reproductive anatomy anatomy of the maternal pelvis and the fetal skull
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REPRODUCTIVE ANATOMY ANATOMY OF THE MATERNAL PELVIS AND THE FETAL SKULL. Dr. Shaima Abozeid Arab& Libyan Board. Objective An understanding of the development and anatomy of the female genital tract is important in the practice of obstetrics and gynaecology.

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reproductive anatomy anatomy of the maternal pelvis and the fetal skull

REPRODUCTIVE ANATOMY ANATOMY OF THE MATERNAL PELVISAND THE FETAL SKULL

Dr. Shaima Abozeid

Arab& Libyan Board

slide2
Objective

An understanding of the development and anatomy of the female genital tract is important in the practice of obstetrics and gynaecology.

Both the urinary and genital systems develop from a common mesodermal ridge , so it is important to remember that congenital anomalies of the genital tract may be associated with congenital anomalies of the urinary tract.

This is a reminder and not a comprehensive review of anatomy and embryology.

slide3
Anatomy of the female pelvis and fetus relevant to labour

It is difficult to practise perceptive and accurate medicine without a reasonably sound knowledge of the way structures relate to each other , the nature of their nerve and blood supply , and what happens when the area is cut or traumatized.

slide4
THE GENITALIA

Is divided into external and internal genitalia.

External Genitalia

These are visible on inspection and together with the vagina, serve the function of coitus.

The female external genitalia, commonly referred to as the vulva, include the mons pubis, the labia majora and minora , the vestibule, the clitoris and the greater vestibular glands.

Structures within the urogenital triangle arise in both sexes from common embryonic origins. In the female these include the vulva and external genitalia.

slide5
This is a fibro fatty cushion lying anterior and superior to the junction of the pubic bones (symphysis pubis).

Inferiorly it divides to become continuous with the labium majus on each side of the vulva. It is covered by hair, the distribution of which in the female does not extend upward onto the abdominal wall.

Mons Pubis

slide6
Labia Majora

These are hair-covered fibro fatty folds that extend from the mons pubis above the perineum below.

They have both sweat and sebaceous glands, and a few specialized apocrine glands and are homologous with the scrotum in the male.

In the deepest part of each is a core of fatty tissue continuous with that of the inguinal canal and the fibres of the round ligament end there.

A rich plexus of veins is present which may rupture with trauma to form a haematoma.

slide7
Labia Minora

The lesser labial folds are enclosed by the labia majora and are smaller and more delicate. They contain no hair, but contain sebaceous glands and a few sweat glands. They are richly vascular and plentifully supplied with nerve endings, and become turgid during sexual excitement.

Superiorly, they divide into two to form the prepuce and frenulum of the clitoris and inferiorly merge to form the fourchette (or posterior ring of the vaginal introitus).

They are not well developed before puberty and atrophy after menopause. In the male, these structures form part of the penile urethra.

slide8
Clitoris

This is the homologue of the male penis. It is composed of a vascular plexus (erectile tissue) arranged in a central corpus with two crura, the corpora cavernosa, which are attached to the inferior rami of the pubis. It is covered by the ischiocavernosus muscle while the bulbospongiosus muscle inserts into its root.

It measures 1.5-2 cm, the terminal 0.5 cm is called the glans, has a highly developed nerve supply and is extremely sensitive. The folds of the labia minora enclose the clitoris, forming the prepuce above and the frenulum below.

slide9
Vestibule

This is the area enclosed by the labia majora, and the cleft between the labia minora.

The urethra and vagina open into it, as do the paired Bartholin glands and Skenes glands.

It represents the lower part of the embryological urogenital sinus.

Urethral Meatus

The external urinary orifice is 1-1.5 cm below the clitoris. It is often covered by the folds of the labia minora, which must be separated to expose it , for example, for passing a urinary catheter.

The urethra is a membranous tube 3-5 cm long, for the passage of urine. The proximal two-thirds are lined with stratified transitional epithelium and the distal third by stratified squamous epithelium.

slide10
The vagina

It is a fibro muscular tube which links the uterus to the vestibule. It is longer in the posterior wall around 9 cm than anteriorly approximately 7 cm. The vaginal walls are normally in apposition except at the vault where they are separated by the cervix.

The vault of the vagina is divided into four fornices, posterior, anterior and two lateral.

