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Extracted from Survival Guide to Midwifery, 2nd Edition (2012) Diane M. Fraser and Margaret A. Cooper, Oxford; Churchill Livingstone: 2012. Courtesy Elsevier
The non-pregnant uterus is a hollow, muscular, pear-shaped organ. It is 7.5 cm long, 5 cm wide and 2.5 cm in depth, each wall being 1.25 cm thick. The cervix forms the lower third of the uterus and measures 2.5 cm in each direction. The uterus consists of the following parts:
The uterine (fallopian) tubes extend laterally from the cornua of the uterus towards the side walls of the pelvis. They arch over the ovaries, the fringed ends hovering near the ovaries in order to receive the ovum. Each tube is 10 cm long. The lumen of the tube provides an open pathway from the outside to the peritoneal cavity. The uterine tube has four portions:
The cavity is a potential space between the anterior and posterior walls. It is triangular in shape, the base of the triangle being uppermost.
The isthmus is a narrow area between the cavity and the cervix, which is 7 mm long. It enlarges during pregnancy to form the lower uterine segment.
This protrudes into the vagina.
The layers of the uterus are called:
This forms a lining of ciliated epithelium on a base of connective tissue or stroma.
In the uterine cavity this endometrium is constantly changing in thickness throughout the menstrual cycle. The basal layer does not alter, but provides the foundation from which the upper layers regenerate. The epithelial cells are cubical in shape and dip down to form glands that secrete an alkaline mucus.
The cervical endometrium does not respond to the hormonal stimuli of the menstrual cycle to the same extent. Here the epithelial cells are tall and columnar in shape and the mucus-secreting glands are branching racemose glands. The cervical endometrium is thinner than that of the body and is folded into a pattern known as the 'arbor vitae'. The portion of the cervix that protrudes into the vagina is covered with squamous epithelium similar to that lining the vagina. The point where the epithelium changes, at the external os, is termed the squamocolumnar junction.
This layer is thick in the upper part of the uterus but sparser in the isthmus and cervix. Its fibres run in all directions and interlace to surround the blood vessels and lymphatics that pass to and from the endometrium. The outer layer is formed of longitudinal fibres that are continuous with those of the uterine tube, the uterine ligaments and the vagina.
In the cervix the muscle fibres are embedded in collagen fibres, which enable it to stretch in labour.
This is a double serous membrane, an extension of the peritoneum, which is draped over the uterus, covering all but a narrow strip on either side and the anterior wall of the supravaginal cervix, from where it is reflected up over the bladder.
The uterine artery arrives at the level of the cervix and is a branch of the internal iliac artery. It sends a small branch to the upper vagina, and then runs upwards in a twisted fashion to meet the ovarian artery and form an anastomosis with it near the cornu. The ovarian artery is a branch of the abdominal aorta. It supplies the ovary and uterine tube before joining the uterine artery. The blood drains through corresponding veins.
Lymph is drained from the uterine body to the internal iliac glands and also from the cervical area to many other pelvic lymph glands.
This is mainly from the autonomic nervous system, sympathetic and parasympathetic, via the inferior hypogastric or pelvic plexus.
Physiological changes in pregnancy
The body of the uterus
After conception, the uterus develops to provide a nutritive and protective environment in which the fetus will develop and grow.
After embedding of the blastocyst there is thickening and increased vascularity of the lining of the uterus, or decidua. Decidualisation, influenced by progesterone and oestradiol, is most marked in the fundus and upper body of the uterus.
Uterine growth is due to hyperplasia (increase in number due to division) and hypertrophy (increase in size) of myometrial cells under the influence of oestrogen. The dimensions of the uterus vary considerably, however, depending on the age and parity of the woman.
The three layers of the myometrium become more clearly defined during pregnancy.
Uterine activity in pregnancy
The myometrium is both contractile (can lengthen and shorten) and elastic (can enlarge and stretch) to accommodate the growing fetus and allow involution following the birth.
Uterine activity can be measured as early as 7 weeks' gestation, when Braxton Hicks contractions can occur every 20-30 minutes and may reach a pressure of up to 10 mmHg. These contractions facilitate uterine blood flow through the intervillous spaces of the placenta, promoting oxygen delivery to the fetus. Braxton Hicks contractions are usually painless but may cause some discomfort when their intensity exceeds 15 mmHg.
In the last few weeks of pregnancy, prelabour occurs:
Further increases in myometrial contractions cause the muscle fibres of the fundus to be drawn up.
The actively contracting upper uterine segment becomes thicker and shorter in length and exerts a slow, steady pull on the relatively fixed cervix.
This causes the beginning of cervical stretching and ripening known as effacement, and thinning and stretching of the passive lower uterine segment, but there is no cervical dilatation at this time.
The perimetrium is a thin layer of peritoneum that protects the uterus. It is deflected over the bladder anteriorly to form the uterovesical pouch, and over the rectum posteriorly to form the pouch of Douglas. The double folds of perimetrium (broad ligaments) become longer and wider with increasing tension exerted on them as the uterus enlarges and rises out of the pelvis.
The uterine blood flow progressively increases from approximately 50 ml/min at 10 weeks' gestation to 450-750 ml/min at term.
Changes in uterine shape and size
For the first few weeks the uterus maintains its original pear shape, but as pregnancy advances the corpus and fundus assume a more globular form.
Formation of the upper and lower segments
By the end of pregnancy, the body of the uterus is described as having divided into two anatomically distinct segments.
The muscle content reduces from the fundus to the cervix, where it is thinner. When labour begins, the retracted longitudinal fibres in the upper segment pull on the lower segment, causing it to stretch; this is aided by the force applied by the descending presenting part. A ridge forms between the upper and lower uterine segments, known as the physiological retraction ring.