The success of Call the Midwife is well documented. On its launch in 2012, the series overtook Downton Abbey's record for the largest first season audience on British TV for a drama this millennium. Today, it is regularly watched by 10 million viewers.
The appeal of Heidi Thomas' series, originally based on the memoirs of Jennifer Worth, a nurse practising in the poverty-stricken East End of London in the 1950s, is clear. Call the Midwife harks back to seemingly kinder times, with the nostalgia factor amped up by its crooner-heavy soundtrack and retro fashions, often sported by Helen George's modish Trixie.
The Daily Telegraph praised its ability to "tickle the middle of the brow while touching the most anguished parts of the human condition".
Not only does it punch surprisingly hard emotionally, it also tackles the big health issues. Female genital mutilation, cleft palates and sickle cell disease have all found their way into Thomas' scripts, and the next series will cover fistula and tuberculosis.
Thomas began researching medical history once she ran out of source material (Worth's memoirs only run to three short books). She explains: "Once we started addressing issues of disability or broader health practice, we started to hear from the audience, including members of the nursing profession. It was a rare day when I didn't receive a letter or message from a member of the public saying that an episode had touched or helped them."
But perhaps the most extraordinary thing about Call the Midwife is its influence in practical terms. In the UK, there has been a surge in interest for midwifery courses. For example, in 2013, applicant numbers at Anglia Ruskin University rose 11.6 per cent.
This year, an episode aired in which a patient died after an illegal backstreet abortion. Professor Louise Kenny, of the Faculty of Health and Life Sciences at the University of Liverpool and a consultant obstetrician at Liverpool Women's Hospital, spearheaded the campaign to overturn Ireland's ban on abortion in the 2018 referendum, emphasising how the law prevented women with cancer from getting terminations they might not want but needed.
Kenny credits Call the Midwife with influencing public opinion about the dangers of illegal abortion. "When I was knocking on doors prior to the vote, I would often open the conversation by asking: 'Do you watch Call the Midwife?'" she says.
When the series started to air in the United States the perception of midwifery started to change. There, midwives are present at only 10 per cent of births (compared to more than half in the UK), since most expecting mothers opt for an obstetrician.
Professor Michelle Collins, a midwife who teaches at Rush University in Chicago and is former director of the nurse-midwifery course at Nashville's Vanderbilt University, says: "I think people in the US are more curious about how universal healthcare works, as we see the dissolution of our healthcare system.
"Seeing the midwives doing home visits, sitting down to have a cup of tea with a patient, going out of their way to assist a family with some social situation — these experiences endear the midwives to US viewers."
This has had a practical knock-on effect as the high rates of caesareans have led to an increased interest in the midwifery model, which is less reliant on technology, less invasive and less inclined towards intervention unless there is a clear medical need.
Another point of influence can be traced back to Call the Midwife's second series, when nitrous oxide was used in childbirth. The show's Facebook page was inundated with messages from American women, asking: "What is that gas and how can I get it?"
Nitrous oxide declined in popularity because of epidurals and general anaesthetic. However, Collins introduced t it in childbirth at Vanderbilt at around the same time as the series aired. Then, it was one of two places in the US where you could receive the treatment during labour. Now, there are more than 1000 such clinics.
Meanwhile deep in the jungle of India, community midwife Tanya Burchell is using episodes of the drama to demonstrate compassionate care to her students.
"It reminds us that the core of caring is made up of many things; compassion, rapport, using humour and continuity of care, support and encouragement."
There is a strange irony to a period drama effecting change in the 21st century. "The more complex the 60s become, the more the Call the Midwife world seems to reflect life in the present day," says Thomas. "Maybe the show has more to say than ever before."