Physiology is the study of the way that the body functions.

The following video explains the changes occurring in the ovaries and uterus during the menstrual cycle and the hormone changes that control the cycle.

Why did I develop into a girl?

The reproductive organs of boys and girls are determined by chromosomes (genes). Human cells have twenty-three pairs of chromosomes. One of each pair comes from your mother and the other from your father. The chromosomes determine how the body develops and functions. The length of each pair of chromosomes is identical with the exception of the sex chromosomes of the male. The chromosomes are numbered 1- 23. The mother will provide an X chromosome to all her eggs but the father provides either an X-chromosome from his mother or a Y-chromosome from his father. The sex chromosomes (X and Y) are the chromosome pair number 23.

Ordinary cells duplicate themselves completely, including the pairs of chromosomes, before dividing into two, by a process called mitosis. During production of egg cells and sperm cells, only one of each pair of chromosomes is replicated, this process being called meiosis. As a result, the gametes, the eggs and sperm, have twenty-three single chromosomes; they are not in pairs. The fertilised egg, which is now called the embryo, will have twenty-three pairs of chromosomes - the twenty-three from the egg and the twenty-three from the sperm. If the fertilised egg has two X-chromosomes, the resulting child will be a girl and if there is one X-chromosome and one Y-chromosome the child will be a boy.

Until the eighth week after conception, there is no obvious difference between the external genital area of the two sexes. Over the next few weeks the differences become apparent. 

Egg and sperm development.

All the eggs available to a woman have developed in her ovaries whilst she was a baby. They lie dormant until they go into an active phase (Q 2.3). Sperm development (spermatogenesis) is an ongoing process throughout a man's adult life. It takes approximately three months for a sperm to be produced and reach maturity. It has been estimated that each testicle is producing 1000 new sperm every second. 

What is a hormone?

A hormone is a chemical substance that is produced in a special tissue within a gland. Hormones are released into the bloodstream and then travel to distant responsive cells where they exhibit characteristic effects. A hormone acts like a key fitting into a lock and this produces a characteristic response. We call the 'locks' hormone receptors. Hormones are essential chemical regulators of body functions. Well known examples include insulin, which prevents our blood sugar rising too high after a meal, and thyroxin, the thyroid hormones, which regulates the speed of body activities.

Hormones are extremely potent. Although they are released in tiny amounts, their effects are profound. An oestradiol implant, which is introduced under the skin, usually when the ovaries are no longer functional after the menopause (Q 28.6), is smaller than an airgun pellet and most of it is composed of inert material which slowly releases the hormone. A typical oestradiol implant will contain only one twentieth of a gram of oestradiol. This may be enough for one year to maintain generalised well-being, to relieve menopausal symptoms and, in the long-term, to reduce the risks of cardiac disease and osteoporosis significantly.

Endocrinology is the medical discipline involving hormone investigation and treatment.

What is a steroid hormone?

Steroid hormones are a subgroup of lipids ('fat' chemicals), which share a chemical structure characterised by four fused rings (Figure2.5). Each ring has five carbon atoms (C). Cholesterol (chole - bile, and steros- solid) was the first of this group to be purified and gave us the group name 'steroids'. There are complex chemical pathways from cholesterol through to the sex steroids - progesterone, testosterone and the oestrogens. Testosterone is one of the male hormones, called androgens, and oestradiol is one of the female hormones called oestrogens. Men and women have both androgens and oestrogens although in women oestrogens predominate whereas in men there are more androgens. There is only one progesterone although synthetic 'progestogens' used in treatment regimens are similar in structure.

What is sex hormone binding globulin (SHBG)?

Hormone molecules are bound to proteins in the blood. Bound hormone molecules are inactive until they are released and become free. The main protein produced by the liver and released into the blood is albumin, which is similar to the egg white of a chicken egg. Sex hormone-binding globulin (SHBG) is one of the proteins produced by the liver. The binding capacity of this protein for testosterone is 30,000 times greater than that of albumin. During their reproductive years, women have double the concentration of SHBG when compared to men as oestrogens encourage SHBG production. Androgens, such as testosterone, suppress SHBG production. Women with hirsutism (excess body hair Q8.6) associated with hyperandrogenism have relatively low SHBG levels resulting in increased levels of free, actively available, androgens. 

What controls reproductive hormone release?

There are a number of biological rhythms controlled by a pacemaker near the hypothalamus. Releasing hormones are discharged episodically in pulses from the hypothalamus rather than as a continuous flow. External stimuli including darkness and light (circadian rhythm) and the sleep-wake cycles influence the releasing hormone pulse Frequency. Abnormality of these biorhythms may be responsible for some types of abnormal reproductive function.

There are usually negative feed back systems that reduce stimulating hormone release. As oestrogen levels rise, the gonadotrophin, the hormones that influence the production of the sex steroids ovaries, output falls (Figure 2.6). the whole setup is known as the hypothalamic-pituitary ovarian axis.

Figure 2.6 Hypothalamic-pituitary ovarian axis

How do oestrogens and androgens affect me?

Oestrogens are fundamental in the development of your feminine characteristics including physical shape, breasts, uterus, external genitalia, skin texture and emotion.

