Obstetric Anatomy Professor of Obstetrics Gynecology Ain Shams

Obstetric Anatomy Professor of Obstetrics & Gynecology Ain Shams Faculty of Medicine

The Fetal Skull • Anatomy • Diameters • Molding • Caput Succedaneum • Cephalhematoma

• The vault : From the orbital ridge to the nape of the neck (frontal, parietal, occipital bones). It is compressible. • The Face: Root of the nose to junction of head and neck.

Transverse Diameters of the Fetal Skull Biparietal Diameter 9. 5 cm Between the 2 parietal eminences Bitemporal Diameter 8. 5 cm. Bimastoid Diameter 7. 5 cm. Between the 2 mastoid processes (Not reducible nor destroyable even by destructive procedures Supra-subparietal 8. 25 - 9 cm. Asynclitic head

6 Length Presentation 1 -Suboccipito-bregmatic 9. 5 cm. Flexed vertex 2 -Suboccipito-frontal 10. 5 cm. Partially deflexed vertex 3 -Occipito-frontal 11. 5 cm. Deflexed vertex 4 -Mento-vertical 13. 75 -14 cm. Brow 5 -Submento-bregmatic 9. 5 cm. Face 6 -Submento-vertical 11. 5 cm. Face Not fully extended

Length Presentation 1 -Suboccipito-bregmatic Nape of neck to centre of bregma 9. 5 cm. Flexed vertex 2 -Suboccipito-frontal Nape of neck to 2. 5 cm. In front of bregma 10. 5 cm. Partially deflexed vertex Diameter distending the vulva after crowning 3 -Occipito-frontal Root of nose to occipital protuberance 11. 5 cm. Deflexed vertex Diameter distending the vulva in face presentation 4 -Mento-vertical Point of chin to above posterior fontanelle 13. 7514 cm. Brow 5 -Submento-bregmatic From below chin to centre of bregma 9. 5 cm. Face 6 -Submento-vertical 11. 5 cm. Face Not fully extended From below chin to infront of post. fontannelle

Fetal Skull Circumferences The Suboccipito-Bregmatic X Bipareital (28 cm. ) These are the engaging diameters of well flexed vertex presentation. Occipito-frontal X Biparietal (33 cm. ) These are the engaging diameters in deflexed vertex presentation ( OP position). Mento-vertical X Biparietal (35. 5 cm. ) This is the largest head circumference ( Brow presentation)

Engaging Diameters of Fetal Skull Well Flexed Head Circle of 9. 5 cm. The engaging Diameter is the Suboccipito-Bregmatic diameter A deflexed Head An oval The longer occipito-frontal diameter Of 11. 5 cm. Is exposed. Greater Deflexion of the Head Full Extension of the Head An oval The longer mento vertical diameter of 13. 75 -14 cm. is exposed A circle of 9. 5 cm. The engaging dimeter is the submento-vertical diameter

Moulding… Reshaping of the fetal skull: Obliteration of the sutures. Overlapping of the bones of the vault: One parietal bone overlaps the other. Both overlap the occipital bone. It accounts for diminution of the biparietal diameter and suboccipitobregmatic diameters by 0. 5 -1 cm. 0 r even more.

A: Well flexed Head B: Partially Flexed Head C: Deflexed Head D: Face Presentation E: Brow presentation

Overmoulding Occurs in case of obstructed labor. There is overstretch of the falx cerebri which tears from its attachment at the tentorium cerebelli. Subsequently there is injury of the vein of Galen with ICH. Superior long. Sinus Falx cerebri Inferior long sinus Vein of Galen Tentorium Cerebelli

The Scalp Tissues There are Five layers of scalp tissue Skin: The outer covering containing hair. Subcutaneous tissue Muscle Layer: containing the tendon of Galae. Connective tissue: a loose layer. Periosteum: covers the skull bones and attached at the suture line

Caput Succedaneum Diffuse scalp edema resulting from venous congestion due to prolonged pressure on the fetal head by the pelvic bones. It is soft and boggy to touch It usually disappears Localized caput…? It is usually few mm. Thick but may be large and lead to misinterpretation of the station of the head. The presence of caput may have medico-legal implication: The baby was living Labor was difficult D. D…Cephalhematoma

Cephalhematoma This swelling is due to bleeding between the skull bone and periosteum. Bleeding occurs due to friction between the overriding bones and periosteum during molding. It is just as likely to occur during a normal delivery as during more difficult labor. A low prothrombin level is probably a contributory cause

Caput Succedaneum Cephalhematoma is not present at birth but appears 2 -3 days. The swelling is limited by the periosteum. It therefore can NOT lie over a suture. The head is more red ad bruised in appearance than in caput succedaneum. The swelling may increase and it takes 6 weeks at least to disappear.

