Frost became firm friends with Moss in the late 1990s and the pair was at a concert together in Los Angeles in 2000 when she went into premature labour.
Her husband Jude Law and sister Holly rallied around to ensure Frost made it to hospital in time - but wild woman Moss had a detour in mind.
In her upcoming autobiography, Crazy Days, Frost reveals, "Seven weeks before my due date we went to a gig with a group of friends, including Kate and my sister Holly, when I started feeling contractions. 'Sadie, I think we need to go the hospital,' Holly said.
"She and Jude helped me to the waiting limo, with Kate rushing in, determined to be with me at the birth. As we headed towards Santa Monica, Kate spotted a convenience store.
"'Stop!' she shouted to the driver. 'We're going to need supplies.' The limo stopped and the others dived out to buy champagne and cigarettes before rushing on to the hospital.
"While waiting at the hospital, Jude put some Bob Marley on and sat and ate bananas because he was so nervous, while Holly and Kate paced about the hall. Almost seven hours later, on October 25, 2000, our daughter Iris Tallulah was born."
Iris was a little sister for Frost's two sons - Finlay, from her marriage to Gary Kemp, and Rafferty. The actress and Law later welcomed son Rudy to the family before their divorce in 2003.