I can vividly remember the morning that my wife discovered she was pregnant with our first child. We were in France. It was midwinter, just after New Year’s Day, and we were on the way to the supermarket. As we wrote our shopping list, I had an odd feeling that something was up.

‘Let’s get a pregnancy test,’ I said.

‘Let’s get two,’ she replied, before we discovered, like everyone else, that pregnancy tests that promise 99.9999 per cent accuracy first time round are actually sold in pairs. So monumental is the news, we all demand a second opinion.

When we got home, she rushed off to do the test(s). But then she nipped back. ‘I was just thinking,’ she said, ‘if it is positive, I won’t be able to eat that delicious pate we just bought. So I might just have a few bites now. You know, before I’m officially pregnant.’

‘Righto.’

Is it going too far to suggest that this vignette, with its mixture of crafty self-delusion and virtuous planning, the promise of good behaviour tomorrow and the indulgence of today, sums up human nature?

Parenthood tests us as do few other trials. We all know about how hard things can get once babies arrive: the sleep deprivation, the colic, the tantrums and the nappies. The endless worry about little rashes and infections. The occasional feeling of utter helplessness. Unless you are struggling to conceive, however, we talk less about how planning to become parents can and should stretch us, too — about how the factors that in so many ways define life’s opportunities begin at conception, not at birth.

But we should. Because new evidence shows that ensuring your child gets off to the best possible start, physically and cognitively, means planning for pregnancy, not delivery.

Getting nutrition right is still the best way to ensure a healthy pregnancy
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And if that sounds draconian, another guilt cosh to slug women over the head with, it’s not. It’s actually the incredibly optimistic fruit of a groundbreaking study that crushes ethnic or genetic determinism under the weight of its findings. No matter what you look like, or where you live, you can give your children the best start. A healthy child is a healthy child, and can hit the same neuro-developmental benchmarks, whether it is born in Somalia or Sao Paulo or Somerset.

‘Nurture is far more important than nature,’ says Stephen Kennedy, professor of reproductive medicine at the University of Oxford, who with Jose Villar, professor of perinatal medicine, has run Intergrowth-21st, which followed tens of thousands of babies around the world. They found that the right nutrition and the right health care in the first 1,000 days, from conception to second birthday, can make all the difference. And if they get their way, Intergrowth-21st will change the very nature of antenatal and neonatal care around the world.

Why? Because of the crucial revelation that it is parental behaviour and health, not parental DNA, that really matters. ‘What it means is that you should be as healthy as you can be before you get pregnant,’ Kennedy says. ‘You wouldn’t dream of standing on the starting line of a marathon having smoked 20 a day for the last couple of years, with a body mass index of 35. And diabetes. You’d try to get all those problems under control before you ran the marathon. Pregnancy is no different.’

The impact on babies whose mothers were unhealthy from the outset of pregnancy is striking, he says. ‘Obesity,’ which is now associated with a host of health risks, ‘begins in the womb.’

Meanwhile, in low-income countries, being underweight ‘leads to greater morbidity and mortality, both in the short and long term; and to impaired neuro-development, which translates into reduced economic and human capital’.

So watch what you eat from well before you plan to conceive. That doesn’t have to mean yucky food, or hugely expensive food, says nutritionist Sally Beare. Around conception, the importance of specific nutrients, such as zinc and folic acid, is well documented. But for the most part, she says, expectant mothers can aim for and stay at a healthier weight with a simple, balanced diet.

It is parental behaviour and health, not parental DNA, that really matters in antenatal and neonatal care
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‘Loads of veg, a couple of pieces of fruit a day, healthy — not fatty — proteins in beans and fish, and maybe some free-range organic chicken or game.’

Drop the white bread, white rice and sugar, in favour of wholegrains and sweet potato with the skin on instead. ‘And don’t gorge on pasta.’

Fat is not a dirty word. ‘Good’ fats — like omega 3 and omega 6 — can be found in olive oil, avocados, fish, nuts and seeds. ‘Flax seed is a great one,’ Beare says.

‘If in doubt, go Mediterranean — the diet there is so well studied and it’s not too expensive: casseroles with beans, not too much meat, a bit of lamb on the bone, salads with raw cabbage, carrots, lettuces, spinach and basil.’

Obvious enough, you might say. But what should you be aiming for? This is the key question: exactly what is healthy? What size and weight should developing embryos be, and what healthy weight gain among expecting mothers helps achieve that?

Before this study, the extraordinary answer is that no one knew for sure. There were no international standards. Instead, to this day, the NHS uses different weight and size charts depending on the ethnicity of the mother. Yet being bang in the middle of the healthy range for weight and size — a critical determinant of reaching neuro-developmental milestones — has nothing to do with ethnicity.

Rather, it is, the Intergrowth-21st project shows, a fixed, not a relative, figure: 80.6mm crown to rump length at 14 weeks; 172.5mm head circumference at 20 weeks; 1,755g at 32 weeks; 13.7kg weight gain for mothers at 40 weeks; 6.65kg/m weight/length ratio for girls born at 40 weeks.

Even so, the UK’s NHS continues to make allowances for women of different ethnicities. ‘So it’s acceptable to have underweight children because, say, you’re of Indian ancestry — and it shouldn’t be,’ says Kennedy. ‘Health care professionals in UK are wedded to the notion that you have to take the woman’s ethnicity into account when assessing the growth of her foetus.’ Given that health care is the second vital element of raising healthy infants, he says, ‘that is hugely problematic in a multicultural society’.

Changing attitudes among doctors and midwives will take some doing, Kennedy reckons. But prospective parents might bear in mind that it is a revolution that needs to happen, if we are to prevent ourselves normalising chronic problems.

That normalisation is already happening. In official statistics from 2011, for example, 54,449 babies out of 509,332 born in England, when assessed using current charts, were over the 90th percentile by birthweight, the point at which foetuses are deemed Large for Gestational Age (LGA), which can be associated with health problems and may lead to special medical observation or attention.

However, when the international Intergrowth-21st standards were applied to the same 509,332, it turned out that another 42,988 babies were LGA — almost twice the initial number, or one in five of babies born.

‘Those babies are at risk of childhood obesity but are currently being ignored,’ says Kennedy.

So the final lesson of the study for prospective parents may be to forget what is normal. Amid what Kennedy and his fellow researchers at the Nuffield Department of Women’s and Reproductive Health describe in a letter to The Lancet as an ‘unprecedented rates of obesity in pregnant women and children in England’, normal is actually becoming a problem. Instead, focus on optimal. And for the first time, you can access the figures to show what optimal is: ‘Our charts describe optimal growth,’ says Kennedy. ‘This should be the aspiration for all children everywhere.’

Get nutrition and health care right, his team notes, and the impact is astonishing. Neuro-developmental differences between infants melt away. Wonderfully, the ability to hit cognitive milestones turns out to be universal. Human beings are not pre-programmed for success or failure. ‘Across a comprehensive set of indicators of physical and early child neuro-development,’ notes Villar, ‘less than 10 per cent of the variability was based on the child’s genes (nature); the rest is environment (nurture).’

It really is up to us parents. That may mean more guilt, more self-recrimination, more pressure — as if parenting doesn’t come with enough of that. But it is better, surely, than your child’s development depending purely on genetic pot luck. The power is in our hands.

The Daily Telegraph