But when it comes to a baby's health, it is not only these tangible variables that matter. Now a collection of new research further explores the roots of asthma and allergies, tracing them back to the womb.
"It's almost natural to continue this way," said Dr. Andy Liu, director of Pediatric Allergy and Immunology at National Jewish Medical and Research Center in Denver. "It's where causation starts."
Four studies, presented this weekend at the meeting of the American Thoracic Society in Toronto, explore how some of the less-obvious aspects of a mother's life during pregnancy -- such as her living environment or whether she delivers by Caesarean section -- can affect the baby's developing immune system and the risk of having asthma and allergies.
Among children under 18 years old in the United States, 9 million have been diagnosed with asthma and allergies, according to the American Academy of Allergy, Asthma, and Immunology.
"Increasingly, research suggests that exposures that you might encounter during early development in utero may predict a number of health outcomes," said Dr. Rosalind Wright, assistant professor of medicine at Harvard Medical School.
Wright and colleague Junenette Peters conducted a first-of-its-kind study that examined the effects of a mother's stress levels on the immune system of her babies before they were born.
Wright and Peters used a detailed questionnaire to determine how many stressors, including financial problems, relationship problems and health worries mothers-to-be experienced. They also measured the amount of dust mites, a common allergen, in the mothers' homes. Once the babies were born, the researchers measured blood levels of a particular antibody that acts as a marker for the child's allergic immune response, the IgE antibody.
Wright and Peters found that higher IgE levels correlated with higher stress levels during pregnancy, and the child was more likely to develop asthma and allergies. This effect was even more pronounced when the mother was exposed to low levels of dust mite allergens in her home during pregnancy, suggesting that the stress was the primary factor affecting the child's immune system.
Biologically, a mother has an enormous effect on her child's immune system. The baby is exposed to allergens the mother encounters through the placenta. During the third trimester, the mother begins to transfer her antibodies to the baby as well, building up the baby's immune system so that it has some measure of protection after it is born.
"The genetics are really an important part of asthma," said Dr. Devang Doshi, director of Pediatric Allergy, Immunology, and Pulmonology at William Beaumont Hospital in Royal Oak, Mich., adding that identifying it in children is key. "Research is really shifting in that direction."
But some believe there are other ways to protect the baby and boost its developing immune system even before it is born.
Dr. Bianca Schaub, leader of the research team at University Children's Hospital in Munich, Germany, conducted a study that showed that mothers who spent time on a farm, where they were exposed to animals and local milk, conferred more protection against allergies and asthma to their babies than mothers who were not exposed to farms. Schaub proposed that the greater number of microbes in a farm environment passed on to the baby via the mother, might build up the baby's immune system.
"The fetus in the womb is really an extension of mom," Doshi said. "Everything the mom is exposed to the fetus is exposed to."
Nature Versus Nurture
But genetic makeup remains the classic starting point in determining what a child will and will not be susceptible to.
In one study combining genetics with circumstance, researchers from the University of South Carolina, determined that birth order correlates with the likelihood of having asthma or allergies. First-born children are more likely to have a different version of a gene involved in allergic development, one that leaves the oldest sibling more susceptible to developing asthma or allergies than their younger siblings. The chance of developing asthma or allergies persisted in the oldest child until at least age 10.
But one study showed that certain situations, not just physical environments, could also affect a child's immune system such that they were more or less susceptible to asthma and allergies.
Researchers at the University of California at San Francisco found a correlation between birth by Caesarean section and a weaker immune response that could put children at risk for developing asthma and allergies.
According to Doshi, it will also be key to continue these studies on the children as they grow older. Patterns of asthma or allergies in babies or young children are different than in older children. Continuing this research over time could help researchers understand how severe or lasting the prenatal effects are, could be the first step in developing effective preventive strategies, or even vaccines, for asthma and allergic diseases.
"We're beginning to learn more and more that genetics plays a v large role in asthma and allergies. And so does environment," Doshi said. "When you start combining [them] together, we are finding out that these go more and more hand in hand."