In general, reproduction concerns women more profoundly than men in the physiological and social senses, because conception and development of a fetus happen in woman's body: she gives birth to the child, usually cares for it, and motherhood is culturally more important social and personal identity and institution than fatherhood. However, a woman and a man are both needed for conception (or at least female and male gametes—female egg and male sperm—are needed in assisted reproduction). A couple that has achieved a pregnancy is considered fertile. A woman and man must be sufficiently healthy to conceive. A healthy, fertile woman has approximately 400,000 immature eggs in her ovaries at the time of her birth. From puberty until menopause a woman's body goes through ovulation, in other words, periodic cycles of physical and chemical change during which an egg matures and is released from an ovary into one of the two fallopian tubes (Royal Commission on New Reproductive Technologies 1993). A woman must have at least one functional ovary and her tubes must be open and function to transport the egg to the uterus. The probability of conceiving a surviving pregnancy is highest on the two days before ovulation.
A healthy, fertile man produces 2.4–5 milliliters of semen containing between 200 million and 500 million sperm at each ejaculation. In order to conceive the anatomies of the man and woman have to be normal and their physiological and hormonal systems must function normally at the right time. The hypothalamus at the base of the brain orchestrates the body's hormonal reproductive function. Next, natural conception requires timely oocyte release, sexual intercourse, and the transport of gametes through the female reproductive tract. The woman's hormone level must be sufficient to stimulate the production of eggs and normal cervical mucus. Proper cervical mucus is necessary both near the time of ovulation and later to support implantation of the embryo and maintain pregnancy. Correspondingly, a man must be able to produce an adequate number of normal sperm and must be able to deposit them in the woman's vagina at the appropriate time during the female ovulation cycle (Office of Technology Assessment 1988). Only one sperm is needed to join with an egg in a woman's uterus for fertilization to occur. For this to happen, however, the sperm must be physically and functionally normal, active, and capable of swimming through the woman's reproductive tract.