The research in July 2017 suggests that stress in pregnancy makes women more vulnerable to smoking and air pollution.
A review of one study found that pregnant women who smoke and who are very stressed and who smoke are significantly more likely to have babies with low birth weight than those who have more relaxed smokers.
The combination of high stress and air pollution also increases the risk of having a baby with low birth weight, the research adds.
The lead author, Professor Tracey Woodruff of the University of California, San Francisco, said: "It seems that stress can amplify the health effects of exposure to toxic chemicals, which means that for some people, the substances toxic chemicals become more toxic. "
Co-author and professor Rachel Morello-Frosch of the University of California, Berkeley, added: "The bottom line is that stress related to poverty can make people more susceptible to the negative effects of environmental health hazards, and that must be a consideration. " For policy makers and regulators.
The researchers analyzed 17 studies in humans and 22 animal trials that investigated the link between stress, chemicals and fetal development.
Stress was defined by factors such as socioeconomic status.
Professor Morello-Frosch added: "While the evidence on the combined effects of chemicals and stress is new and emerging, it clearly suggests an important issue of social justice."