Kathleen Walsh, 26, from Southfields, and her partner Daniel Collict woke up on Thursday (August 17) to a hissing sound "like someone had lit a rocket in the room".
In fact, they say, a Samsung Galaxy S5 phone that was charging on the duvet had ignited and then exploded next to the couple, burning Kathleen's hand and tearing a hole in the mattress.
Daniel says if his girlfriend had been lying in any other position, their unborn baby and her could have suffered "a horrendous fate".
Daniel said: "We are so grateful to be alive. We have all been told the dangers of sleeping with our phones, tablets and vapes under pillows or next to us, but please don't assume it won't happen to you."
After hitting the snooze button on their phone's wake-up alarm, Daniel and Kathleen were lying in bed with their dog on Thursday morning around 6.20am. It was then they heard "a really loud hiss".
Daniel said: "It sounded like someone had lit a rocket. There was a really loud bang."
"The battery on Kathleen's charging phone had set alight. It was glowing and melting into the mattress."
They extinguished the scorched mattress with a bucket of water. But then Kathleen noticed the burns to her hand.
She was taken A&E where she was treated for her injuries. Doctors say she will not need skin grafts.
Daniel said: "Kathleen is pregnant and if she had been sleeping two inches closer she and our unborn child could have suffered a horrendous fate.
"I just couldn't believe it happened to us."
The couple has also had to throw away their mattress after the blast damaged the springs.
A spokeswoman for Samsung said: "There are no known safety issues with Galaxy S5 devices. Samsung takes all customer queries extremely seriously and this incident is currently being investigated and our customer services team is in direct contact with the customer regarding the matter.”
Northamptonshire Fire and Rescue Service have also issued advice on safely charging devices.
A spokeswoman said: "Don’t leave items on charge for long periods of time or overnight. Unplug devices and chargers when not in use, and only use the correct manufacturer-approved charger for your phone or device.
"Don’t leave devices on soft items such as beds or sofas while they are in use or on charge. Regularly check charging plugs, wires and cables for any wear, fraying or scorch marks. Discard immediately if any damage is found. Never overload plug sockets.
!If you discover a fire, don’t risk your safety trying to tackle it. Get out, stay out and call 999."