The mid-vagina is a transverse slit and the lower portion is H shape in the transverse section.

slide11
The upper posterior vaginal wall forms the anterior peritoneal reflection of the pouch of Douglas.

The middle third is separated from the rectum by pelvic fascia and the lower third abuts the perineal body.

Anteriorly ,the lip of the vagina is in contact with the base of the bladder while the urethra runs down the midline to open to the vestibule.

Its muscles fuse with the anterior vaginal wall. Laterally , at the fornices , the vagina is related, to the attachment at the cardinal ligaments.

slide12
Below this are the levator ani muscles and the ischiorectal fossa.

The cardinal ligaments and the uterosacral ligaments which form posteriorly from the parametrium , support the upper part of the vagina and the ischiorectal fossa.

Age changes

At birth, the vagina is under the influence of maternal oestrogens so the epithelium is well developed.

After a couple of weeks the effects of the oestrogens disappear and the pH rises to 7 and the epithelium atrophies.

At puberty the reverse occurs and finally at the menopause the vagina tends to shrink and the epithelium atrophies.

slide13
Uterus

This is the centrepiece of the reproductive apparatus. It has two functional elements:

(1)- a lower cervix- which functions as a passageway, a barrier and a reservoir, and

(2)-an upper body, in which the fetus develops.

The uterus measures 7.5x 5x2.5 cm and the length of the cavity is 5-6 cm in the mature woman.

slide14
Cervix

There is a vaginal and supravaginal component.

The cervix is a strong pivotal point for uterine stability being attached to the pelvic walls by pubocervical ligaments anteriorly , uterosacral ligaments posteriorly and the transverse cervical ligaments laterally.

Premature bearing down in labour may seriously damage and weaken these ligaments causing uterine prolapse.

The cervix is 2-3 cm long , and is delineated by the external os inferiorly and the interior os superiorly.

slide15
The internal os separates the proximal cervix from the uterine body while the external os separates the distal cervix from the vagina.

The shape of the external os is spherical pinpoint in a nullipara and transverse (slit-like) in a multipara.

The reddish columnar epithelial lining the endocervix may be seen (exaggerated by ectropion formation), and also the small orifices of the cervical glands. When the ducts of the glands are blocked by inflammation small retention cysts form and are obvious as Nabothian follicles.

slide16
The uterine body

(A)Corpus- the larger upper body above the isthmus.

(B)Isthmus- the transverse constriction between the corpus and the cervix.

(C)Cornu -isthe lateral part of the uterine body at the point of entry of the fallopian tubes. The uterus is covered externally by peritoneum, except the lower part anteriorly, where the peritoneum is reflected onto the bladder. It is at this loose attachment that the incision is made in the lower segment Caesarean section operation (Bandl ring).The lower segment lies at the junction of the uterus and cervix and expands during pregnancy and labour.

In pregnancy the uterus is usuallydextrorotated(rotated to the right side).

slide17
The uterus is globular in shape, but flattened in the anteroposterior direction.

Normally ,it is bothanteverted (rotated forward) and anteflexed (bent forward on itself), in more than 50% of women. In about 20% of women it is rotated backwards lying more in relation to the rectum than the bladder (retroverted) and (retroflexed)(bent backwards on itself).

It is in this group that the rare complication of incarceration of the uterusoccurs in the late first trimester of pregnancy (the uterus is caught in the hollow of the sacrum). The remaining percentages of women have a midposition uterus. 

The fallopian tubes (representing the unfused proximal parts of the Mullerian ducts) are continuous with the uterus, (representing the fused distal portions of the Mullerian ducts).

Occasionally various duplications and deletions can occur resulting in a variety of uterine congenital anomalies.

slide18
Layers

(1)Mesometrium(serosa) - is formed by the peritoneal covering and its associated blood vessels, lymphatics and nerves and it covers the surface of the uterus.

(2)Myometrium(smooth muscle) - is the middle muscular layer and is composed of several interlacing layers of smooth muscle. During pregnancy under the influence of oestrogen, great enlargement of the muscle fibres occurs (10-20 Xs increase in length), ready for the task of expelling the fetus in labour. The content of muscle in the cervix is small (10%).

(3)Endometrium(mucosa) - is the inner lining composed of columnar epithelium and branched tubular glands. Both are responsive to oestrogen and progesterone. The thickness of the lining depends on the stage of the menstrual cycle- ranging from 0.05-0.5 cm.

slide19
The two layers of the inner lining undergo changes during menstruation;

1-Zona functionalis; is the layer shed during each menstrual cycle.

