Puberty is the process of reproductive and sexual development and maturation which changes a child into an adult. It is the time when one becomes functionally capable of reproduction and includes both physical and psychological development. In females, It includes breast development, axillary and pubic hair growth, growth spurt, menstruation and finally ovulation. Menarche refers to the first menstrual flow or menstrual period of a woman. It is in contrast to menopause which is the last menstrual flow or menstrual period of a woman, signifying the end of a woman’s natural reproductive career.

Menstruation is the process of cyclical and regular discharge of blood and dead endometrial tissue from the inner lining of the uterus/womb of a woman through the vagina, due to failure of conception at the end of each reproductive or menstrual cycle. The discharged content of menstruation is referred to as menstruum, menses, menstrual period or menstrual flow. A normal menstrual flow is one which lasts for two to eight days.

Secondary sexual characters: are features which appear during puberty in humans that distinguish a man from a woman. They are formed due to the biological effect of the major sex hormones produced by either the male or the female sex organs. In females, the hormone oestrogen produced by the ovary results in breast development, axillary and pubic hair growth, an earlier growth spurt seen in young girls compared to boys of the same age and finally menstruation. Alhough, they are not directly parts of the reproductive organs, they are important as accessory organs necessary for reproduction.

The reproductive or menstrual cycle is a natural process seen only in women of reproductive age group that prepares the female body for pregnancy but results in menstruation everytime a pregnancy fails to occur. It is orchestrated by the interaction of three major organs that include the following: 

The hypothalamus is an organ located in the central part of the brain tasked with the regulation of many systems of the body and coordination of the activities of the pituitary gland. It is important in reproduction as it is the organ responsible for the kick-starting puberty and it is important in the maintainance of  the normal integrity of the reproductive cycle via the activity of its hormone known as the gonadotrophin releasing hormone (GnRH)

The pituitary gland or the master gland is a gland located in the central part of the brain below the hypothalamus tasked with the production of numerous hormones necessary for normal body function hence the name “master gland”. Under the influence of the hypothalamus, it produces the hormones follicle stimulating hormone (FSH) and leutinizing hormone (LH) which are both responsible for coordinating the activities of the ovaries.

The ovaries are paired organs that serves as the female main organ of reproduction. Under the influence of the two glands described above, They secrete the female sex hormone oestrogen at puberty, which results in the development of the female sexual characteristics. They also serves as the reservoir for the female sex gametes called oocytes. These oocytes, each stored in its respective follicles are formed in their millions  within the ovary of a female child in the uterus many months before birth where they are stagnated in their development. While some die as the years roll by, others after puberty are selected in small numbers every month, under the influence of the hormone FSH produced by the pituitary galnd, undergoing further development and maturity until only one (in most instances) is finally chosen for ovulation following the release of the  hormone LH produced by the pituitary gland in anticipation of fertilization by a sperm. Follicles are fluid filled cavities surrounded by aggregates of hormone secreting cells each containing one undeveloped oocyteThough about 1 – 2 million in number at birth of a female child, due to gradual disintegration of some of the follicles and their containing oocytes, about 400,000 are present in the female ovaries at puberty, each with the potential to release an ovum during ovulation. Their function is to maintain and nurture the oocyte and also to produce the female sex hormones.

Ovulation is the cyclical release of oocytes or eggs from its respective follicles within the ovaries. In humans, only one oocyte is programmed to be released monthly in a natural cycle. Every cycle, some follicles containing immature oocytes are pooled out of the ovarian store and are prompted to undergo further growth and development, during which they increase in size and undergo genetic maturation, in anticipation of ovulation. Among this cohort of follicles, one of them known as the dominant follicle, begins to outgrow and out-develop the others until a point is reached when it ruptures releasing the now matured oocytealso known as the matured ovum by the process of ovulation. Immediately after ovulation till about 24 hours after, the egg is available for fertilization by a sperm after which it disintegrates or dies off, kick starting a series of events that results in menstruation.The remaining follicles and their partially matured oocytes also disintegrate and die off. If pregnancy fails to occur, the whole cycle is repeated every month till it is paused at pregnancy, lactation or till it totally stops at menopause. This is what is known as the reproductive or menstrual cycle.

Fertilization is the process by which the ovum combines with a sperm to form a zygote, which in turn grows to become an embryo and then a fetus and at delivery a baby all within a space of nine months in humans..

The menstrual cycle can therefore be described as the cyclical regular natural change that occurs in the female reproductive system that makes pregnancy possible resulting in ovulation and in the absence of pregnancy, menstruation. Because the normal process of the menstrual cycle depends on the normal interaction of the hypothalamus, pituitary gland and the ovaries, the Hypothalamo-pituitary-ovarian (HPO) axis is used to refer to the three organs acting as if they were a single entity and abnormalities of this axis can result in disorders of ovulation and menstruation. Basically, women with a normal HPO axis experience ovulatory cycles because the menstrual cycles are associated with regular ovulation and the menstrual periods are predictable and are usually normal. On the other hand, disorders of the HPO axis result in anovulatory cycles asssociated with irregular menstrual cycles due to irregular ovulation. Here, the menstrual periods are usually unpredictable, irregular and occaionally may be totally absent and are often abnormal.