Oestrogens are characterised by their relationship to oestrus, or sexual heat, in many species. Libido, the sexual drive, is more complex in women than in men with emotion probably having a more significant role than hormones. The androgens play a major part in the libido of a man but they do not seem to be quite as important for a woman.

An interesting variation of hormone action is the effect of oestrogens, which increase SHBG levels and this reduces the free, actively available, androgen level. This oestrogenic action is of benefit in the treatment of hirsutism (excess body hair).

What are progesterone and progestogens?

Fourteen days before the next period is due, the dominant follicle releases its egg at ovulation. The follicle changes its structure and becomes a corpus luteum (Figure 2.3) in the luteal phase of the menstrual cycle. It continues to function but now it not only releases oestrogens but also progesterone. The progesterone causes a change in the lining of the womb from a proliferative to a secretory pattern. In pregnancy, the lining of the uterus changes its structure to become the decidua which in turn develops into the placenta or afterbirth. Progesterone (pro -in favour and gestation pregnancy) is essential for the development of the decidua. The decidua produces a hormone called HCG (human chorionic gonadotrophin). This HCG is essential to keep the corpus luteum functioning. A cycle is set up with the decidua producing HCG which results in the corpus luteum producing progesterone and this in turn maintains the decidua. From about the twelfth week of pregnancy, the placenta has formed and is self-sufficient in the production of progesterone.

Figure 2.3 The Menstrual Cycle - Hormone Changes

In non-conception cycles, there is no HCG to maintain the corpus luteum and it gets smaller and disappears. When the corpus luteum stops releasing oestrogen and progesterone, the endometrium is shed and a period is the result.

Progesterone has effects on the endometrium and the breasts but only when they are primed by oestrogen.

Progestogens are drugs that have progestogen-like effects.

How do my menstrual cycles occur?

From your menarche, the first period, to your menopause (Greek: men- month; pausis cessation) the endometrium (lining of the womb) changes its structure cyclically under the influence of the sex steroids hormones, oestrogen and progesterone with menstruation marking just one of these cyclical changes. The menstrual cycle is interrupted by pregnancy when specific endometrial changes occur (decidualisation). The menstrual cycle is controlled by hypothalamic and pituitary hormones (the hypothalamus and pituitary gland are situated at the base of the brain) and by ovarian hormones. (Figure 2.6)

The hypothalamus provides the link between the brain and pituitary gland by producing releasing and inhibiting factors which control the pituitary gland's secretion of hormones into the blood. There are five releasing hormones each composed of a chain of peptides, which are the basic unit for proteins. Gonadotrophin releasing hormones, which controls the sex hormone stimulating hormone output of the pituitary gland, is a decapeptide - a chain of 10 peptides. Thyrotrophin releasing hormones, which controls the thyroid gland through thyroid stimulating hormone of the pituitary, consists of just three peptides. Reproductive dysfunction, an abnormality of the control of the menstrual cycle and fertility, can result from functional abnormality of any of these releasing hormones.

The pituitary gland functions as two separate areas the anterior pituitary, which is at the front of the gland and the posterior pituitary which is the part at the back. The anterior pituitary produces the two gonadotrophic hormones known as follicle stimulating hormone (FSH) and luteinising hormone (LH).

Ovarian control of pituitary function is principally through a negative feedback by oestradiol . A rise in LH level, the LH surge, triggers egg release (ovulation) 36 hours later. At the end of a menstrual cycle, oestradiol production falls with the demise of the corpus luteum (Figure 2.3) and FSH levels begin to rise. FSH release is inhibited by oestradiol. At the menopause, the ovaries run out of functional eggs and oestradiol levels fall. As a result, the pituitary releases high levels of FSH in an attempt to gain a response from the ovaries (4). There are other regulators of pituitary gonadotrophin release. Current interest is focusing on one called inhibin.

A third important pituitary hormone involved in reproduction is prolactin. This hormone derives its name from the fact that it encourages milk production. Levels of prolactin normally rise after childbirth.

Why do menstrual cycles stop - (amenorrhoea)?

Table 6. 1 Some of the more frequent causes of amenorrhoea:

System or organ


Question Number

Physiological (natural)

(Pre-puberty,pregnancy, lactation and menopause


Generalised ill health



Weight loss e.g. anorexia




Thyroid disease




Pituitary / Hypothalamus



Gonads (usually ovaries)

Polycystic Ovary Syndrome


Hormone secreting tumours


Turner's Syndrome


Testicular Feminisation


Resistant Ovary Syndrome


BSO (Removal of both ovaries)



Congenital Absence




Asherman's Syndrome



Imperforate Hymen


Congenital Absence



Pregnancy is the most common cause of secondary amenorrhoea. A pregnancy test is usually recommended for women whose menstrual periods have stopped, even if the results of a home pregnancy test are negative.

With most types of ovarian failure, pregnancy can be achieved using donor eggs.

Please click on the required question.

Women's Health

This is the personal website of David A Viniker MD FRCOG, Consultant Obstetrician and Gynaecologist - Specialist Interests - Reproductive Medicine including Infertility, PCOS, PMS, Menopause and HRT.

I do hope that you find the answers to your women's health questions in the patient information and medical advice provided.

The aim of this web site is to provide a general guide and it is not intended as a substitute for a consultation with an appropriate specialist in respect of individual care and treatment.

David Viniker retired from active clinical practice in 2012.
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