2-Zona basalis; is the layer from which regeneration of the endometrium occurs, (removal of which during over-curettage results in intrauterine adhesions, or Asherman’s syndrome).

An additional feature is the typical coiled arteries which are also under hormonal influence. During pregnancy they enlarge especially in the region of the placenta, and they form the maternal contribution to the placental blood supply.

slide20
Age changes

The disappearance of maternal oestrogens after birth causes the uterus to decrease in length by around one-third and in weight by about one-half. The cervix is then twice the length of the uterus.

At puberty, however, the corpus grows much faster and the size ratio reverses. After the menopause the uterus atrophies, the mucosa becomes very thin, the glands almost disappear and the wall becomes relatively less muscular. These changes affect the cervix more than the corpus, cervical loops disappear and the external os becomes more or less flush with the vault.

slide21
Supports and uterine attachments

Broad ligaments–are folds of peritoneum that extend between the uterus and the pelvic organs to the lateral pelvic walls. In the upper part lie the round ligament and the fallopian tubes and at the base the uterine vessels and ureter, and the remainder delicate areolar tissue, vessels and nerves and embryological remnants related to the Wolffian ducts.

The embryological importance of the Wolffian duct remnants is that they may become cystic and enlarge (Gartner’s cyst).

Uterine perforation or rupture may occur into the broad ligament, and similarly an ectopic pregnancy may rupture downwards into it.

The tissue adjacent to the uterus in the broad ligament is called the parametrium; its importance is that it represents one of the pathways in the spread of uterine infection (parametritis).

slide22
Round ligaments- are continuous with the ovarian ligaments, representing an embryonic structure called the gubernaculum, and extend from the fundus to the pelvic walls and into the inguinal canal.

The round ligaments provide some anterior support for the uterus, especially during pregnancy, when they enlarge markedly. Stretching may cause pain, the round ligament syndrome.

Cardinal ligaments–are condensations of subserous fascia that extend from the uterus to the lateral pelvic walls. Functions include containing the uterine blood supply and ureter, and providing support for the middle and upper thirds of the vagina and cervix.

Uterosacral ligaments– are condensations of fascia that extend from the sacrum around the rectum to the cervix.

Uterovesical ligaments– are connective tissue attaching the bladder to the lower uterine segment.

slide23
Blood supply of the uterus
  • (1)-Uterine artery
  • (2)-Ovarian artery and an anastomosis between them.
  • Lymphatic drainage
  • (1)-Aortic
  • (2)-Lumbar
  • (3)-Internal iliac lymph nodes
  • Nerve supply
  • This passes through the uterosacral ligament.
  • Afferent pain fibres (T11-12) cause referred pain from the uterus to the lower abdomen.
  • Sympathetic innervation arises from the hypogastric and ovarian plexus.
  • Parasympathetic innervation from the pelvic nerves(S2-4).
slide24
Fallopian tubes

These are 10-14 cm long and their function is indicated by their other name (oviduct) that is to transfer the fertilized ovum to the uterus.

Parts of the fallopian tubes:

1.Interstitial- 1 cm segment that penetrates the myometrial wall into the uterine cavity.

2.Isthmus– is the narrow proximal end with simple mucosal folds and a thick muscular wall.

3.Ampullary – is the relatively dilated lateral half of the tube with a wide lumen and complex mucosal folds.

4.Infundibular – is the distal segment that terminates in mobile tentacle-like fimbriae that become turgid at ovulation entrapping the ovum.

Attachments

Medially they are attached to the uterine cornu.

Laterally they are attached to the pelvic side wall (infundibulopelvic ligament).

The mesosalphinx attaches the oviducts to the broad ligament.

slide25
Layers -vary in size and thickness.

(1)-Serosa is derived from the visceral peritoneal folds of the broad ligament.

(2)-Loose adventia contains lymphatics and blood vessels.

(3)-Smooth muscles are mixed among the outer longitudinal and inner circular layers and spiral bands.

(4)-Lamina propria is composed of vascular connective tissue elements.

(5)-Ciliated columnar epithelium produces tubal fluid and secretions that nourish the dividing blastocyst. Cilia beat in the direction of the uterus.

