Research has shown that stress can affect an unborn baby, and extreme stress can increase the incidence of very low birth weight (less than 3.3 pounds).
Led by Professor Vivette Glover at Imperial College London and Dr Pampa Sarkar of Wexham Park Hospital, Berkshire, researchers tested levels of the stress hormone, cortisol, in pregnant women's blood and in the amniotic fluid surrounding their unborn babies. As early as 17 weeks gestation a correlation was drawn between cortisol levels in the mother's blood and in the amniotic fluid. Since amniotic fluid is largely produced by the fetus the presence of cortisol levels in amniotic fluid is an indication of the unborn baby's exposure to the stress hormone. As gestation age increased so did the strength of the correlation.
Other studies have shown that babies of mothers who were under stress during pregnancy have shown IQ's averaging 18 point lower than other babies, higher rates of ADHD, hyperkinesis, and behavior problems.
An article, "Extreme Stress and Tiny Babies" (Linda Formichelli, Psychology Today) cites the research of Ralph Catalano, Ph.D., professor of public health at the University of California at Berkeley, and Terry Hartig, a docent at the Institute for Housing Research at Uppsala University in Sweden. Studies on the effects of communal bereavement (such as the widespread of grief following the September 11, 2001, attacks on the World Trade Center) showed higher rates of premature and very low birth-weight babies.
Two possible explanation of the connection between extreme stress and very low birth weight are offered: 1) It is suspected that evolutionary mechanisms may result in premature delivery when those mechanisms sense a threat to the fetus (and elevated cortisol levels essentially represent a threat). 2) There is the possibility that when elevated stress hormones compromise the expectant mother's immune system latent infections are activated.
All the effects of stress on the fetus, and the degree of those effects, are not yet fully understood. It is important to note that rises in cortisol levels in response to short-term stressful situations are normal, and studies on the effects of maternal stress on unborn babies have focused on stress that is present over a substantial period of time. (In other words, a stressful afternoon here or there is very different from living under chronic, unrelenting, stress.)
Expectant mothers need to keep in mind that they often have some degree of control over their response to, and management of, long-term stress. The presence of stressful circumstances in the mother's life is not the thing that affects the fetus. What affects the fetus is the mother's response to those stressful circumstances. Expectant mothers are encourage to discuss high stress levels with the physicians, and to learn relaxation techniques that can reduce the stress response.