Conception as a biological matter and a part of human reproduction means fertilization in which a sperm makes contact with an egg, fuses with it, and develops into the zygote, embryo, and fetus (Birke, Himmelweit, and Vines 1990). The monthly probability of conception without contraception is called fecundability, which for fertile couples is approximately 25 to 30 percent per month (Spira 1986), a remarkably low figure. Nevertheless, among humans fertilization can occur any given month, whereas among many other species fecundity is usually limited only to certain time of the year. An individual woman's fecundity varies as a function of her age, menstrual cycle, ovulation, and the functional status of her genital organs. Women in their 20s usually show the highest fecundability, which decreases among those over 35 years old (Day Baird and Strassmann 2000). Both female oocyte, or female egg, and uterine quality worsen with age, as do a woman's capacity to become pregnant and deliver a healthy child. In the era of birth control methods, for example, women in the United States and Finland reproduce relatively rarely, because only about 6 percent of them (15–44 years of age) give birth annually. A man's fecundity correspondingly varies with his age and seasonal and environmental factors. There is ample proof that environmental pollutants and occupational exposures to hazardous substances, such as radiation, heat, solvents, and pesticides, have adverse effects on male fertility (Feichtinger 1991). Fewer studies have inquired into the environmental effect on female fertility (Baranski 1993).