A report today had suggested that the Coronation Street actress had admitted herself to Manchester Royal Infirmary on Tuesday night after going into early labour.
It has now been announced that Marsh and her fiancé welcomed their daughter yesterday (Wednesday) evening.
"Jamie and Kym are delighted to announce that they are the proud parents of a baby girl Polly Lomas," a spokesperson from the ITV1 soap confirmed. "Mum and baby are doing well."
The new arrival, who weighed 4lb 1oz, is a sister to Marsh's two children David and Emily and Lomas's son Billy.
The 34-year-old, who took early maternity leave from the ITV1 soap, previously revealed that she had been diagnosed with an incompetent cervix which can lead to a premature birth.
The couple lost their son Archie in 2009 after he was born 18 weeks early.
> Kym Marsh: 'Pregnancy has been tough'