Blood supply

Through the mesosalphinx from the ascending uterine artery and ovarian artery.

Function (A) -Facilitate sperm migration from the uterus to the ampulla to fertilize the ovum.

(B) -Transport the fertilized ovum toward the uterus.

Partial obstruction of the lumen whether congenital or acquired or delay in transport of the fertilized ovum for other reasons may result in an ectopic pregnancy.

slide26
Ovaries

They are paired structures which are situated on the back of the broad ligaments attached by a mesentery (mesovarium).

Each ovary is almond shaped and measures 2-4 cm in length. Its functions are production of ova during the woman’s reproductive years and the secretion of the key hormones during the early months of pregnancy. They are attached medially to the uterine fundus by the ovarian ligaments, and laterally to the pelvic side wall by the suspensory ligament. The mesovarium attaches the ovaries to the broad ligament.

Blood supply

Ovarian arteries (from the aorta), which anatomise with the uterine arteries in the mesosalphinx.

Venous drainage– ovarian veins drain on the left to the left renal vein and to the right to the inferior vena cava.

Lymphatic drainage–through the infundibulopelvic ligaments to the pelvic and para- aortic lymph nodes.

slide27
The bladder

The average capacity of the bladder is 400 ml. The bladder is lined with transitional epithelium. The involuntary muscle of its wall is arranged in an inner longitudinal layer, a middle circular layer and an outer longitudinal layer.

The ureters open into the base of the bladder after running medially for about 1 cm through the vesical wall. The urethra leaves the bladder in front of the ureteric orifices; the triangular area lying between the ureteric orifices and the internal meatus is known as the trigone.

The base of the bladder is related to the cervix, with only a thin layer of connective tissue intervening. It is separated from the anterior vaginal wall below the pubocervical fascia, which stretches from the pubis to the cervix.

The urethra

It is about 3.5 cm long , lined with transitional epithelium. The smooth muscle is arranged in outer longitudinal and inner circular layers. The upper part of the urethra is mobile but the lower part relatively fixed.

slide28
Ureters

They cross the lateral pelvic wall at the bifurcation of the internal and external arteries; it runs on the lateral pelvic wall, passes inwards and forwards, attached to the peritoneum of the back of the broad ligament, to pass beneath the uterine artery. It then passes through a fibrous tunnel, the ureteric canal, in the upper part of the cardinal ligament.

Finally, it runs close to the lateral vaginal fornix to enter the trigone of the bladder. The ureters are inferior and posterior to the pelvic blood supply and traverse the entire route retroperitoneally.

Its blood supply is from branches of the ovarian artery, the uterine artery and the vesical arteries.

Because of its close relationship to the cervix, the vault of the vagina and the uterine artery, the ureter may be damaged during hysterectomy.

slide29
The rectum

The rectum extends from the level of the third sacral vertebra to a point about 2.5 cm in front of the coccyx, where it passes through the pelvic floor to become continuous with the anal canal. Its direction follows the curve of the. Its direction follows the curve of the sacrum and is about 11 cm long. The front and sides of the upper third are covered by peritoneum of the rectovaginal pouch; in the middle third only the front is covered by the peritoneum. In the lower third there is no peritoneal covering and the rectum is separated from the posterior wall of the vagina by the rectovaginal septum.

Lateral to the rectum are the two uterosacral ligaments, beside which run some of the lymphatics draining the cervix and the vagina.

Perineum

It is outlined by the vaginal fourchette anteriorly and the anus posteriorly. Deep to it is the perineal body which lies between the anal canal and the lower one third of the posterior vaginal wall. It is the area that is incised in the operation of episiotomy, where the introitus is enlarged to facilitate the birth of the baby or where lacerations can occur.

slide30
Pelvic diaphragm

This muscular layer forms the inferior border of the abdominal-pelvic cavity and extends from the pubic bone to the coccyx and between the pelvic walls.

The main muscle is the levator ani which forms the floor of the pelvis and roof of the perineum.

It arises from the lower part of the body of the pubis, the internal surface of the parietal pelvic fascia along the white line, and the pelvic surface of the ischial spine.

It inserts into the pre-anal raphe, the wall of the anal canal, where its fibres join the external sphincter muscle, the post-anal raphe and the lower part of the coccyx.

