But according to a provocative new field of research, what happens during pregnancy can have lasting consequences that emerge decades after the child leaves the hospital. Studies are finding that adult illnesses like heart disease, stroke, cancer and diabetes can have roots in the mysterious months we spend in the womb.
For example, scientists have found that a diet containing excessive protein can suppress fetal growth and lead to adult-onset hypertension. Expectant mothers who starved during their final trimester as a result of the Dutch famine of 1944-45 were more likely to have babies who later developed Type 2 diabetes. And the children of obese mothers also are at high risk of Type 2 diabetes and metabolic syndrome.
"Human beings break down the same reason cars break down; they're either driven on bad roads or made badly in the first place," said David Barker, a professor of clinical epidemiology at England's University of Southampton, who in 1989 initially advanced the idea that coronary heart disease might originate in fetal life. "Some people are just strong and some are not. Being made bad means, biologically, that you have fewer functioning units."
Experts stress that this field of study is relatively new and that the physical mechanisms that might explain the correlations between stressors in the womb and mechanical problems down the road are unclear.
It is also not lost on researchers that some pregnant women already are wracked with guilt over forgetting their prenatal vitamins or eating hot dogs instead of broccoli.
"I feel like a walking bomb," said Chicago's Amy Elstein, 28, who is five months pregnant and fears that her stress levels are affecting her baby. "It's like my body is not my own. Everything I put into it — what I eat, what I breathe — I worry that will have an effect on my child."
"Pregnancy feels like a period in your life when you want very badly to do the right thing, but you don't have control of what's going to happen, so women look for areas they can control," said Dr. Ann Borders, an assistant professor of obstetrics and gynecology at Northwestern University. "We're trying to help women be aware of unhealthy stresses but not freak out that they're hurting baby for long term."
The current advice for pregnant women still stands: Eat nutritious foods, exercise, reduce stress and avoid smoking and drinking.
But Barker and other scientists in the field want to step up prenatal care radically because they believe the diets of girls and young women are determining the health of the next generation.
Eventually, this area of research "will make a huge impact on not just what we tell women during pregnancy, but what our children's health will be," said Alan Guttmacher, director of the National Institute of Child Health and Human Development.
It was once widely assumed that, aside from cigarette smoke, drugs and excessive alcohol, the uterus, or womb, sheltered the fetus from environmental influences. Scientists also thought that the growing fetus could siphon off necessary nutrients from a mother like a parasite to ensure its survival.
Two decades of research into the fetal origins of disease, however, have challenged both assumptions and led to a revolutionary shift into the thinking about health and development.
According to Barker's widely accepted fetal origins theory, also referred to as the developmental origins of health and disease, stressors in the womb can permanently change a fetus's body structure, physiology and metabolism. Those changes then can lead to a higher risk of illness in the future.
Though some research looks at the effects of environmental exposures and psychological stress, much attention centers on nutrient deprivation, which occurs when the mother isn't getting amino acids, glucose and lipids to the developing baby quickly enough to meet its growing needs.
For example, most pancreatic beta cells — they produce the insulin that regulates blood sugar — are produced during fetal life, said Dr. Susan Ozanne, a British Heart Foundation senior fellow at the University of Cambridge. If the environment in the womb is suboptimal, as when a mother isn't getting enough protein to the fetus, the baby can wind up with fewer beta cells.
"This causes us problems later on, particularly if we place high demands on our body to produce insulin by becoming obese or eating high-sugar diets. Then we don't have enough beta cells to produce the amount of insulin we need," Ozanne said.
Nourishing a fetus, however, involves more than just eating the right foods. A woman's body composition also can affect how well her placenta transfers nutrients to the fetus.