The Female Pelvis • Anatomy • Pelvic Diameters • Pelvic Types

The Female Pelvis Four Bones articulated at Four Joints. False pelvis: above the pelvic brim and has no obstetric importance. True pelvis: below the pelvic brim. It is the bone defined tunnel that the infant must traverse at birth.

Ilio-pectineal line Ischial spine SP Ischial tuberosity

SP Ischial Tuberosity Ischial Spine

The Planes of the pelvis Plane of of the the pelvic inlet. cavity: Plane of greatest Pelvic Dimensions mid pelvis (plane of obstetric outlet) Anatomical outlet

Plane Of The Pelvic Inlet passing with the boundaries of pelvic brim and making an angle of 55 o with the horizon (angle of pelvic inclination).

Plane of the Pelvic Cavity It is the plane of greatest pelvic dimensions. It passes between the middle of the posterior surface of the symphysis pubis and the junction between 2 nd and 3 rd sacral vertebrae. Laterally, it passes to the centre of the acetabulum and the upper part of the greater sciatic notch. It is a round plane with diameter of 12. 5 cm. Internal rotation of the head occurs when the biparietal diameter occupies this wide pelvic plane while the occiput is on the pelvic floor i. e. at the plane of the least pelvic dimensions.

Plane Of Obstetric Outlet It is the plane of least pelvic dimensions. It passes from the lower border of the symphysis pubis anteriorly, to the ischial spines laterally, to the tip of the sacrum posteriorly. It is the plane of the pelvic floor. The head is considered engaged if the vault reaches it. This is the plane where the pelvic axis turns forwards.

Plane Of Anatomical Outlet It passes with the boundaries of anatomical outlet and consists of 2 triangular planes with one base which is the bituberous diameter. Anterior sagittal plane: its apex at the lower border of the symphysis pubis. Anterior sagittal diameter from the lower border of the symphsis pubis to the centre of the bituberous diameter: 6 -7 cm Posterior sagittal plane: its apex at the tip of the coccyx. Posterior sagittal diameter from the tip of the sacrum to the centre of the bituberous diameter: 7. 5 -10 cm

The consequences of walking upright… When a women stands erect: The pelvic inlet makes an angle of about 55° with the horizon. The pelvic outlet makes an angle of 15° with the horizon If the angle made by the inlet is greater than 55° this may make the descent of the fetal head in the pelvis difficult.

The Obstetric Pelvic Axis This represents the path that the presenting part must follow for delivery to occur: The upper part moves downward approximately in a straight line till the level of the ischial spine. The trajectory then changes to become a curvilinear path directed forward and downward

At the level of the Ischial Spine The plane of obstetric outlet (plane of the least pelvic dimensions). The levator ani muscles. The obstetric axis of the pelvis changes its direction. The head is considered engaged when the vault is felt vaginally at or below this level. Internal rotation of the head occurs when the occiput is at this level. Forceps is applied only when the head at this level (mid forceps) or below it ( low and outlet forceps). Pudendal nerve block is carried out at this level. Normal level of the external os of the cervix.

Four types of Female Pelvis The Caldwell-Moloy’s classification They differ in: Shape of the pelvic inlet Shape of the side-walls Character of the subpubic arch Four types do exist: Gynecoid: 50%. Android: 20%. Anthropoid: 25%. Platypelloid: 5%. The truth is that the majority of the pelves are a mixture of all the 4 types.

