Call the Midwife creator: 'People tell me birth stories in the supermarket'

Thomas manages to cram in tough storylines into a cosy 8pm, Sunday night slot, often shared with shows such as Antiques Roadshow.

There have only been two instances where she's been asked to change something in the show, which was initially set in 1957 and is about to hit the 1970s.

"One was for the haemorrhage machine," she recalls, describing a contraption which recreates post-birth blood loss.

"I think it's a unique piece of equipment to us.

"We were told the blood was too noisy on the lino [on the floor] - so that was literally about a sound effect."

The other was "a tussle" over the use of the word bastard, a dated and offensive way of describing a child born to unmarried parents.

Thomas says they had to do a bit of "creative cutting" to remove it.

Thomas has also managed to write endless fresh storylines, reflecting the health and social issues of the era.

"People ask me every year, 'Where do you get your stories from?'

"And the simple answer is, I go into the British Newspaper Archive, or I go into the medical archive, or people come up to me in the street."

This is what made her decide to write about sexually transmitted diseases in the upcoming series in January.

"In the health report for Poplar [in London's East End], there was this brilliant description of how gonorrhea was getting out of control, and the Greater London Council instigated tracing teams of middle-aged women," she says.

"I just thought, 'this is Miss Higgins' - I love Miss Higgins."

Miss Higgins, played by Georgie Glen, is the prim surgery receptionist with a poignant family backstory, which revealed her hidden strengths.

"I think all good drama takes you by surprise," Thomas says.

"The stories I choose to tell are the ones that grab me by the throat or the heart.

"I never know which organ is coming under pressure.

"But there's this kind of 'Wow' moment, when I see things that actually happened, and then that feeds into the fictional process of developing the drama."

She talks about the impact of the series abroad, including the stories it tells about illegal abortion.

"We've had requests from young American women to use clips from our show on their Instagram feeds, so they can warn their peers of what may happen if the laws in America are changed.

"You realise you are touching people's lives and perspectives in a very profound way."

She also speaks about the importance of kindness, which is in abundance on the show, and about her brother David.

He was born in 1970 with Down's syndrome and serious heart complications, and died in 1985.

"I've lived quite a long life now. I'm 62 and I grew up with quite a severely disabled brother, and that opened me to the goodness of people," she says.

"To the people who just met him as he was, accepted him as he was and loved him for what he was - that, at a very early, informative age, was my introduction to humanity," she says.

"And I honestly think that having somebody severely disabled at my side for the first 20 years of my life changed me for the good."