The idea that we can plan our lives is fairly new and very western. For thousands of years people assumed that their destinies were not in their own hands, but in the unpredictable grasp of gods or demons or chance or family history or destiny. Man proposes but God (or something ineffable) disposes – that was how everybody thought.

Now, however, we in the rich world imagine we can choose our fates; we can eliminate diseases, double our harvests, split the atom, uncover the mysteries of the moon and even hold back time.

This feeling of being in control began slowly, not much more than 300 years ago in the West with the scientific revolution, but in the 20th century it suddenly burst forth as a new religious belief. We got antibiotics, we got vaccines, we got mass transport, mass communication and the mass information revolution and we got contraception. That, particularly, seemed to make women the mistresses of their own destinies. Those over 50 will remember that contraception used to be called family planning; we women thought we could plan our lives and our brilliant careers. We were mistaken.

Last week the Royal College of Obstetricians and Gynaecologists put out a statement about the best time to have babies. Aware of the trend for women to have their first baby when older (and of its painful consequences), their advice is that women who put off motherhood into their middle thirties and beyond are taking unwise risks and the best age for childbearing is between 20 and 35. It is much easier to get pregnant during this time, on average, and much harder later, even with IVF; older mothers are much more likely to miscarry or to have babies with problems.

Well, yes. We all knew that. Every courageous granny and good GP has been saying that for the past 40 years. Just because many of us found, luckily, that we could easily have babies in our thirties, it does not mean it’s a good risk for everyone or that the age of motherhood can move up bit by bit as medical technology advances. Of course it would in theory be good for women to have babies early. Lots of commentators came forth last week, in view of all this, to urge young women to “have that baby now”.

This is completely, cruelly unrealistic. The truth is that however much one wants children, for an ambitious woman there is never a good time to have a baby. And even if there were, it certainly is not something that can easily be planned. All that one can plan is when not to have a baby and that, of course, is a great freedom. However, there are so many uncontrollable variables.

Take the case of Ms Hopeful Youngthing. At 25 or 26 she has finished a second degree, essential if she is to become a high-flying businesswoman, lawyer, doctor or whatever. She is as poor as any student and couldn’t afford a baby, childcare etc. More important, she hasn’t got a babyfather: Mr Right-Enough hasn’t come along yet. Besides, she has to put her toes on the first rung of the career ladder and climb; she needs to do a professional apprenticeship, make contacts and get experience. Starting out like this, working terrible hours as ambitious young people do, is incompatible with having babies. When you have everything to prove, serious ambition is not a part-time thing.

If Ms Youngthing is lucky she may get sent abroad for a while to gain valuable experience, but when she comes home she will need time to re-establish herself and start looking again for Mr Right-Enough – sadly, gorgeous Herr Ganz Genug was not prepared to leave Munich and follow her to her new job in London. Then her love life may become rather fitful, given the long hours in her competitive world. By the time she’s 30 or 31 or 32, she feels broody but still uncertain about her current lover, Mr Almost Right-Enough, her professional equal, and he goes off with someone younger, less stressed and easily impressed.

Alternatively she moves in with him and what with work and their enormous mortgage and debts, they don’t feel in a position to take on the cost of having babies. Then suddenly she’s successful, exhausted and 37 and he has gone off with someone younger and less career-driven, having worked out that you cannot have two demanding careers in one family. Where does planning fit into any of that?

There was never a good or obvious moment for Ms Youngthing to have a baby and it’s cruel and unrealistic to tell her, as she sits in the IVF clinic, that she should have done so at twentysomething. Besides, what if she had, having providentially met Mr Right-Enough when very young?

Miraculously he was somehow able to provide for her – although they couldn’t afford childcare, since her earning power was so low – and she produced three babies by the age of 30, as medically recommended. Then at thirtysomething, still the clever, ambitious woman of before but now with no work experience, no contacts and not much confidence – a side effect of staying at home – she is not very marketable and certainly not in the world she once aspired to. Not earning a great deal, she still can’t afford much childcare so she hopes for flexi-time – something like hen’s teeth for a working woman who isn’t already established.

Ms Youngthing might rise above all these obstacles but then again, in a competitive world, she might not. It is difficult for a mother to go back to a competitive work environment without having established herself before having babies.

She is now of no more use than a 22-year-old but with less commitment and less time. Meanwhile, her younger sister of 24 is miserable that her responsible 26-year-old husband is not prepared to start a family for several years, until he can get on top of their student debts, get established in his stressful career and have just a little child-free fun while still young. So she can’t have an early baby either. Again, she might rise above such handicaps or she might not.

These uncertainties apply most harshly to the most ambitious and career-minded women. But to some degree they apply to most women, except to those whose jobs can really be taken up and put down very easily.

Life is full of countless imponderables which can’t be wished away and the idea that we or our doctors or our government are in charge of planning our destinies is largely an illusion. On the other hand, if there is no right time to have a baby, perhaps there’s no particularly wrong time either, assuming there is at least a father available. Happy Father’s Day.

minette.marrin@sunday-times.co.uk

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