Overview of the biological aspects of the fertile period.

The fertile period of the human menstrual cycle consists of those days on which sexual intercourse can result in a pregnancy. Its duration is determined by the functional life span of the gametes within the female reproductive tract. Various mechanisms control gamete transport and survival in the reproductive tract of the human female. The ovarian hormones estradiol and progesterone have an important role in regulating these mechanisms. The nature of cervical mucus and its governing influences on sperm transport and survival following coitus are of prime importance in defining the fertile days of the menstrual cycle. Man's early concepts of the fertile period were often based on erroneous theories of the female reproductive cycle. It is only since the late 1920's that a true understanding of ovulation and the menstrual cycle has evolved. Current approaches in natural family planning to recognizing the fertile and infertile days of the menstrual cycle are discussed and evaluated.