Gynecoid Rounded Trans. Diameter Slightly behind the centre Anthropoid AP diameter>Trans. Android Heart shaped Trans. Diameter Near the sacrum Platypelloid Wide Trans. diameter

Types of female Pelvis Inlet Gynecoid Female 50% Rounded Android Male-like 20% Triang. Anthropoid Platypelloid Ape-like Flat 25% 5% AP-oval Trans-oval Cavity Wide and Narrow and shallow deep Wide Subpubic angle Wide Narrow >90 <70 Not Inward prominent projection Wide Reduced Parallel Convergent <90 >90 Prominent Not prominent Wide Divergent Ischial Spines I. S. D Walls Sacrum Smooth Jutting Reduced Parallel

The Ideal Obstetric Pelvis Brim Round or Oval transversely No undue projection of sacral promontory. AP diameter: 12 cm. Transverse diameter: 13 cm The plane of pelvic inlet not more than 55°. Cavity Shallow with straight side-walls. No great projections of ischial spines. Smooth sacral curve Outlet Pubic arch rounded Subpubic angle >80°. Intertuberous diameter of at least 10 cm.

The True Conjugate = 11 cm The Obstet. Conjugate = 10. 5 cm The Diagonal Conjugate = 12 cm

Diameters of the Inlet Antero-posterior Diameters True Conjugate from the tip of the sacral promontory to the upper border of the symphysis pubis. 12 cm. Obstetric Conjugate from the tip of the sacral promontory to the most bulging point on the back of symphysis pubis which is about 1 cm below its upper border. It is the shortest antero-posterior diameter 10. 5 cm. Diagonal Conjugate From the tip of sacral promontory to the lower border of symphysis pubis. 12 -12. 5 cm. External Conjugate 20 cm.

Transverse Diameters Anatomical Transverse Diameter between the farthest two 13 cm. points on the iliopectineal lines. It lies 4 cm anterior to the promontory and 7 cm behind the symphysis. It is the largest diameter in the pelvis. Obstetric Transverse Diameter It bisects the true conjugate and is slightly shorter than the anatomical transverse diameter. 12 cm.

Oblique Diameters Right and left oblique diameters Right and left Sacro-cotyloid diameters From the right Sacro-iliac joint to the left ilio-pectineal eminence and vice-versa. From the right iliopectineal eminence to the promontory of the sacrum (rt. ) 12 cm. 9 -9. 5 cm.

Interspinous diam. = 10. 5 cm. Anato. Ant. Post diam= 11 cm. Obstet. Ant. Post diam= 13 cm.

Diameters of the Outlet Antero-Posterior Diameters Anatomical antero-posterior diameter Obstetric antero -posterior diameter From the tip of the coccyx to the lower border of symphysis pubis. 11 cm From the tip of the sacrum 13 cm to the lower border of symphysis pubis as the coccyx moves backwards during the second stage of labour.

Transverse Diameters Anatomical Transverse Diameter (Bituberous) Extends between the inner aspects of the ischial tuberosities. Obstetric Transverse Diameter (interspinous) Extends between the tips of the ischial spines. It is the smallest diameter of the pelvis. 11 cm 10. 5 cm. Tom’s Dictum: If the sum of the Bituberous diameter and Post. Sagittal diameter is less than 15, the pelvic outlet is contracted. This is an indication of performing a Cesarean section.

The Plane of the Outlet Anterior Sagittal Plane Posterior Sagittal Plane

Pelvic Soft Tissues • The Formation Of The Lower Uterine Segment • The Levatores Ani • The Perineal Muscles • Formation of the birth canal during labor • The Episiotomy

The formation of the lower uterine segment It is the part between the vesico-uterine fold of peritoneum superiorly and the cervix inferiorly. It develops as early as the 16 th week by incorporating the upper part of the cervix in the lower part of the uterus to accommodate for the presenting part of the fetus.

Differentiation of the Uterine Segments The passive lower segment is derived from the isthmus. The physiologic retraction ring develops at the junction of upper and lower uterine segments. The Pathologic retraction ring develops from the physiologic ring in case of obstructed labor

Formation of the Birth Canal During Labor The lower uterine segment, cervix and vagina become a single canal that allow for the passage of the baby to the outside. Hypertrophy of the vaginal muscle layer and unfolding of the rugae allow for the accommodation of the fetus without damage.

Formation of the Birth Canal During Labor Level of Internal os The cervix is obliterated, taken-up or effaced: It is reduced from a length of 2 -2. 5 cm to a mere paper thin circular orifice. The lower uterine segment, cervix and vagina become a single canal that allow for the passage of the baby to the outside. Hypertrophy of the vaginal muscle layer and unfolding of the rugae allow for the accommodation of the fetus without damage.

