While these sobering statistics are relatively easy to calculate, it's a far more complicated matter to cite their causes. And one has to consider the effects of 'environmental' factors such as viruses, medicines, illegal drugs, chemicals, tobacco and alcohol. The impact of environmental exposure is thought to depend on the stage of development of the fetus when exposure occurs. A given exposure during the first trimester might result in a birth defect while the same exposure during the second trimester might cause low birthweight. But it is important to point out that such generalities may understate the complexity of the problem because the amount, duration and route of exposure also relevant.
Birth defects are but one point of interest in medical and scientific communities.
Reproductive health issues also involve infertility, miscarriage, premature birth, low birthweight, developmental disorders, stillbirth and childhood cancers. Japanese women who consumed methylmercury-contaminated fish in 1970 produced children with mental retardation and cerebral palsy. Several cases of Swedish and Iraqi women who consumed seed treated with mercury had children with impaired motor skill. Although there are only about 40 agents recognized to cause human birth defects, many pesticides test positive in the evaluation of reproductive parameters of laboratory animals. It is these pesticides that continue to spark public debate and propel scientific research centering on low dose exposures in human populations.