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Folic acid, vitamin A and other nutrients reach developing embryos and fetuses by crossing the placenta, the remarkable temporary organ produced by the blastocyst that develops from the fertilized egg. The outer ring of cells that compose the placenta are extremely aggressive, behaving very much like tumor cells as they invade the uterine wall and tap into the pregnant woman's blood vessels. In fact, these cells actually go in and replace the maternal cells that form the lining of the uterine arteries, says Susan Fisher, a developmental biologist at the University of California, San Francisco. They trick the pregnant woman's immune system into tolerating the embryo's presence rather than rejecting it like the lump of foreign tissue it is.
In essence, says Fisher, "the placenta is a traffic cop," and its main job is to let good things in and keep bad things out. To this end, the placenta marshals platoons of natural killer cells to patrol its perimeters and engages millions of tiny molecular pumps that expel poisons before they can damage the vulnerable embryo.
Alas, the placenta's defenses are sometimes breached--by microbes like rubella and cytomegalovirus, by drugs like thalidomide and alcohol, by heavy metals like lead and mercury, and by organic pollutants like dioxin and PCBs. Pathogens and poisons contained in certain foods are also able to cross the placenta, which may explain why placental tissues secrete a nausea-inducing hormone that has been tentatively linked to morning sickness. One provocative if unproved hypothesis says morning sickness may simply be nature's crude way of making sure that potentially harmful substances do not reach the womb, particularly during the critical first trimester of development.
Timing is decisive where toxins are concerned. Air pollutants like carbon monoxide and ozone, for example, have been linked to heart defects when exposure coincided with the second month of pregnancy, the window of time during which the heart forms. Similarly, the nervous system is particularly vulnerable to damage while neurons are migrating from the part of the brain where they are made to the area where they will ultimately reside. "A tiny, tiny exposure at a key moment when a certain process is beginning to unfold can have an effect that is not only quantitatively larger but qualitatively different than it would be on an adult whose body has finished forming," observes Sandra Steingraber, an ecologist at Cornell University.
Among the substances Steingraber is most worried about are environmentally persistent neurotoxins like mercury and lead (which directly interfere with the migration of neurons formed during the first trimester) and PCBs (which, some evidence suggests, block the activity of thyroid hormone). "Thyroid hormone plays a noble role in the fetus," says Steingraber. "It actually goes into the fetal brain and serves as kind of a conductor of the orchestra."