“Wanting a child" is a strangely mild phrase to cover a desire that can punch one in the gut, and leave a constant thrumming in one's ears, hands and knees.

It is warm and deliciously tactile, yet comparable to a war-like rage. Like an obscenely powerful hoover, it vacuums up your inner rational voice, leaving you reckless of loss, care and peace.

But the inner rational voice fights back with reminders of easy delights, such as the day-to-day agency whereby it is you who decides where to go, and when. At work or among friends, the clarion calls of career ambition cause us to pause and reconsider these bodily urges. So we offer ourselves an argument and a promise: “Yes, but not yet".

Biology always ticks away: as children we may want to speed it up; as adults we might want to slow it down, or turn it back. The timeline for female fertility is less visible, and more difficult to gauge, but it is there. Increased opportunities for further education and training make early childbearing difficult. In turn, the optimal age for having a child continues to be pushed back.

In the UK, the average age at which a woman has her first child is now thirty. Yet career progression is not speeding up and career pressures are not easing. Instead, the deep-seated negotiations, often barely articulated, between baby hunger and the rational voice inside our head get more and more complicated and prolonged. Ten years ago, Sylvia Ann Hewlett blasted a warning that continues to lodge in the female psyche: in thinking they can delay motherhood, women have been “sold a bill of goods" by a media intent of hyping miracle babies; as a result, many would be infertile by the time the time was ripe for children.

Hewlett says that your priorities in your thirties should be to find a suitable partner and have children and later, you can find an on-ramp back into a career. While, Hewlett is right to point out the relentless nature of the biological clock, the advice she gives young women is often impossible to follow. It is an excellent blueprint, but life does not always match the blueprint we make of it. And it smacks of making do, which is particularly difficult for Generation Y, who have higher expectations than previous generations.

In my early thirties, when my life was constrained by young children and a fledgling, vulnerable career, I wrote a book called Working Women Don't Have Wives, in which I argued that the persistent stalemate in equality arose from the fact that a good career was structured by the assumption that the worker had a wife at home servicing all domestic and family responsibilities. What I did not quite admit to then, was that the visceral need to be with my children, to watch them, to care for them and be their primary person, was more powerful than any structural bias.

The biological clock does not stop with childbirth. As children grow, our need for them and theirs for us changes but persists. As my own daughter became a mother, I saw how little things had moved on. In spite of huge social change, the dilemmas persist. In fact, I now see them spread across generations, as I once again consider how I might re-assess my time commitments to support my child who is now both mother and career woman. If this sounds soft or indulgent, then I draw on evolutionary psychology to defend my gut response: menopause, it is now believed, serves an evolutionary purpose. It allows women who are still young enough to be active contributors to family and society, to be free of child bearing so that they can help their own children bring up their children.

Some believe that societies did not settle and gel until older women lived long enough to contribute to the care of their grandchildren. So now I look back and see the long sweep of that clock: how sensible to have your own children at a young age so that you have a fighting chance to be fit and active when they have children.

We inhabit a culture that offers age-defying promises and this is why we often ignore the relentless pressure of our biology. What is, or should be, more amenable to change, are assumptions about what a good career looks like and assessments based on where a successful woman should be at a given time in her life. But career pressures and insecurities persistently defy respect for the biological clock, and women who want to become mothers have to be very lucky to follow life's best blueprint.

Buy Sylvia Ann Hewlett's book Baby Hunger: The New Battle for Motherhood here.

Buy Terri Apter's book Working Women Don't Have Wives here.