The pubococcygeus is the most significant component of the levator ani and has attachments to the urethra, rectum, and vagina which all pass through it. Other components include the pubovaginalis muscle, the puborectalis and iliococcygeus muscles.

slide31
Functions of the levator ani include flexing the coccyx and constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.

The perineal body

This is the perineal mass of muscular tissue that lies between the anal canal and the lower third of the vagina. Its apex is at the lower end of the rectovaginal septum, at the point where the rectum and posterior vaginal walls come into contact.

Its base is covered with skin and extends from the fourchette to the anus.

It is the point of insertion of the superficial perineal muscles and is bounded above by the levator ani muscles where they come into contact in the midline between the posterior vaginal wall and the rectum.

slide32
Skeletomuscular supports

Supporting the genitalia are the bony and fibro muscular structures that make up the birth canal.

The bony pelvis

This is made up of 4 bones joined together by ligaments. At the sides are the paired hip bones.

These are joined in front at the symphysis pubis and, behind, they articulate with the ala of the sacrum forming the sacroiliac joints. The fourth bone the coccyx, is loosely articulated with the lower border of the sacrum.

The hip bone is composed of 3 separate elements- pubis, ischium and ilium.

The sacrum is composed of 5 fused vertebrae, and is directed backwards and downwards, and this throws its superior border into prominence as the sacral promontory, an important bony landmark for assessing the size of the pelvis, especially the antero-posterior diameter, its pelvic aspect, providing in part the characteristic curve of the birth canal.

slide33
The pelvis is divided into a true and false pelvis, delineated by the iliopectineal line.

The canal is made up only of the symphysis pubis and is short while posteriorly, there is the sweep of sacrum and the coccyx (11-13 cm), which when added to the fibro muscular perineal body describes the curve of Carus.

The canal is made up only of the symphysis pubis and is added to the fibro muscular perineal body describes the Curve of Carus.

Pelvic brim

The pelvic brim is formed by the superior aspect of the pubic crest, the pectineal line of the pubis, the arcuate line of the ilium, the alae of the sacrum, and the sacral promontory. The normaltransversediameter in this plane is 13.5 cm and is wider than the anterior-posterior diameter, which is normally 11cm. The angle of the inlet is normally 60 degrees to the horizontal in the erect position but in the Afro-Caribbean woman this angle may be 90 degrees. This increased angle may delay the head entering the pelvis in labour.

slide34
Pelvic inclination

The lateral view of the pelvis indicates that the pelvic brim makes an angle of 40-50 degrees with the horizontal; this is called the angle of inclination.

The inclination lessens as the birth canal is descended, being about 30 degrees in the midpelvis and 10 degrees at the outlet.

Pelvic cavity

It is the area between the inlet above and the outlet below. It is bounded by the pubic bones anteriorly, the curve of the sacrum posteriorly, (the second and third pieces of the sacrum) and parts of all 3 components of the hip bone laterally.

The cavity is almost round as the transverse and anterior diameters are similar at 12 cm.

The ischial spines are prominent in the android pelvis.

The ischial spines are important landmarks, not only as indicators of the type of pelvis and size, but also as a reference point for the station of the presenting part. They are also used as landmarks for providing an anaesthetic block to the pudendal nerve.

The pelvic axis describes an imaginary line that shows the path that the centre of the fetal head takes during its passage through the pelvis.

slide35
The pelvic outlet

It is outlined by the subpubic arch, the descending ramus of the pubic bone, ischial tuberosities, the sacrotuberous ligaments and the coccyx . The anterior-posterior diameter of the pelvic outlet is 13.5 cm and the transverse diameter 11 cm.

Where the subpubic arch is narrow, as in the android pelvis the angle may be 60-7O˚, compared with the normal angle of 90˚.

The pelvic floor

This is formed by the two levator ani muscles, which with their fascia form a musculofascial gutter during the second stage of labour.

slide36
The female pelvis

It differs from the male pelvis in that;

(1 )-the female pelvis is wider

(2)-the female pelvic brim is transversely oval (less prominent sacral promontory) while the male pelvic brim is heart shaped.

(3 )-the outlet is wider and the subpubic arch is round while the male subpubic angle is acute.

The major obstetric interest in the bony pelvis is that it is not distensible and minor degrees of movement are possible at the symphysis pubis and sacroiliac joints.

Its dimensions are critical at childbirth.

slide37
Four basic types of pelvis have been described;

(1 )- Gynaecoid type – The classical female pelvis has an oval transverse inlet and a wide pelvic cavity.

