The uterus is a pear-shaped organ, with the narrow cervical part extending from the vaginal roof. It widens out into the vault. A narrow canal penetrates the cervix. This is called the cervical canal, and the external part that joins up with the vagina is called the external os (or hole), and the inner one is termed the internal os.
Inside, the cavity of the uterus widens out, although in normal everyday life its thick walls lie very closely together. The uterus measures about nine centimetres long, six centimetres in width at its widest part and four centimetres in thickness from front to back. It weighs about 40 grams to 60 grams, and its walls (when not pregnant) are one to two centimetres in thickness.
It sits in the pelvis in the midline, and tilts forward. Behind is the bowel, and in front is the bladder.
The uterus is lined with very special tissue called the endometrium. The thickness of this varies from day to day, according to the day of the menstrual cycle. This is under the direct control of
chemicals called hormones, which are produced by the ovaries, as well as by some other parts of the body. Every 28 days (this varies a little with each woman) menstruation takes place. During this time, which extends anywhere from two to nine days, the lining of the uterus is gradually shed. This is recognised by the woman in the form of bright red bleeding occurring on a regular basis. It is termed the menstrual period, but goes by many colloquial names. Whenever a woman uses quaint names for it, it is usually not difficult to understand what she is referring to.
The main part of the uterus is called the body, and the very uppermost part is referred to by doctors as the fundus. This part is often quoted during examinations in the course of a pregnancy, for the height of the fundus gradually rises as the pregnancy advances. It can give a rough estimation of the age of the developing foetus inside.
At the upper corner of the uterus (called the cornu or horn) on each side, a narrow canal leads out and joins with the oviduct. This is a tube, and on each side extends outwards for about 10 centimetres.
The oviduct is also known as the Fallopian tube. At the far end, the oviduct comes into close approximation with the ovary, a whitish-grey organ about the size and shape of an almond. The ovary is packed with microscopic, partially developed eggs, which have been present since birth. By the time puberty is reached (anywhere from nine years onwards), nearly a quarter of a million of these eggs are contained in the ovary. This is an enormous number, and only a fraction of the total will ever be utilised during a woman’s normal reproductive life cycle.
The far end of the oviduct has numerous fingerlike tentacles that largely overhang the ovary. They are present for a very special and important reason. Approximately each 28 days (this varies in different women, but averages this figure), a developing ovum suddenly works its way to the surface of the ovary, and bursts through the surface. It leaves a minute space on the surface of the ovary that quickly fills with blood, and this becomes known as the corpus luteum. Very rapidly this organ commences to produce a special hormone, or chemical, called progesterone, which is pumped into the general bloodstream, and has a profound effect on the uterus.
Meanwhile, the released egg, or ovum, is suddenly swept up by the fingerlike tentacles of the oviduct. It makes its way rapidly into the free end of the tube. The cells lining the tube have special hair like projections on them called cilia. These move with a sweeping motion in the direction of the uterus.
So the egg has little choice but to be carried along toward the uterus. Should the egg encounter a male reproductive cell (called a sperm) while in the tube, then fertilisation takes place as the two cells unite. At this instant, conception has occurred, and the woman is then pregnant.
The fertilised egg continues on its way to the uterus, which under the powerful influence of progesterone (from the corpus luteum of the ovary) is rapidly preparing for pregnancy.
The uterine walls thicken, its blood supply is increased, and all is in readiness. The single cell of the egg quickly subdivides, then redivides, and continues to divide, the number of cells doubling on each occasion. By the time it reaches the uterus it has developed into a mass of cells. This then becomes embedded in the thick lining (the endometrium) of the uterine walls, and the pregnancy is under way.
But if the egg fails to contact a male cell, a different situation occurs. It will reach the uterus, and fails to become embedded in the walls, even though they are ready to receive it.
Suddenly the whole process comes to an abrupt halt. The corpus luteum senses that pregnancy has not occurred, so it turns off its progesterone production. Suffering from this lack of chemical, the uterine wall gradually commences to crumble. At first this is represented by a tiny trickle of bright red blood, which escapes to the outside through the vagina.
Over the next several days, this increases, until finally the entire endometrial lining is shed in this manner. A normal menstrual period takes place; the uterine wall is reduced to its. normal state, and once more another cycle commences. Menstrual bleeding starts about 14 days following the release of the egg from the ovary, if pregnancy has not taken place.
Meanwhile, back at the ovary, another follicle gradually makes its way to the surface, and the whole process is repeated. Nature is very persistent. Even though the chances of pregnancy in any one menstrual month are very slim, the process is repeated from the time of the menarche (when menstruation occurs in young females, often around the age of 10 – 16 years) until it ceases, at the so called change of life, or menopause, which is anywhere from the age of 45 – 50 years, or maybe even later.
Ovulation, the moment the egg is released from the ovary, takes place 14 days before the onset of a menstrual period. Pregnancy may take place only in the 24 – 48 hours immediately following this event. This, of course, means that sexual intercourse must occur during that time.
With the enormous number of unplanned, and apparently unwanted babies that are conceived, it is incredible what can take place on a universal basis during that critical 24-48-hours time segment!
Nature’s chief aim is to guarantee reproduction of the species. Therefore, it uses every wile to ensure that this will take place. Just as the system’s hormones help to secure pregnancy internally, so they have profound psychological and emotional effects as well.
Many women have a heightened libido (a desire for lovemaking) at the crucial time when pregnancy is most likely. They are more responsive, more sensuous, more amorous, more “desirable,” as many a faltering male has later on discovered to his dismay (or joy, whatever his attitude to the outcome happens to be).
The act of intercourse means that the male organ of reproduction, the penis, penetrates the vagina. Erection occurs, and this is stimulated by psychological overtones, all inherent in the lovemaking process.
At the point of orgasm (frequently referred to as the climax), millions of male reproductive cells (called spermatozoa, or sperms for short) pour forth from the end of the penis, to be deposited high up in the vaginal tract.
The uterine cervix is bathed temporarily in the seminal fluid, and the cells actively swim up the cervical canal into the uterine cavity. Many are casualties by the way, but a few stalwarts ultimately penetrate into the oviducts (the Fallopian tubes), and work their way along the duct in the quest for an egg.
Should one be encountered, the remaining sperms will actively surround the egg. Finally, one with greater sustaining powers than his fellows will penetrate the outer layers of the egg, enter it, and immediately fuse with it inside to produce a fertilised egg. At this instant, conception has taken place, and the woman is then pregnant. The egg continues on its way down the tube as described previously.