Josef Fritzl, the sick Austrian dad took Kerstin, his daughter, to hospital while his wife was away, it is reported.
The 73-year-old sex beast wasted vital time waiting for Rosemarie, 69, to leave before carrying 19-year-old Kerstin from her dungeon home for the first time.
Rosemarie, who was completely unaware of the unfolding drama, sent home a postcard from her break at Lake Maggiore, Italy.
It was postmarked April 21, two days after Kerstin arrived at the clinic with her father in Austria.
"Dear family. My holiday has been lovely, although I'm really busy every day. I fall into bed dead tired, but I'll soon be home. Love, Mama," The Sun quoted Fritzl's wife, Rosemarie, as writing in the letter.
It is believed Fritzl was trying to get his daughter treated in hospital and back into the house before Rosemarie returned.
But medics put out a TV appeal for Kerstin's mum Elisabeth, 42 - who saw it and persuaded her dad to take her to the hospital.
It was the first time she had emerged from the cellar in 24 years, during which she was repeatedly raped by her father and bore him seven children.
Suspicious medics called cops - who raided the family home in Amstetten and freed two of Elisabeth's other kids, Felix, five, and Stefan, 18, who were also kept in the cellar.