Asked to summon two words to describe Call the Midwife, most British viewers would offer “heart-warming” or “nostalgic”. It’s a surprise, then, to learn that the show is considered rather more edgy in the US. “An American paper recently said it’s the Breaking Bad of midwifery drama,” says its delighted writer and producer, the vivacious Heidi Thomas.

“In America, midwifery is a minority profession,” she explains. “Over here 82 per cent of babies are delivered by midwives, but in America only 8 per cent of babies are – and they’re often [from families who are] Christian or follow an alternative hippy way of life. It’s seen as quite radical.”

It’s certainly an interesting new perspective on this adaptation of Jennifer Worth’s memoirs about a group of midwives working with nuns in London’s East End in the 1950s, which returns to our screens for a full third series next month following last week’s Christmas special. The BBC drama, which features Miranda Hart and Jenny Agutter among its ensemble, has been one of the corporation’s greatest success stories of recent years: regularly drawing nine million viewers in the UK, it has also proved an international hit, not least across the Atlantic. There, it screens on the PBS network and further chimes with audiences as a result of the fierce debate raging in the US about women’s reproductive rights, and President Obama’s struggles to legislate on health care. “A timely Valentine to socialised medicine” is how the LA Times has described it.

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“I think it’s helped fan the flames of the debate about publicly-funded healthcare in America,” says Thomas, adding that the show’s unforeseen consequences aren’t confined to the US. “In Bangladesh there’s a women’s healthcare initiative which has launched a copycat drama. They’ve got four Bangladeshi midwives cycling round this rural community where midwife-led delivery has been vilified because women are tended usually by their mother-in-law. Under other circumstances you could say that’s a bit of a rip-off but I think it’s marvellous.”

Another element of the show that has earned attention across the world is its in-built feminist dynamic. “Our young actresses Bryony (Hannah), Jess (Jessica Raine) and Helen (George) have said to me that they are very rarely offered parts where they are nice to other women,” says Thomas. “[People] say there aren’t enough roles for older women – maybe there aren’t enough roles for younger women where women are respected.”

We’re sitting in the former RAF officers’ mess – a somewhat dilapidated gothic pile on the western fringes of the M25 – that doubles as Call the Midwife’s nursing convent Nonnatus House. This is a new location for the show, the former production base – a disused missionary college in north London – having been sold off for development. “In some ways it was a relief because there were pigeons and fungus … it was freezing and people were getting a lot of coughs and chills,” says Thomas. “It was becoming an inappropriate environment for working with children – or adults for that matter.”

The new series promises all the usual character plotlines for its midwife heroines – there’s Chummy’s new baby, Jenny’s new boyfriend and Trixie’s new haircut – but it will also grapple with weightier issues than ever before: subjects covered include mental illness, childbirth in prison, adoption and the emergence of the hospice movement. “I always say we are a medical drama,” says Thomas, who adds that, despite there clearly being no new material from the late Jennifer Worth, there is no shortage of fresh storylines. “A lot of midwives and former nurses who were in Poplar in that period have contacted us and given us their stories,” she says. “And I have an on-going friendship with the real nuns – the Community of St John the Divine who are now based in Birmingham. I feel the connection to Jennifer’s world remains a very active and alive one.”

Worth herself died in May 2011, just weeks before the first series went into production – and staying true to her vision is paramount to Thomas. “We could easily have hated each other,” she says. “We didn’t have much in common on paper – but I think what crept into our relationship is respect. I let her stories speak and she let me add my own voice into the mix. The last time I saw her she was on her deathbed. We had to talk some business, but also I had an hour alone [with her where] I just sat and held her hand and made certain promises to her. I found this lovely note she left saying ‘I leave it to you in confidence’ and if ever my confidence is low, I look at that.”

It was Worth who first suggested casting Hart as upper-crust Camilla “Chummy” Cholmondely-Browne, in fact. “She didn’t even own a television but she’d been with her granddaughter and seen an episode of Miranda. She said ‘I’ve found this marvellous girl I don’t think you would have heard of her … her name is Miranda Hart, she’s six foot two and she falls over splendidly….”

Thomas, who is from suburban Liverpool, has been a writer for three decades. She first came to widespread attention in 1985 with a play, Shamrocks and Crocodiles, which she completed while recuperating from viral meningitis. Then, after years concentrating on the stage, she made the move into television in 1999, when she was asked to adapt Madame Bovary for the BBC. “I’d found my niche then and have not been budged from it since,” she says. “I think Call the Midwife has been my most contemporary show.”

Indeed, her skill at producing popular period drama was proven in 2007, when she scored a massive hit with Cranford, her adaptation of three Elizabeth Gaskell novels; she is about to deliver the script for a new version of Gaskell’s Mary Barton, she says, though the demands of Call the Midwife have slowed her. She was also behind the BBC’s two-series re-boot of Upstairs Downstairs, which suffered from the fact that a blasé Beeb allowed ITV’s similarly-themed Downton Abbey to launch first.

Thomas lives in Saffron Walden with her 17-year-old son, Dominic, and her husband of 23 years, actor Stephen McGann. Keeping things in the family, McGann plays Call the Midwife’s Dr Turner, although Thomas says she never provides a sneak preview: “He gets the scripts through the letterbox. He then finds out what I’ve been doing in the upstairs back bedroom for the past six months.”

Broadway is Thomas’s next stop: she has written a new stage version of the Lerner & Loewe musical Gigi, which will hit the New York stage in January 2015. She is clearly in great demand, so how long will Call the Midwife monopolise her services? “People say ‘Oh but in 1961, you’ll get the pill and there’ll be no more babies or Call the Midwife.

“That’s so not the case. The pill was licensed in 1961 but it wasn’t available on the National Health Service until 1974. But if it [the show] ever became soap, I would question our authenticity … the moment we do Siamese twins, that’s the moment to shut up shop.”

‘Call the Midwife’ returns to BBC1 in January