This website is intended for healthcare professionals.
LOGIN
JOIN US
Subscriber log in
Trial log in
  

Uterus

Extracted from Survival Guide to Midwifery, 2nd Edition (2012) Diane M. Fraser and Margaret A. Cooper, Oxford; Churchill Livingstone: 2012. Courtesy Elsevier

F37.jpg

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 body or corpus: this forms the upper two-thirds of the uterus.
  • The fundus: the fundus is the domed upper wall between the insertions of the uterine tubes.
  • The cornua: These are the upper outer angles of the uterus where the uterine tubes join.

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 interstitial portion
  • The isthmus
  • The ampulla
  • The infundibulum (Figure 37)

The cavity

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

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.

The cervix

This protrudes into the vagina.

  • The internal os is the narrow opening between the isthmus and the cervix.
  • The external os is a small round opening at the lower end of the cervix. After childbirth it becomes a transverse slit.
  • The cervical canal lies between these two ora and is a continuation of the uterine cavity.

Layers

The layers of the uterus are called:

  • The endometrium
  • The myometrium
  • The perimetrium.

The endometrium

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.

The myometrium

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.

The perimetrium

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.

Blood supply

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.

Lymphatic drainage

Lymph is drained from the uterine body to the internal iliac glands and also from the cervical area to many other pelvic lymph glands.

Nerve supply

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.

Decidua

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.

  • The decidua is believed to maintain functional quiescence of the uterus during pregnancy; spontaneous labour is thought to result from the activation of the decidua with resultant prostaglandin release following withdrawal of placental hormones.
  • The decidua and trophoblast also produce relaxin, which appears to promote myometrial relaxation, and may have a role to play in cervical ripening and rupture of fetal membranes.

Myometrium

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.

Muscle layers

  • The outer longitudinal layer of muscle fibres is thin. It consists of a network of bundles of smooth muscles. These pass longitudinally from the front of the isthmus anteriorly over the fundus and into the vault of the vagina posteriorly, and extend into the round and transverse ligaments.
  • The thicker middle layer comprises interlocked spiral myometrial fibres that are perforated in all directions by blood vessels. Each cell in this layer has a double curve so that the interlacing of any two gives the approximate form of a figure of eight. Due to this arrangement, contraction of these cells after birth causes constriction of the blood vessels, providing 'living ligatures'.
  • The inner circular layer is arranged concentrically around the longitudinal axis of the uterus and bundle formation is diffuse. It forms sphincters around the openings of the uterine tubes and around the internal cervical os.

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.

Perimetrium

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.

Blood supply

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.

 

Weeks Changes in the pregnant uterus
10 weeks The uterus is about the size of an orange
12 weeks The uterus is about the size of a grapefruit
It is no longer anteverted and anteflexed and has risen out of the pelvis and become upright
The fundus may be palpated abdominally above the symphysis pubis
The globular upper segment is sitting on an elongated stalk formed from the isthmus, which softens and which will treble in length from 7 to 25 mm between the 12th and 36th weeks
20 weeks The fundus of the uterus can be palpated at the level of the umbilicus
As the uterus continues to rise in the abdomen, the uterine tubes become progressively more vertical, which causes increasing tension on the broad and round ligaments
30 weeks The fundus may be palpated midway between the umbilicus and the xiphisternum
38 weeks The uterus reaches the level of the xiphisternum
As the upper segment muscle contractions increase in frequency and strength, the lower uterine segment develops more rapidly and is stretched radially; along with cervical effacement and softening of the tissues of the pelvic floor, this permits the fetal presentation to begin its descent into the upper pelvis
This leads to a reduction in fundal height known as lightening, relieving pressure on the upper part of the abdomen but increasing pressure in the pelvis. In the majority of multiparous women, however, engagement rarely occurs prior to labour

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 upper uterine segment is formed from the body of the uterus
  • The lower uterine segment is formed from the isthmus and the cervix, and is about 8-10 cm in length

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.

Return to index