Call the Midwife is perhaps the BBC’s most unexpected long-running series of the 2010s. A cynic might be forgiven for asking, “But who would want to tune in to a guaranteed hour of women screaming and babies crying every week, year after year?”

An audience of over ten million people per episode from the get-go, that’s who. As Call the Midwife enters the home stretch of its seventh season – with two more and several Christmas specials already commissioned – it must seem that the show has capitalised on a handy recipe of nostalgia, quaint characters and happy endings. But while Call the Midwife possesses many of those things, it also offers something more.

Created by Heidi Thomas (recently seen adapting the BBC’s Christmas edition of Little Women) from the memoirs of real-life midwife Jennifer Worth, anyone who is only seeing pure nostalgia in Call the Midwife’s recreation of London’s East End in the 1950s and early 1960s must have their eyes closed. They’d be looking past rampant poverty, poor sanitation, widespread misogyny, class divisions – all of which are confronted almost weekly by the fictional sisters and health workers of Nonnatus House. The issues it takes on are not just medical – the introduction of the pill, the choice of hospital or home birth, the thalidomide scandal – but social, from city prostitution to the alcoholism of otherwise glamourous midwife Trixie (Helen George).

Perhaps it is more accurate to say, then, that what Call the Midwife has cracked is a recipe for introducing change while appearing to stay the same. Its themes feel authentic but move with the times; each episode introduces ‘stories of the week’ characters whose concerns and experiences are treated with detail and respect. Even its cast changes are handled with unusual ease. Original lead Jessica Raine left in series three and fan favourite Miranda Hart in series four, but the show didn’t even seem to miss a stride with the arrival of the likes of bustling Phyllis (Linda Bassett) and the eminently practical Patsy (Emerald Fennell, sporting bombshell red hair). And while they acknowledge realities such as racism and stigma, Call the Midwife doesn’t use a conservative time as an excuse to constantly perpetuate myths, either. There was a vintage lesbian love story for Patsy and fellow nurse Delia (Kate Lamb), and when the actresses left the series, they escaped ‘bury your gays’ when they were sent off on a round-the-world trip. The vibrant ethnic makeup of the East End is prominent and there’s practically an unmarried mother featured every other week.

In season six, Nonnatus took the opportunity to open up its primarily middle-class ranks to working-class East End nurse Valerie (Jennifer Kirby). In season seven, new midwife Lucille (Leonie Elliott) arrives from the West Indies as part of the Windrush generation of Caribbean immigration to the UK. The show is even holding its own in the marital absence of Jack Ashton’s dishy curate Tom (known round our place as ‘The Hot Reverend’) and Barbara (Charlotte Ritchie), who are set to return later this year. Through all this, head nun Sister Julienne (Jenny Agutter) and Sister Monica Joan (Pam Ferris), an elderly, cake-loving eccentric who was one of the first qualified midwives in the country, have been a steadying presence, while in secular matters that role has been fulfilled by Dr. Turner (Stephen McGann) and nursing nun turned multi-tasking housewife Shelagh (Laura Main).

Nonnatus House never feels like a rotating door of new faces and slim backstories, because Call the Midwife’s characters do a lot more with their on-screen time than the eight-hour series run time would suggest. It showcases a gentle build-up of well-rounded, believable, multi-faceted characters. Barbara may seem shy and prim, but she grew up poor, and is good in a crisis. Sister Julienne shepherds her charges with a firm but temperate touch, showing an immense capacity for compassion.

Indeed, if there’s anything that Call the Midwife makes people nostalgic for, it’s probably this. There is an intensely personal compassion to such one-on-one, face-to-face scenes. These are characters who visit new mothers sometimes twice a day for weeks, getting to know them, to know their families, sharing and halving their problems. It is a world that puts value on things like knowing your neighbour’s name and putting your empty glass bottles out for the milkman in the morning. The diverse communities of Poplar can be overcrowded and overbearing, but they evoke a sense of belonging; the midwife’s uniform is the audience’s rare golden ticket through their doors.

Though it lightens the mood with cosy camaraderie and occasionally comical side plots, Call the Midwife is a tearjerker, frequently dealing with love and loss, birth and death. It is a show that only occasionally opts for the happy ending, preferring a hopeful or bittersweet or sometimes even heartbreaking one. Yet viewers remember not what is lost but what is shared in those most hazardous moments. Encouragement through pain and peril, the squeezing of a hand, the wait for the first tearful wail. It’s the husband stepping up to the plate to help deliver a baby in the back of a van. It’s the couple who want to stay in their own home until the very end, even as the street is demolished around them. The father who wants another boy opening his eyes to his daughter for the first time.

It is women among other women, doing important, life-altering work. Rarely has a fictional creation so thoroughly centred women of all kinds – old and young, childless and childbearing, working and non-working – in a cast and community that is almost entirely female-led. It is this story that draws in ten million people every week. Perhaps it is a sign that we should be filling our screens with other female-led stories more often, too.