If you're living on salads and sweating it out every other day at the gym, you might be rather discouraged to learn that to some extent it may be too late.

A growing body of evidence suggests that our health in later life is not simply a matter of genes and lifestyle but is intimately linked to what happened to us in our mother's womb.Numerous studies around the world confirm that the first nine months of our lives may be the most important. Extensive research by David Barker, director of the British Medical Research Council's Environmental Epidemiology Unit in Southampton, has found that retarded growth in the womb is strongly linked to an increased risk of various killer diseases, including heart disease, diabetes and stroke.

When Barker's team examined the unusually detailed birth records for 16,000 people born between 1911 and 1930 in Hertfordshire, Sheffield and Preston, it discovered that babies weighing 51/2 pounds or less were 50 percent more likely to die of heart disease than those weighing in at 71/2 pounds and twice as likely to die as those weighing 91/2 pounds.

Finnish research of 3,300 men also revealed that those who were thin at birth with small placentas had high death rates of coronary heart disease.

Babies deprived of nutrients in the last months of pregnancy, for instance, tend to have larger heads and shorter bodies with smaller abdomens and are more at risk of heart disease. Those experiencing a shortage earlier in fetal life, on the other hand, are often proportionally smaller with larger placentas and are more prone to strokes; those affected during the middle months are commonly thin at birth and likely to suffer from diabetes. High blood pressure is associated with retarded growth at any stage of pregnancy.

"What is clear is that the babies who grow least in the uterus are subsequently at risk of these diseases in later life," says Dr. Christopher Martyn, clinical scientist at the Southampton unit.

But why should this be? Put crudely, when resources are in short supply, the fetus adapts by protecting the essentials, such as the brain or the growth of the placenta, at the expense of other parts of the body. This can reduce the number of cells or produce other physiological alterations in various organs and body parts - changes we may not be able to reverse later in life.

"If you have a baby who is growing fairly well in the uterus until the last part of gestation, for instance, it makes a number of adaptations directed towards maintaining growth of the brain at the expense of the rest," explains Martyn. Hence the larger head in proportion to the rest of the body.

So, should pregnant women reach for the multivitamins? Sadly, it's not that simple.

"It depends not so much on what mothers eat when they're pregnant, but what she stored before and how well the placenta formed," says Martyn. "Rather than stuff mothers full of food, we need to make sure that the young women who will become mothers are well fed."

Unfortunately, attempts to make up for lost ground later in life may do more harm than good. A study of 150,000 Swedish army recruits in their late teens found those with the highest blood pressure were those who were lightest at birth but heaviest in adulthood, suggesting that trying to maximize growth postnatally may make matters worse.

Research is also revealing the role of fetal life in other disorders beyond the womb. A Southampton trial of 300 families with a family history of asthma is currently determining whether reducing a pregnant mother's exposure to allergens - house dust mites, cat fur and tobacco smoke - will protect her child from asthma in later life.

"We know from samples of the baby's blood from the umbilical cord at birth that its T-cells (part of the immune system) are already able to respond to common allergens," says Dr. Cathy Jones, researcher at the University of Southampton's Child Health unit.

A recent U.S. study of 177 young boys also found that mothers who smoke more than half a pack of cigarettes a day during pregnancy were four times as likely to have sons with behavioral problems.

But what happens in the womb amounts to predisposition rather than predetermination. Being the runt of the litter is no excuse for throwing out your jogging shoes and smoking two packs a day; on the contrary, a healthier life may be more important than you ever imagined.