Melissa Merrony collapsed and died just nine weeks after the birth of her son Keegan, in August 2011.
Over a year after her death an inquest took place at Pembrokeshire Coroners Court this week.
Coroner Mark Layton recorded a narrative verdict that Melissa died from 'cardiac arrest caused by sudden unexplained arryhthmic death syndrome.'
The rare heart condition is an umbrella term for around a dozen conditions that kills 800 people under 35 a year in the UK.
Speaking after the verdict Wendy's mum, Wendy Moody, 47, said her daughter had dreamed of being a mum all her life, and was 'wonderful' with baby Keegan.
"It's so tragic. It's on every mum's mind her baby dying suddenly from cot death and there are monitors and all sorts to prevent it," she told Wales Online, "But you never imagine the mother dying."
Wendy is caring for Keegan, who is now nineteen months old, but says she still 'can't believe' her daughter has gone.
"I miss her every single day so much," she says, "She was only 20 years old but she was such a natural mother. She adored Keegan so much but now she will never see him grow up and that breaks my heart."
Wendy said that Melissa had 'so wanted to be a mummy' that after discovering she was pregnant she had bought enough clothes to last Keegan until his first birthday, and even some Christmas presents.
"As soon as she knew she was expecting a boy, she named him Keegan and she had everything planned and organised. She was so proud when he was born. He was 6lb 12.5oz and so cute."
The tragic young mum died at the flat she shared with Keegan and her fiance Daniel. Mrs Moody recalled the moment she 'feared the worst' when she heard the paramedics had been called.
"I scrambled inside the house, Melissa was laid out on the floor. I hugged her tightly and shook her to wake up. But nothing. She was gone."
A post mortem examination revealed Melissa died of natural causes from sudden arrhythmic death syndrome (SADS), and the coroner at the inquest into her death recorded a narrative verdict.
Wendy says that all the family – including little Keegan – have now been tested for heart problems and are all OK.
"He's doing really, really well. He's such a lovely, good baby. I'm just so sad that Melissa isn't here to see him. it just seems so unfair and cruel," she said, adding that the best tribute to her late daughter is to tell Keegan how 'his little life brought the brightest sunshine into hers'.