And, due to a cruel trick of the brain, they both have false memories of dropping their children at day care on the respective mornings their sons died.

Lyn – a US army reservist who served twice in Iraq – left her nine-month-old son, Bryce, in the car for seven hours while she was at work in March 2004.

She went about her day, looking at pictures of her son, talking about her baby and planning to pick him up from day care. It wasn’t until she went to leave that she saw him in the backseat.

“I was hoping that it wasn’t too late so I started screaming for someone to call 911 and I took him out of the car and I began CPR,” Lyn told Seven’s Sunday Night.

Related: “This week, I left my baby in the car for five hours”.

“I prayed and I prayed to God to take me instead, to please, please not take him and after about another ten minutes another doctor came in and she told me that they couldn’t save him, that he was gone.

“And I sat there and said to myself, ‘How am I going to tell my husband that I killed his son?’

“If you would have asked me or given me a lie detector test, even today, I would tell you that he was at the day care provider’s house and that he was okay and that he was fine,” she said.

It’s a scenario eerily similar to that which caused baby Gannon’s death in July 2009.

Stephanie Salvilla was also convinced she had dropped the five-month-old at day care before continuing to work.

“I know now what really happened, but still to this day it’s the only memory I have,” she told Sunday Night.

Instead, her son sat in a car in the Florida sun for eight hours.

It doesn’t take long for a car in the sun to heat up. Seventy five per cent of the temperature rise occurs within five minutes of closing the car and ninety percent occurs within 15 minutes.

The temperature inside a car can be 30 degrees warmer than that outside and babies are particularly vulnerable, heating up at a rate three to five times faster than adults.

See what happens when the Sunday Night reporter sits inside a hot car in the sun:

Both mothers were charged; Stephanie with aggravated manslaughter and Lyn with second-degree murder (which carries a penalty of up to 40 years’ jail).

International memory expert Dr David Diamond has given evidence in many cases of ‘forgotten baby syndrome’, including that of Bendigo mother Jayde Poole, who was acquitted of the manslaughter of her baby girl in 2012.

He said the neurological explanation for the phenomenon involves two parts of the brain competing and causing a memory failure.

Dr Diamond said the habit-forming part of the brain can override the multi-tasking and fact-based part of the brain, resulting in us forgetting to do something we had intended to, like buy milk on the way home from work, or drop a child off at day care.

He said three factors, fatigue, stress and a change of routine – all of which were at play in Lyn Balfour’s case – create the perfect storm for forgotten baby syndrome.

“I don’t consider this poor parenting,” Dr Diamond said.

“It’s just a matter of chance that some people happen to be caught up in this combination of events that results in good parents forgetting their children, so I don’t categorise it as bad parenting.

“If you have no awareness, then what you have is a tragedy, but not a crime.”

Lyn’s murder charge downgraded to manslaughter, but she was acquitted of the crime unanimously by a jury.

Stephanie was not so fortunate. She was convicted of leaving Gannon unattended causing great bodily damage and served five years’ probation.

Lyn has dedicated herself to educating others about the dangers of leaving your child in the car. She says she made a promise to Bryce to “make sure his death wasn’t for nothing”.

She works with Kids in Cars – a group lobbying for legislation to have a baby seat alarm built into new cars.

“Somehow all of our cars tell us if we’ve left our headlights on because nobody wants a dead battery but it’s OK to have a dead baby?” the group’s founder Janette Fennell said.

“We’re talking about an industry who already knows we’re forgetful.

“We have a reminder to put on our seatbelt, we have a reminder if we leave our key in the ignition, we have a reminder if our gas is getting low, we have a reminder if we left the door open.

“I’ll take a dead battery any day of the week if I can get a protection so that little babies don’t get inadvertently left behind.

“These are not failures of love, this is a failure of our memory and we should all be working together to do whatever we can to save these babies’ lives.”

You can watch the full Sunday Night report here.