Between the time of conception and birth, there is a huge vulnerability to the effects of environmental pollutants, which may be either short term or long term. The fetus undergoes rapid growth and organ development. The actual effect of a given contaminant depends on the exact sensitive time periods of exposure and the concentration of the environmental agent. The majority of these sensitive time periods are found in the first trimester of the pregnancy. Environmental exposures can reduce birth weight and cause premature birth or lead to certain birth defects. These babies are more likely to die in infancy. If they do survive, they are at high risk of brain, respiratory, and digestive problems in their early lives, as well as learning disabilities.
The fetus may be more prone to genetic damage from much lower concentrations of chemicals than the adult. The fetus clears the toxicants much less efficiently than the adult does. The developing nervous system is extremely sensitive and the specialized organs and tissues are highly vulnerable to deficits in oxygen and nutrients, as well as to toxic chemicals. A number of polycyclic aromatic hydrocarbons are reproductive and developmental toxicants, as well as mutagens and carcinogens. They can bind to the DNA and damage it. They can disrupt the endocrine system by altering the metabolic pathways of the natural hormones or interfere with their activities. Polycyclic aromatic hydrocarbons are found in ambient air indoors and outdoors, attached to particulate matter, at the workplace, in food, in water, and associated with a variety of activities.
The fetal brain is unusually vulnerable because it lacks a blood-brain barrier and so does not have the ability to detoxify chemicals. Learning and behavioral disabilities may be the result of a complex interaction of genetic, social, and environmental factors, including low-level exposure to toxic chemicals.
Exposure to lead within the uterus during this time period causes more damage to the nervous system than at any other time of development. The lead exposure may cause hyperactivity, compulsive behavior, reduced IQ, and aggression. Exposure to ethanol can lead to brain malformation and fetal alcohol syndrome. Exposure to methyl mercury has caused cerebral palsy and severe mental retardation. Even small amounts of mercury can impair IQ, language development, visual-spatial skills and memory. Elevated levels of manganese can cause hyperactivity and are associated with Parkinson’s disease. PCBs can decrease reflexes and IQ, and delay mental and motor skill development. Tobacco smoke and nicotine can reduce IQ and lead to learning disorders.
Other chemicals are endocrine disruptors, altering and interfering with normal hormonal function by binding to receptors, blocking them or interfering with proteins which regulate the production, transport, metabolism, and activity of hormones. Endocrine disruptors can work at very low dose levels. The effects of the chemicals may not be seen for years. The fetus can experience acute toxicity to carbon monoxide at levels that are harmless to healthy children. Other chemicals, which have teratogenic properties, can cause birth defects, congenital anomalies and congenital malformations, and abnormalities of structure, function, or metabolism.
Fetal exposure to environmental pressures such as heat, noise, and ionizing radiation may occur from outside of the placental barrier. Also, exposure of the parents prior to, or at the time of conception, to anesthetic gases and some solvents can produce adverse reproductive effects. Exposure to lead within the uterus during this time period causes more damage to the nervous system than in any other time of development.