To get a proper understanding of infertility one needs to first understand the human reproduction system in detail and way it functions. Humans reproduce through fertilization of the sperm with an egg. For fertilization to occur, sperm must be deposited in the vagina within a few (5) days before or on the day of ovulation. Sperm transfer is accomplished by copulation.
However, often couples are unable to conceive even after having regular unprotected sex. This is referred to as infertility. This could be because of the biological inability of an individual to contribute to conception, or because the female cannot carry a pregnancy to full term. Men and women have different underlying reason that can make pregnancy not possible.