Keishawn Williams is already talking to her baby, although her child isn't due until November.

"What are you doing?" asks Williams, 22. "Are you awake? Are you asleep? Why are you sitting on my bladder?"

Although Williams may not realize it, her body and baby are also conducting a separate, even more important conversation that may influence her child's health for the rest of its life. Although neither mother nor child is aware of this crucial dialogue, Williams' body already is telling her baby about what to expect from the world outside, says Mark Hanson, a professor at the University of Southampton in England.

And thanks to those biological signals, the choices that Williams makes today — by getting good prenatal care, eating nutrient-packed vegetables and avoiding alcohol, tobacco and caffeine — may help her baby long after birth, Hanson says.

Research into the "developmental origins of adult disease" suggests that Williams' healthy living may help her child avoid problems such as cancer, heart disease, depression and diabetes not just in childhood, but 50 years from now.

Though adults still need to eat right and exercise, a growing number of studies now suggest the best time to fight the diseases of aging may be before babies are even born, says Peter Gluckman of the University of Auckland in New Zealand.

Williams' baby is still too young to kick, let alone ponder its future. But its body is already adapting and preparing for its specific environment, Gluckman says, by reading cues sent through Williams' blood and amniotic fluid.

"Every baby in fetal life is adjusting its pattern of development according to the world it predicts it will live in," he says.

CUES IN UTERO

During the crucial "window of opportunity" before birth and during infancy, environmental cues help "program" a person's DNA, says Alexander Jones of Great Ormond Street Hospital in London and the University College of London Institute for Child Health. This happens through a delicate interplay of genes and the environment called epigenetics, which can determine how a baby reacts for the rest of its life, Jones says.

Through epigenetics, chemical groups attach to DNA. Although they don't change the order of the genes, the chemical groups can switch those genes on or off, Jones says.

Many things, such as chemical contaminants, can cause epigenetic changes. So babies exposed in the womb to synthetic hormones may begin responding abnormally to the natural hormones later made by their own bodies, says Hugh Taylor of Yale University School of Medicine.

That's why, doctors believe, many babies exposed before birth to a drug called DES, or diethylstilbestrol, later developed rare cancers or fertility problems, Taylor says.

Doctors stopped prescribing DES, which had been used for decades to prevent miscarriages, in 1971. But Taylor and other scientists are concerned that "hormone-disrupting" chemicals, such as those used in pesticides and even common plastics, could cause similar problems.

Babies and children also can develop abnormal reactions to stress, says Jack Shonkoff of Harvard University, co-author of a June paper on early influences in health in the Journal of the American Medical Association.

In the short term, reacting to typical, everyday difficulties can help people develop a healthy response to stress.

But persistent, "toxic" stress — such as neglect or extreme poverty — may program a child's nervous system to be on perpetual high alert. Over time, this can damage the immune response and lead to chronic ailments, such as heart disease and depression, the study says.

DIET AS PREDICTOR

A pregnant woman's diet tells a fetus a lot about its future environment, including how much food will be available after birth, Jones says.

A baby conceived during a famine, for example, might learn to be "thrifty," hoarding every calorie and packing on fat rather than muscle, even at the expense of developing vital organs, such as the kidneys, liver and brain. Because of a lack of calories, the baby also may be born small.

In a famine, those early adjustments and predictions about the future could mean the difference between survival and starvation, Jones says.

But babies may run into trouble if the world doesn't match their predictions, Jones says.

A baby who has learned to hoard calories, for example, may grow up to be fat or diabetic once he or she finally gets enough to eat, Jones says. Doctors believe this occurs not just with babies whose mothers are starving, but with those who are malnourished because of a mother's medical problems, poor nutrition or exposure to tobacco smoke, which damages the placenta.

It's well known, Taylor says, that women who smoke are more likely to have low-birth-weight babies, who are in some ways "starved" for nutrients in the womb. Babies born too small are at risk for many immediate problems, such as underdeveloped lungs and bleeding in the brain.

If they survive, these youngsters also face long-term risks.

Studies show that small babies who gain weight rapidly in infancy or childhood — a sign that bodies are already making the most of every calorie — also have higher rates of adult heart disease and diabetes, Jones says.

Specialized X-rays have shown babies of young mothers with poor diets in India, for example, are born with extra belly fat, even though they seem to be a normal weight. Once these babies start getting an adequate diet, they are likely to put on weight, Gluckman says.

"Even by the time of birth, they're on a different pattern of development."