FETAL SKULL Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull. AREAS OF SKULL: The skull is arbitrarily divided into several zones of obstetrical importance (Fig. 9.1). These are: Vertex: It is a quadrangular area bounded anteriorly by the bregma and coronal sutures behind by the lambda and lambdoid sutures and laterally by lines passing through the parietal eminences. Brow: It is an area bounded on one side by the anterior fontanel and coronal sutures and on the other side by the root of the nose and supraorbital ridges of either side. Face: It is an area bounded on one side by root of the nose and supraorbital ridges and on the other, by the junction of the floor of the mouth with neck. Anterior fontanel-or bregma Sinciput Frontal bone Glabella- Mentum. Coronal suture Temporal bone Base ➤ Physiological Enlargement of Pelvis during Pregnancy and Labor Sinciput is the area lying in front of the anterior fontanel and corresponds to the area of brow and the occiput is limited to the occipital bone. Flat bones of the vault are united together by non-ossified membranes attached to the margins of the bones. These are called sutures and fontanels. Of the many sutures and fontanels, the following are of obstetric significance. SUTURES (Figs. 9.1 and 9.2) The sagittal or longitudinal suture lies between two parietal bones. The coronal sutures run between parietal and frontal bones on either side. The frontal suture lies between two frontal bones. The lambdoid sutures separate the occipital bone and the two parietal bones. Importance: (1) It permits gliding movement of one bone over the other during molding of the head. Vertex Parietal eminence Vault Parietal bone Lambdoid suturea phenomenon of significance while the head passes through the pelvis during labor. (2) Digital palpation of sagittal suture during internal examination in labor gives an idea of the manner of engagement of the head (asynclitism or synclitism), degree of internal rotation of the head and degree of molding of the head. FONTANELS: Wide gap in the suture line is called fontanel. Of the many fontanels (6 in number), two are of obstetric significance: (1) Anterior fontanel or bregma and (2) Posterior fontanel or lambda. Anterior fontanel (Fig. 9.2): It is formed by joining of the four sutures in the midplane. The sutures are anteriorly frontal, posteriorly sagittal and on either side, coronal. The shape is like a diamond. Its anteroposterior and transverse diameters measure approximately 3 cm each. The floor is formed by a membrane and it becomes ossified 18 months after birth. It becomes pathological, if it fails to ossify even after 24 months. Importance: Its palpation through internal examination denotes the degree of flexion of the head. ⚫ It facilitates molding of the head. As it remains membranous long after birth, it helps in accommodating the marked brain growth; the brain becom-ing almost double its size during the first year of life. Palpation of the floor reflects intracranial status-depressed in dehydration, elevated in raised intracranial tension. Collection of blood and exchange transfusion, on rare occasion, can be performed through it via the superior longitudinal sinus. Cerebrospinal fluid can be drawn, although rarely, through the angle of the anterior fontanel from the lateral ventricle. suture lines-sagittal suture anteriorly and lambdoid Posterior fontanel: It is formed by junction of three suture on either side. It is triangular in shape and Subme lentot 9.5 cm Occipitofrontal 11.5 cm Face measures about 1.2 x 1.2 cm (1/2" x 1/2"). Its floor membranous but becomes bony at term. Thus, truly in nomenclature as fontanel is misnomer. It denotes th position of the head in relation to maternal pelvis. Sagittal fontanel: It is inconsistent in its presence When present, it is situated on the sagittal suture at the junction of anterior two-thirds and posterior one-third. I has got no clinical importance. DIAMETERS OF SKULL (Fig. 9.3): The engaging diameter of the fetal skull depends on the degree of flexion present. The anteroposterior diameters of the head which may engage are shown in Table 9.1. The transverse diameters which are concerned in the mechanism of labor are (Fig. 9.2 and Table 9.1): Biparietal diameter-9.5 cm (3%"): It extends between two parietal eminences. Whatever may be the position of the head, this diameter nearly always engages. Super-subparietal-8.5 cm (3½"): It extends from a point placed below one parietal eminence to a point placed above the other parietal eminence of the opposite side. Bitemporal diameter-8 cm (3½"): It is the distance between the anteroinferior ends of the coronal suture. Bimastoid diameter-7.5 cm (3"): It is the distance between the tips of the mastoid processes. The diameter is incompressible and it is impossible to reduce the length of the bimastoid diameter by obstetrical operation. CIRCUMFERENCES: Circumference of the plane of the diameter of engagement differs according to the attitude of the head (Table 9.2). forecoming head while passing through the resistant MOLDING: It is the alteration of the shape of the birth passage during labor. There is, however, very littleSabuccipitubregmatic extends from the nape of the neck to the center of the bregma Sextends from the nape of the neck to the anterior end of the anterior fontanel or center of the sinciput alabella) extends from the occipital eminence to the root of the nose Mensavertical extends from the midpoint of the chin to the highest point on the sagittal suture Submentovertical extends from junction of floor of the mouth and neck to the highest point on the sagittal suture. Submentobregmatic extends from junction of floor of the mouth and neck to the center of the bregma. Table 9.2: Circumferences of the head in different attitudes. Attitude of the head Circum-ference Plane of engagement Complete flexion Biparietal-suboccipitobregmatic (almost round shape). 