The current second series of Call the Midwife will be the last to be based closely on the books of the late Jennifer Worth - but Heidi Thomas, who has brought Worth's tales of 1950s midwives to the screen to create one of BBC1's biggest dramas, says the characters now have a life of their own.
"I will have [used up all Worth's stories] by the end of the second series,” Thomas said, in an interview for the new issue of Radio Times magazine. "But don’t worry. It doesn’t mean the show will end. The characters will be well developed by then and Jennifer was happy for us to continue."
Call the Midwife's third series, plus a Christmas special, have just officially been commissioned by the BBC.
Thomas, who developed a strong bond with Worth before the author died in 2011, and received her blessing for the TV series, discussed how she has already fleshed out characters from the books, such as homeless woman Mrs Jenkins in last year's Christmas special. "Mrs Jenkins is a very powerful presence in the books, but her story was not resolved. I decided she needed some redemption so I wrote in that scene."
In the interview, Thomas also opened up about her brother David, who had Down's syndrome and died at the age of 15: "[David] was one of the greatest gifts of my life," Thomas said. "If there’s anything in me that’s kind or strong or good, it’s because of those childhood years when I had him as a vulnerable, funny, outrageous sibling."
And the writer of a string of hits (Lilies, Cranford, Ballet Shoes) said Call the Midwife was a project that particularly interested her - because she's addicted to birth programmes on TV: "I love birth programmes. There’s a great American series I’m Pregnant and… having a dwarf or homeless or on drugs. It’s a compulsion.”
Read the full interview in the new issue of Radio Times magazine, in shops now.