Uterus is the Latin word for the womb, though to the ancient Romans it also meant a man’s belly or paunch. These days the word ‘uterus’ is more commonly used than ‘womb’, the old English word. The uterus is remarkable: its growth and activities are very obedient to the commands of hormones. It can change its size and function more than any other organ.
An adult woman’s uterus is about the size and shape of the average pear, weighs around 60 g and is empty. During pregnancy it enlarges to weigh 1000 g or more and its contents – foetus, membranes, amniotic fluid and placenta – weigh around 5 kg. Within two weeks of delivery it has returned to pear size.
The wall of the uterus is composed of powerful muscle called the myometrium, which is capable of pushing a baby weighing 4 kg or more through the birth canal (the passageway through the cervical canal and vagina to the outside). The inner surface of this muscle wall is lined with endometrium, a layer of tissue consisting of glands embedded in a network of special cells. The endometrium undergoes remarkable changes with the rise and fall of hormones in the menstrual cycle and during pregnancy. The upper, wider two-thirds of the uterus is called its body, and its very top is its fundus. The tubes enter on each side near the fundus.
The cavity of the uterus is flattened from front to back into a slit-like triangular space that usually contains nothing except a thin film of fluid. This cavity continues into the tubes and through the cervical canal into the vagina.
At its lower end the uterus narrows into the cervix (neck), where the cavity (cervical canal) becomes narrow and tubular. The lower third of the cervix projects into the upper end of the vagina, and the cervical canal opens into the cavity of the vagina through the cervical os (mouth). The muscle cells in the wall of the cervix are separated by more fibres and tissue fluid than those in the wall of the uterus: this arrangement gives the cervix its astonishing capacity to dilate during the first stage of labour.
Positions of the uterus
The uterus is held in place by a number of ligaments and membranes that tether its top and sides to the walls of the pelvis. These tethers are not tight: they allow considerable movement of the uterus. They also grow and stretch as the uterus enlarges during pregnancy, quickly returning to their normal size and length after delivery.
Below, the uterus and vagina (plus the other pelvic organs – the bladder and the rectum) are surrounded and supported by an arrangement of muscles and connective tissue, the very important pelvic floor. The muscles of the pelvic floor fan out from the organs they surround to become attached to the walls of the pelvis. If the pelvic floor is weakened or damaged, the pelvic organs can prolapse (sag downwards) and problems can develop with bladder and bowel control.
Because it isn’t tightly tethered, the uterus can lean this way or that, depending on what else is happening in the pelvis. For example, as the bladder fills it lifts upwards and backwards; if the lower bowel is full the uterus can move forward; during sexual arousal it lifts upwards a little. In most women, the uterus leans forwards at an angle of about 90 degrees to the vagina when the bladder is empty, so the cervix points backwards. This position is described as anteverted. However, I would say at least two out of ten women have a uterus that normally leans backwards (retroverted), and in others it occupies mid position in the pelvis. Any of these positions is normal.
Previously it was considered a serious abnormality to have a retroverted uterus, believed to cause infertility, pelvic pain, backache, constipation and numerous other problems. This is now known to be untrue. In the past women were subjected to various surgical procedures to ‘correct’ the retroversion. A retroverted uterus is only abnormal if it is stuck (fixed) in the back of the pelvis by scar tissue arising from infection or endometriosis. A uterus that is fixed in front or to one side is similarly abnormal, and the symptoms that arise from a fixed uterus are mainly due to the scarring, rather than the position of the uterus.