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reproductive anatomy anatomy of the maternal pelvis and the fetal

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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.
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.
September 14
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.
September 14
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.
September 14
Mons Pubis
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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 -is the 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 usually dextrorotated
(rotated to the right side).
September 14
The uterus is globular in shape, but flattened in the
anteroposterior direction.
Normally ,it is both anteverted (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 uterus occurs 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.
September 14
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 0.050.5 cm.
September 14
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.
September 14
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.
September 14
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).
September 14
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.
September 14
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).
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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.
September 14
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 postanal 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.
September 14
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.
September 14
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.
September 14
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 normal transverse diameter 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.
September 14
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.
September 14
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 anteriorposterior 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 60-7OЛљ, compared with the normal angle
of 90Лљ.
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.
September 14
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.
September 14
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.
September 14
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 contracted if
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.
September 14
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 symphysiotomy is
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.
September 14
The sacrococcygeal joint is 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 coccydynia or 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
sacrotuberous ligaments.
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.
September 14
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.
September 14
On its deep aspect are two sets of muscles- the
constrictor of the urethra and vagina , and the deep
transverse perinei.
Superficially, there are the ischiocavernosus muscles, 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.
September 14
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.
September 14
Landmarks
•Frontal suture between 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.
September 14
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.
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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.
September 14
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. The caput disappears within a few hours of
birth.
It must be distinguished from cephalohaematoma which 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.
September 14
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.
September 14
September 14
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