It is almost round except for the intrusion of the sacral promontory posteriorly, (55%) .

(2) - Anthropoid type - This is long, narrow and oval.

This results from high assimilation (the sacral body assimilated on the fifth lumbar vertebra , ( 20%)

(3) - Android type - The inlet is heart shaped, and the cavity is like a funnel with a contracted outlet , 20%) .

(4) - Platypelloid type–This is a wide flat pelvis, flattened at the brim with the sacral promontory pushed forwards, (5%) , where the largest diameter is the transverse one.

slide38
These differences in pelvic shape are of more than radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.

In general, the pelvis is considered to be mildly contractedif the diameter is reduced 1 cm, moderately contracted if reduced by 1- 2 cm, and markedly contracted if reduced 2 cm or more.

The anteroposterior diameter (obstetrical conjugate), (11.5 cm) , is measured from the back of the symphysis to the tip of the sacral promontory.

This measurement can only be made accurately by radiography, but an approximate idea is given by the (diagonal conjugate), from the bottom of the symphysis pubis to the sacral promontory as measured during vaginal examination.

The transverse diameter (13.5 cm) is taken as the widest part of the brim. The oblique diameters, right and left , (12.5 cm), run from the right and left sacroiliac joints to the opposite iliopectineal eminences, respectively.

slide39
The pelvic joints

The sacroiliac joints are partly cartilaginous, partly fibrous and are very strong. Despite this, pain is often experienced late in pregnancy as joint mobility increases with softening of the ligaments, and the weight of the pregnant uterus is added to that of the head and trunk.

The lumbosacral joint lies between the fifth lumbar vertebra and the sacrum. Because of the backward inclination of the sacrum, considerable strain occurs here during pregnancy. In extreme cases (spondylolisthesis), the fifth lumbar vertebra projects downwards into the area of the pelvic brim.

Symphysis pubis

The two pubic bones are joined anteriorly by fibrous tissue, although a layer of cartilage remains between them. It is through this cartilage that the operation of symphysiotomyis occasionally carried out to increase pelvic diameters in cases of obstructed labour. In about 1 in 750 women there is an abnormal separation of the pubic bones- usually associated with a rapid second stage of labour.

slide40
The sacrococcygeal jointis much looser than the others, allowing the coccyx to bend backwards as the fetus passes through the birth canal. Undue displacement may, however, overstretch the ligaments, giving rise to the condition of coccydyniaor pain, which is especially noticed on sitting.

Ligaments

These are well developed in the pelvis because of the stresses to which the pelvic bones are subjected.

Apart from the ligaments specifically related to the above joints, there are two others of importance- the sacrospinous and sacrotuberousligaments.

These run from the sacrum to the ischial spine, and the ischial tuberosity, respectively.

Together, they form, with the coccyx and the lowest part of the sacrum, the posterior aspect of the pelvic outlet.

slide41
The pelvic soft tissues

A-The pelvic floor, which comprise the various parts of the levator ani muscles which run on each side from the back of the symphysis pubis around the lateral pelvic wall on the fascia over the obturator internus muscle to the ischial spine and side of the coccyx, together with the puborectalis.

The puborectalis is important in maintaining closure of the outlet by drawing the different structures passing through it anteriorly toward the shelf of the symphysis pubis. Inability to relax this part of the levator ani at the time of delivery is often responsible for delay in the birth of the baby in the second stage.

B-The urogenital diaphragm, which is a triangular diaphragm through which the pass the urethra and vagina, and occupies the space between the inferior borders of the ischiopubic rami and extends posteriorly to the front wall of the rectum.

slide42
On its deep aspect are two sets of muscles- the constrictor of the urethra and vagina , and the deep transverse perinei.

Superficially, there are theischiocavernosusmuscles, the bulbocavernous muscles, the superficial perineal muscles, and the Bartholin glands.

Between the vagina and rectum, the superficial and deep perineal muscles, including the anal sphincter, decussate and join forming the strong perineal body.

Behind the anal canal, the sphincter muscles decussate to form the anococcygeal raphe. It is in this region that the presenting part is felt as it approaches the pelvic outlet.

slide43
THE FETUS

The size, position and attitude of the fetus influence the mechanism of birth- i.e. the movements that the fetus undergoes during negotiation of the birth canal.

