You have encouraged them to eat their greens, battled to get them into the best school and sweated with them over their homework - all to give them the best start in life.

But your children's prospects may have been determined long before all the hard work.

A growing body of research suggests the first 1,000 days of a child's life - the nine months in the womb and the first two years out of it - are vital to their long-term health

That period can permanently affect everything from a child's chances of developing diabetes or having a heart attack in old age, to their future weight and life expectancy.

The theory was developed after decades of research by Professor David Barker and his colleagues at Southampton University.

They believe there are a series of critical stages in a child's development. If conditions are not perfect at each step, problems can occur later.

Many of these danger points lie when the baby is still in the womb. Poor nutrition for a mother affects both the unborn baby's weight and how well the placenta works, while smoking, stress, drugs and alcohol can also take their toll.

Professor Barker believes many health problems can be traced back to poor growth in the womb.

He has shown that the lighter a baby is at birth, the higher its odds of heart disease in later life. On average, a baby weighing less than 5lb 7oz is twice as likely to die from a heart attack than one born at 9lb 7oz.

It is thought that when food is scarce in the womb, it is channelled to the fledgling brain, leaving the heart weakened. The seeds of diabetes may also be sown before birth, as the pancreatic cells which make insulin develop in the womb. Conditions in the uterus can also affect weight for years to come, studies suggest.

Professor Barker said many of these early effects are 'set in stone' and cannot be undone. He added that the key to health is ensuring women eat well throughout their lives.

He said: 'It is about building a body that the baby can live off. The baby lives off the mother's body - not what she snacks on during pregnancy.

'What we are seeing is a window of opportunity where we can make better people.'