Prenatal development is sometimes divided into trimesters, or three equal time periods. At the end of the third month, the first trimester is complete. By the middle of the second trimester, between 17 and 20 weeks, the developing fetus has grown large

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enough that the mother can feel its movements. Already, the fetus is remarkably active, in motion nearly 30 percent of the time. A white, cheeselike substance called vernix emerges on the skin, protecting it from chapping during

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For many years, polychlorinated biphenyls (PCBs) were used to insulate electrical equipment, until research showed that they entered waterways and the food supply. Prenatal exposure to high levels of PCBs results in low birth weight, skin deformities, brain-wave abnormalities, and delayed cognitive development. Even at low levels PCBs are linked to reduced birth weights, smaller heads, persisting attention and memory difficulties, and lower intelligence scores in childhood.
Dioxins are toxic compounds resulting from incineration. Prenatal exposure is linked to thyroid abnormalities in infancy and to increased incidence of breast and uterine cancers in women. Even tiny amounts of dioxin in the paternal bloodstream can cause a dramatic change in the sex ratio of offspring, resulting in nearly twice as many girls as boys due to impairments to the fertility of Y-bearing sperm prior to conception.