The Levatores Ani A hammock of muscle sweeping down from the pelvic brim and investing the urethra, vagina and rectum. Two gaps: An anterior gap bridged by the urogenital diaphragm transmitting the urethra and vagina. A posterior gap transmits the rectum and anal canal.

The resistance and shape of the pelvic floor play an important role in facilitating rotation and flexion of the presenting part. As the presenting part descends: The anterior portion of the pelvic floor is pressed outwards against the SP. The posterior part becomes stretched into a thinwalled tube. The perineal body stretches and thins from 5 cm. To 0. 05 cm. and is displaced downward.

The Episiotomy (Perineotomy) Delivery of the fetus through the musculo-fascial support of the pelvic floor requires significant stretching of these structures and often results in trauma. The purpose of the episiotomy is to substitute a surgical incision limited to a reparable portion of the perineum.

The Following Are Incised… The Fourchette. The vaginal mucosa and ischiocavernosus submucosa. Bulbocavernosus The interdigitating fibers of the suerficial and deep transverse perinii & the pubococcygeus muscle group. The inferior fascia of the Pubococcygeus urogenital diaphragm. In mediolateral Iliococcygeus episiotomy, the medial portions of the Coccygeus bulbocavernosus is also incised Superficial transverse perinii

Feto-Pelvic Relationships • Presentation • Presenting part • Lie • Attitude • Position • station

As the journey progresses… The fetal head descends along the pelvic axis. It must rotate to accommodate the appropriate diameters of the head to the pelvic diameters. The reference points during this journey: The ischial spine is the pelvic reference point The presenting part is the fetal reference point.

Fetal Presentation & The Presenting Part Fetal Presentation: Is the fetal pole that presents at the pelvic inlet: Cephalic: Head First Breech: Feet or Buttocks Shoulder: back or abdomen The Presenting part: Is the part of the fetus first touched by the examining fingers during pelvic examination.

The Fetal Lie Refers to the relationship between the fetal longitudinal axis and that of the mother.

Position It refers to the relationships of a designated point on the presenting part “Denominator” to the walls of maternal pelvis. RA A RT LA LT RP LP P

As the fetal head descends through the birth canal, the suboccipito-bregmatic diameter successively occupies the : Transverse diameter of the inlet. Oblique diameter of the cavity. AP diameter of the outlet

What is the predominant fetal head position? During labor, in 90% of vertex presentation, The head assumes either a LOA or a ROP position The sagittal suture occupies the Right Oblique diameter of the pelvis. The right oblique diameter of the pelvis goes from the left iliopectineal eminence to the Right sacroiliac joint.

Why should the head rotate? The larger transverse diameter of the pelvis is more posterior. However the presence of the sacral promontory pushes the head anteriorly towards a smaller transverse diameter. The head will therefore rotate to take advantage of the greater oblique diameter at that level

Why the LOA or the ROP are favored over the LOP or ROA? The presence of the sigmoid colon in the post left quadrant of the pelvic inlet pushes the head anteriorly towards the pubis. The sagittal suture is tending to occupy the wider Right oblique diameter rather then the left oblique diameter which is encroached upon by the sigmoid colon. Thus a LOA or a ROP positions are favored in 90% of cases.

The Stations of the Fetal Head The location of the presenting part with reference to the ischial spine is designated the station of the presenting part. The head is said to be engaged when the vertex is felt at the level of the ischial spine. In that instance, the biparietal diameter should have negotiated the inlet. This is because: The distance from the plane of the inlet to the spine is 5 cm. The distance from the vertex to the biparietal diameter is 4. 5 or less

The Stations of the Bony Pelvis -5 Station 0 Station +5 0 +5 • The station 1 cm. Below the inlet is station -4. • The station below the spine are numbered from +1 to +5 : The perineum

fifth above the pelvic brim The Fetal Head Has Five Fifths… 0 : Head Not Palpable 1 : Sinciput felt – Occiput Not Felt 2 : Sinciput felt – Occiput Just Felt 3 : Sinciput easily felt – Occiput Felt 4 : Sinciput High – Occiput easily Felt 5 : Complete above pelvic brim -5 0 +5