Infertility is the inability or difficulty of producing children through normal sexual relations. Infertility difficulties have been increasing in modern society, along with a drop in sperm count over the last century. The cause is unknown, but speculation has turned to chemicals in the the atmosphere, food, and water that are having an adverse impact on the human body.
Infertility difficulties are also due to the age of the mothers (and sometimes fathers) when pregnancy is desired. Fertility rates are highest in the early 20's, but decrease over time, dropping steeply starting around age 39. The exact rate of fertility decline will vary from woman to woman. In Western culture it has become common to extend the age when a family is first desired, leading to greater instances of infertility due to age.