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.
Dr. Shaima Abozeid
Arab& Libyan Board
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.
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.
Is divided into external and internal 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.
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.
Labia Majora the junction of the pubic bones (symphysis pubis).
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.
Labia Minora the junction of the pubic bones (symphysis pubis).
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.
Clitoris the junction of the pubic bones (symphysis pubis).
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.
Vestibule the junction of the pubic bones (symphysis pubis).
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.
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.
The vagina the junction of the pubic bones (symphysis pubis).
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.
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.
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.
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.
Uterus ischiorectal fossa.
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.
Cervix ischiorectal fossa.
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.
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.
The uterine body uterine body while the external os separates the distal cervix from the vagina.
(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).
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.
Layers anteroposterior direction.
(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.
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.
Age changes menstruation;
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.
Supports and uterine attachments menstruation;
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).
Round ligaments menstruation;- 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.
Fallopian tubes menstruation;
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.
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.
Layers menstruation; -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.
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.
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.
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.
The bladder menstruation;
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.
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.
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.
The rectum menstruation;
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.
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.
Pelvic diaphragm menstruation;
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.
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.
Skeletomuscular supports constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.
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.
The pelvis is divided into a constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.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.
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.
Pelvic inclination constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.
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.
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.
The pelvic outlet constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.
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.
The female pelvis constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.
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.
Four basic types of pelvis have been described; constricting the rectum and vagina, while the pubococcygeus supports the pelvic and abdominal viscera, including the bladder.
(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.
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.
The pelvic joints radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.
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.
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.
The radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.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.
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.
The pelvic soft tissues radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.
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.
On its deep aspect are two sets of muscles- the radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.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.
THE FETUS radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.
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 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.
Thank you radiological interest, since they determine, in large measure, the mechanism which is adopted by the fetus in passing through the birth canal.