When the actress was pregnant with Polly, now five, Marsh was told by doctors she had an undiagnosed medical condition meaning she could go into early labour.
Marsh, 40, said she was “absolutely terrified” that things would go wrong.
“The thought of losing another child was just simply unthinkable.”
Marsh explained that the specialists at St Mary’s performed a procedure to stop her going into early labour and she was put on bed rest during the end of her pregnancy.
She gave birth to Polly, who weighed just 4lbs 1oz, in March 2011. She had breathing difficulties and had to be put on a ventilator.
“My world started to crumble,” Marsh said. “We were so beyond worried.
“Thankfully Polly only needed ventilating for a short time and we realised quite quickly that she was one of the lucky ones.
“The staff at St Mary’s are true angels and are undoubtedly the reason that our Polly is here today.”
Marsh has two other children from previous relationships: Emilie, 19, and David, 21. She explained her Emilie was the only child she carried full-term. David was born six weeks early.
Kym was speaking at the annual Red Sea Pedestrians ball, which raised £300,000 in aid of St Mary’s NICU Unit and the Rheumatology Department at the Royal Manchester Children’s Hospital.