Professor David Barker and his family - ranging in age from his two-year-old grandson Jack to his 88-year-old father-in-law Bob - sat down to a lunch of roast pork, mashed potato, various vegetables and apple pie. The meal required basic culinary skills and because there were 18 of them, a bit of logistical know-how. What it did not require was familiarity with the tables of the mineral content of all vegetables.

In other words, says Barker, director of the Medical Research Council's epidemiology unit at Southampton University and author of a book about how a woman's diet can protect her child from disease in later life, anyone can do it. "Getting a balanced diet is based on simple principles: a reasonable balance of protein and carbohydrates and an adequate amount of fruit, vegetables and dairy products.

"Families in countries like Italy just know this. Our ancestors knew it too but we've lost the plot. Everything is so prescriptive now. We've got to the stage where the Department of Health is telling us that five asparagus spears - five, note, not four or six - is a portion. The French would laugh us to scorn."

If getting it right is easy, is Barker's good news, then his bad news is that the cost of getting it wrong is higher than had ever been previously imagined. Getting it wrong means that young women are malnourished and their babies are subsequently malnourished, putting them at increased risk of heart disease, stroke and diabetes in adulthood. Mothers-to-be really are eating for two.

The increased risk is caused by a number of factors. One of these is that the human body, apart from the brain and immune system, is largely complete at birth. "We bring into the world all the heart, muscle and kidney cells we will ever have," he explains. "After that we can only enlarge them. Smaller babies have fewer kidney cells, for example, but the organ still has to do the same job. It's like a factory having to produce the same output with half the number of workers. And that strain can lead to raised blood pressure in later life."

Another factor is the "plasticity" of the unborn, the fact that it can be moulded by its environment. If it is getting less than optimal nutrition it makes the brain its priority, since the functions of organs such as lungs, liver and kidneys are carried out by the mother before birth. That is why a large head relative to body size can be an indication of pre-natal undernourishment.

"In the womb we also learn how to handle different food. If we don't get enough sugar, for example, we develop a thrifty way of handling it. So we don't store it because there's no surplus, we just keep it circulating, again in order to feed the brain. This is a good idea when supplies are short but a bad idea if want is subsequently replaced by over-sufficiency. Then the blood becomes flooded with sugar, obesity increases insulin resistance and that can lead to diabetes." It is thought similar, not yet fully understood, mechanisms may operate with cholesterol. The dangers seem particularly exacerbated when a small, thin baby gains weight rapidly in later childhood.

"Problems arise when, over a lifetime, initial want is replaced by excess. That's why heart disease is a disease of affluence. We see hardening of the arteries in Egyptian mummies but coronary heart disease only became an epidemic in the western world 50 years ago and now accounts for one in four deaths among men, and one in six among women."

Small babies also "reset" their stress response. "If a baby is not getting enough, its reaction is going to be 'let's get out of here', and so it gets ready for birth by producing more cortisol, the stress hormone, to make it mature more quickly. It might be a good idea to turn it on but it's then turned on for life."

Doctors have long accepted the need for pregnant women to be well-nourished, says Barker. "But we thought it only mattered in the second half of pregnancy when the foetus is growing. It was assumed that before then, because it was so small, it could just take all it needed. We didn't realise that food is also instructive, that it makes the settings for the body from a very early stage."

The realisation that the embryo and foetus were so responsive has grown mainly in the past five years, due in large part to the upsurge in assisted fertility treatments, when close attention started being focused on life from the moment of conception onwards. "Of course, anyone involved with animal husbandry has known this from the year dot. Every farmer knows you look after your female animals before mating. As a society though we've lost touch with the natural world."

Barker and his team studied 12,000 young women in the UK, the largest project of its kind in the world, measuring their diets and body size before pregnancy, monitoring those who become pregnant and following up their babies after birth.

He calculates that up to 30% of women are not eating balanced diets: not because it is difficult but because they believe it is. They lack time, knowledge and skills: domestic science has virtually disappeared from schools, and TV cookery programmes can make the subject seem even more daunting, just as the abundance of food in supermarkets can be overwhelming. They are also often trying to slim, are manipulated by the food industry and are confused by single-issue campaigns and health messages, into thinking that there are "good" foods such as fibre and "bad" ones such as dairy products.

Moreover, adds Barker, there has been too much emphasis on single issues, but what may be appropriate advice for, say, a group of at-risk middle-aged men should not be allowed to "contaminate" the food advice given to young women. "It's vital that women eat well before they get pregnant because the foetus does not just rely on what its mother eats. That would be too risky as women are often sick so it lives from her body, from her stores of protein and fat."

His study has shown that around a quarter of boys are too thin at birth (boys are more at risk because they have more growing to do). "Most places don't pick up on these because they only take measurements of weight and not length. We have such low expectations of pregnancy that if a baby is between five-and-a-half and 10lbs, and not deformed, we think that's good enough. But it worries me to think that every day babies are being born who could be so much better, that in them we are sowing the seeds of bad health that could follow them all their lives."

As a father and stepfather to four sons and four daughters and grandfather of nine, Barker obviously relishes the pleasures and rewards of parenthood, and the last thing he wants to do, he says, is make pregnancy more burdensome or women more guilty. In fact, he thinks current pregnancy books lay too much emphasis on the prevention of disasters, which, he points, out are extremely rare.

"I'm not being prescriptive about what women should eat. In fact, I'm quite hopeful because we know the guidelines, the food is available and a bad diet is only very marginally cheaper than a good one. I know people can say 'Oh it's easy to eat well when you're middle class and have lots of money and a wife at home. It's different for a single mother on a council estate.'

"But I'm very sympathetic to them. They don't have a chance because they don't know. No one has told them that a good diet is not expensive and not difficult. Now we either have to teach that in school or we have to get it back into our culture."

· The Best Start in Life: How a woman's diet can protect her child from disease in later life by Professor David Barker, Century, £9.99.