THE SKULL

The shoulders normally represent the largest fetal diameter but the skull diameters are the more important since the cranium is less compressible.

The bones of the cranium comprise of 2 frontal, 2 parietal, 2 temporal bones and 1 occipital bone - these are separated by sutures and fontanelles and so the cranium is more compressible than the base of the skull.

slide44
Landmarks
  • Frontal suturebetween the 2 frontal bones.
  • Coronal suture; between the frontal and parietal bones.
  • Sagittal suture; between the 2 parietal bones.
  • Lamboidal suture; between the occipital bone behind and the parietal and temporal bones in front.
  • Temporal suture; between the temporal and parietal bones.
  • The anterior fontanelle or Bregma; is the large diamond shaped depression at the anterior end of the cranium where frontal, coronal and sagittal sutures meet. It allows moulding in labour and growth of the skull after birth. It closes at 18 months of age.
  • The posterior fontanelle; the posterior fontanelle is a smaller triangular space at the posterior end of the cranium where the sagittal suture meets the lamboidal sutures.
slide45
Transverse diameters
  • Bitemporal(8 cm); between the lower ends of the coronal sutures.
  • Biparietal(9.5 cm); between the parietal eminences.
  • Sagittal diameters
  • Suboccipito-bregmatic(9.5 cm); from the foramen magnum to the centre of the bregma. It is the diameter that presents when the head is flexed.
  • Occipitofrontal(11-12 cm); from the occipital protuberance to the root of the nose. This diameter presents when the head is partly deflexed (military ‘’eyes front’’ attitude). Thus deflexion has the effect of increasing the presenting diameter of the fetal head by 20%.
  • Mentovertical(14 cm); from the point of the chin to the centre of the sagittal suture. This diameter is seen with brow presentation, and if the fetal head is of normal size and shape, it cannot negotiate a normal sized pelvis.
  • Submento-bregmatic (9.5 cm); the angle between the neck and chin to the centre of the bregma. This diameter is seen when the head is completely extended, i.e. a face presentation.
slide46
Areas of the skull
  • Vertex; (the top of the skull); is the area between the anterior and posterior fontanelles and the 2 parietal eminences.
  • Sinciput; is that part of the head in front of the anterior fontanelle. It is subdivided into the brow (the area between the root of the nose and the anterior fontanelle), and the face (the area below the root of the nose and the orbital ridges).
  • Occiput:(the back of the head); that part which lies behind the posterior fontanelle.
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Caput succedaneum and cephalohaematoma
  • During labour, pressure of the cervix on the fetal head impedes venous and lymphatic drainage and a serous effusion collects between the aponeurosis and periosteum to form a caput.
  • This is most marked when there is slow dilatation of the cervix and when the woman bears down before full dilatation- it is a sign of incoordinate uterine action and is accompanied by excessive moulding when labour is obstructed. Thecaput disappears within a few hours of birth.
  • It must be distinguished from cephalohaematomawhich is a subperiosteal collection of blood. This presents some hours after birth and may enlarge for 12-24 hours; because of the attachments of periosteum to the suture lines, a cephalohaematoma only overlies a single bone- usually the parietal, i.e. it does not cross the midline.
  • The fetal trunk
  • Diameters
  • Bisacromial(12.5 cm); the distance between the tips of the processes.
  • Bitrochanteric(9.5 cm); the distance between the outer surfaces of the greater trochanters.
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Moulding
  • Moulding is due to compression which the head undergoes during its passage through the birth canal. There is diminution of those diameters most compressed and compensatory elongation of those least compressed. The degree and direction of moulding are determined by a number of factors.
  • Size, shape, elasticity and attitude of the fetal head.
  • Size and shape of the maternal pelvis
  • Presentation and position of the head
  • Strength of uterine contractions and duration of labour.
  • Rate of cervical dilatation and position of the cervix.
  • Grades of moulding
  • Grade 0 – The suture lines are separate. Grade 1 - The suture lines meet.Grade 2 - If the sutures overlap but can be reduced with gentle digital pressure.Grade 3 -When the sutures overlap and are irreducible with digital pressure.
  • Moulding occurs relatively easily in the soft head of the premature fetus, whereas, the hard inelastic fetal head characteristic of prolonged pregnancy resists alteration, the hard inelastic fetal head characteristic of prolonged pregnancy resists alteration in shape.
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