Never underestimate motherhood and apple pie. Never underestimate nurses or nuns, either. That, possibly, is the lesson to emerge from the huge surprise hit of Call the Midwife, the BBC's drama about nurses and nuns and brave birthing mothers, whose last episode went out on Sunday.

Call the Midwife, written by Heidi Thomas and based on Jennifer Worth's memoirs of London's East End during the 1950s, came on screen with hardly any fanfare at all. But within a couple of weeks, it was notching up a Sunday-night audience of 10m.

Suddenly, unknown actress Jessica Raine, who plays rookie midwife Jenny Lee, is a huge star, and Miranda Hart, the comedienne who plays an oversized but sweet-natured toff who has turned to nursing, has proved a wow.

You can see why expectations were initially modest about Call the Midwife. Nurses are neat and tidy and nice girls with nice manners. There is no swearing.

There are no explicit sex scenes, although there are plenty of very explicit scenes dwelling upon the harvest of sex - that is, childbirth, in all its gruelling intimacy.

The few commentators who sounded a critical note have suggested that Call the Midwife touches on an unrealistic 'golden age' syndrome. There may be some truth in this.

And yet the drama doesn't flinch from portraying the hardships of life in the London docklands in 1957. The first episode portrayed a slum mother giving birth to her 24th child. Yet Call the Midwife does not condemn, disparage or patronise the mother of 24 or suggest she should wise up. TV often tends to treat the past as an opportunity to give a politically correct tutorial on how stupid and backward people were in 'those days'.

But what about a word of praise for the heroism and fortitude with which women, often overcame those circumstances?

That's where Call the Midwife is so brilliant - it showed the deprived conditions, but it also shows how brave and gallant people were in such deprived conditions. The other secret of the show's appeal is that its subject is one of humanity's great dramas: the life-and-death struggle that can be involved in giving birth.

The moment of birth, as nurses know, is one of enormous drama: a little bundle of flesh and blood emerges from the birth canal and suddenly draws breath.

One of the saddest storylines involved a young musician who is stricken with pre-eclampsia, has a fit and goes into a coma. The woman and baby die leaving a devastated husband. Jessica Raine had never heard of pre-eclampsia before researching her part.

I remember how my own mother, and her generation, feared pre-eclampsia in childbirth, which usually meant a death sentence.

Because of this possibility, she told me, Catholic women would usually make a - hopefully provisional - last Confession a week or so before they expected to give birth. It brings you up close to the truth that this is an enterprise of great depth and meaning.

And it's why Kipling considered the Female of the Species 'more deadly than the male' - 'She who faces Death by torture for each life beneath her breast / May not deal in doubt or pity-must not swerve for fact or jest.'