How many children should a woman have, and when? It’s a trick question, of course, because the answer is nearly always “none of your damn business”. There is no one perfect solution to this most personal and intimate of dilemmas, no iron rule for getting it right, and yet that doesn’t stop the world and its aunt seemingly having an opinion.

Young women who steadfastly insist they don’t want children are knowingly told that they’ll change their minds when they’re older, even as they’re begging doctors for sterilisations. Mothers of only-children can look forward to years of nosy questions about when they plan to try for another, to the eternal distress of those who for whatever reason can’t or won’t get pregnant again. Even sticking to the conventional two children doesn’t make you immune to criticism, judging by the reaction when Prince Harry let it slip that baby Archie might only be getting one sibling; although the Duchess of Sussex clearly can’t win with some people no matter what she does, the idea of a couple deliberately holding back for the sake of the planet seems to trigger some kind of broader kneejerk hostility.

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Yet somehow, despite all this incessant collective nagging, we seem to be heading for a baby drought. This week brought news of yet another fall in British birthrates, and for the first time they’re falling even among immigrant mothers, whose tendency to have larger families has for years quietly propped up the nation’s declining fertility rates. Perhaps some of that is down to tough economic times, with ludicrously high house prices forcing young couples to put off having children because they can’t afford the space in which to bring them up, or even to niggling Brexit uncertainty persuading some to wait until things feel more settled.

But it also reflects the fact that fertility rates are falling around the world, as the most highly educated generation of women ever to reach childbearing age wrestle with choices that simply weren’t open to their grandmothers. In the US the birthrate hit a 30-year low last year, while Japan suffered the biggest population decline on record. According to the UN, global population will peak at the end of this century before falling thereafter, transforming the way we think about our once teeming planet. Good news for depleted natural resources, maybe, but a red flag for economic growth – although it should be said that a crucial driver of that growth for the last few decades has been the surge of women going out to work. Funny how the economic upsides of female liberation never get quite so much coverage as the downsides.

There is a danger of overreacting to all this, treating the decline in fertility like a lemming-like plunge off the cliff rather than a fairly slow and steady downward trend that gives policymakers decades to adapt (and has led to a welcome fall in teenage pregnancies in the UK to boot). Yet falling fertility rates are becoming a dangerously hot political potato, as legitimate questions about how a shrinking pool of younger workers can support an expanding cloud of older ones start to become dangerously entangled with white supremacist hysteria about the supposed failure of Christian communities to breed fast enough, or with the perennial rightwing anxiety about what women might seek to do with their bodies if they had complete freedom to choose. For some men, it seems the only thing scarier than the prospect of a broody woman trying to trap them into pregnancy is the idea of one wilfully refusing to get into all that.

For years, conventional wisdom has been that policymakers could bump up the birthrate by investing in cheap childcare and flexible working, so that women didn’t face such agonising choices between work and motherhood. (For that is what “leaving it too late” so often boils down to in practice; not couples clean forgetting to have a baby, but women seeing what happens to other mothers in the office and not daring to risk a pregnancy until they feel more professionally established, by which time it’s often harder to get pregnant.) But now birthrates are tapering off even in Sweden, with its world-beating parental leave, heavily subsidised nurseries and an egalitarian culture that firmly encourages men to share the load with their exhausted partners. Neighbouring Norway’s prime minister, too, recently declared that the country “needs more children”, amid warnings that its welfare state model would otherwise be in jeopardy. Either the world of work still hasn’t evolved far and fast enough to meet millennial parents’ expectations, or something more fundamental is shifting.

Next week Channel 4 screens the fictional drama I Am Hannah, part of a series exploring female lives, which centres on a 30-something lawyer being nagged by all and sundry to get on with having children before it’s too late. So far, so wearily familiar, but what makes this one stand out from all the other stories set in an egg-freezing clinic is its central character’s struggle to work out how far she desperately wants to be a mother, and how far it’s simply what everyone else wants for her.

As Gemma Chan, the actress who both plays Hannah and contributed to the storyline, put it in a recent interview: “I feel like we have more freedom these days, but we don’t necessarily feel as free as we should to make different choices for ourselves.” And that’s where the policy debate starts scratching at the surface of dark feelings many parents can’t admit even to themselves: not regret, exactly, so much as suppressed resentment about the inevitable sacrifices demanded by children and the occasional guilty daydream about what otherwise might have been.

The more motherhood comes to be seen as a choice, rather than an unavoidable fact of female existence or some kind of great romantic destiny, the greater the anxiety both about making the wrong choice and about living with the ghost of the life not chosen. Trying to counter all that by nagging young women to knuckle down to it in order to avoid a future global pensions deficit is destined for the failure such emotionally tin-eared tactics deserve.

Better, perhaps, to treat a shrinking population less like an annoying economic aberration to be corrected, and more like a riddle of human happiness. Where people are struggling to have the family lives they want, then of course government’s role is to step in: to create the stable jobs, affordable homes and family-friendly working conditions that make it possible. But if millennials are simply thinking harder than previous generations did before having children – or if some people who would in generations past have felt railroaded into an unwanted family life are now finding the courage to remain childfree – well, that’s very different. Sometimes good governance, much like good parenting, is a question of knowing when it’s really none of your business.

• Gaby Hinsliff is a Guardian columnist