27.5 cm (11") Deflexed Biparietal occipitofrontal (oval shape). 34 cm (13½") Incom plete extension Biparietal-mentovertical (bigger oval shape). 37,5 cm (15") Complete extension Biparietal-submentobregmatic (almost round shape). 27.5 cm (11) Note: Conversion of centimeters into inches is approximate. alteration in size of the head, as volume of the content inside the skull is incompressible although small amount of cerebrospinal fluid and blood escape out in the process. During normal delivery, an alteration of 4 mm in skull diameter commonly occurs. Mechanism: There is compression of the engaging diameter of the head with corresponding elongation of the diameter at right angle to it (Fig. 9.4). Thus, in well-flexed head of the anterior vertex Presentation, the engaging suboccipitobregmatic diameter compressed with of head in mentovertical diameter which is at right angle to suboccipitobregmatic (Figs. 9.5A to D). Fig. 9.4: Diagrammatic representation showing the principle of molding of the head. Measurement in cm (inches) 9.5 cm (3½") 10 cm (4") 11.5 cm (4½") 14 cm (5½") 11.5 cm (4½") 9.5 cm (3½") Attitude of the head Presentation Complete flexion Vertex Incomplete flexion Vertex Marked deflexion Vertex Partial extension Brow Incomplete extension Face Complete extension Face During the process, the parietal bones tend to overlap the adjacent bones, viz, the occipital bone behind, the frontal bones in front and the temporal bones at the sides. In first vertex position, the right parietal bone tends to override the left one and this becomes reverse in second vertex position. Molding disappears within few hours after birth. Grading: There are three gradings. Grade-1-the bones touching but not overlapping, Grade-2-overlapping but easily separated and Grade-3-fixed overlapping. Importance: Slight molding is inevitable and beneficial. It enables the head to pass more easily, through the birth canal. Extreme molding as met in disproportion may produce severe intracranial disturbance in the form of tearing of tentorium cerebelli or subdural hemorrhage. Shape of the molding can be a useful information about the position of the head occupied in the pelvis. CAPUT SUCCEDANEUM: It is the formation of swelling due to stagnation of fluid in the layers of the scalp line (Fig. 9.6). It may be confused with cephalhematoma beneath the girdle of contact. The girdle of contact is either bony or the dilating cervix or vulval ring. The swelling is diffuse, boggy and is not limited by the suture (p. 455). Caput disappears spontaneously within 24 hours after birth. Caput needs to be differentiated from cephalhematoma (Table 9.3). Mechanism of formation: While the head descends to press over the dilating cervix or vulval ring, the overlying scalp is free from pressure, but the tissues in contact with the full circumference of the girdle of contact is compressed. This interferes with venous return and lymphatic drainage from the unsupported area of scalp →stagnation of fluid and appearance of a swelling in the scalp (Fig. 9.6). Caput usually occurs after rupture of the membranes. Importance: of time. It signifies static position of the head for a long periodTable 9.3: Differentiation of caput succedaneum rem he matoma Caput succedaneum Appearance: At birth. Feel: Soft, and compressible Extent: Diffuse, spreads over the suture lines. Position: Changes and moves to the dependent part With observation: Gradually reduces in size and disappears within few hours. ne bones Ding but bles the severe torium out the C D elling scalp ntact The rure oma hin шам nds he d Cephalhematoma Hours after birth. Soft but incompressible. Limited to a bone, does not cross the suture lines. Does not change, fixed to one site. It increases in size with time. Disappears only after weeks or months. Definition: Localized swelling on the scalp due to effusion of serum. Etiology: Pressure by the dilating cervix (Cervical ring) causing obstruction to the venous and lymphatic return resulting in scalp edema It is the hemorrhage under the periosteum of bone(s) of the skull. It is following trauma to the skull due to: (a) prolonged pressure on head during labor, (b) forceps/ventouse delivery. Often associated with the fracture of the skull bone(s). Location of the caput gives an idea about the position of the head occupied in the pelvis and the degree of flexion achieved. In left position, the caput is placed on right parietal bone and in right position, on left parietal bone. With increasing flexion, the caput is placed more posteriorly. PELVIS From the obstetrical standpoint, it is useful to consider the bony pelvis as a whole rather than separately. For descriptive purpose, an articulated pelvis is composed of four bones-two innominate bones, sacrum and coccyx. These are united together by four joints-two sacroiliac joints, sacrococcygeal joint and the symphysis pubis. The pelvis is anatomically divided into a false pelvis and a true pelvis, the boundary line being the brim of the pelvis. The bony landmarks on the brim of the pelvis from anterior to posterior on each side are upper border of symphysis pubis, pubic crest, pubic tubercle, pectineal line, iliopubic eminence, iliopectineal line, sacroiliac articulation, anterior border of the ala of sacrum and sacral promontory (Fig. 9.7). FALSE PELVIS The false pelvis is formed by the iliac portions of the innominate bones and is limited above by the iliac crests. It has got little obstetric significance except that its measurements can to a certain extent, predict the sizeconfiguration of the true pelvis. Its only obstetric function is to support the enlarged uterus during pregnancy. Its boundaries are: posteriorly-lumbar vertebrae, laterally-iliac fossa and anteriorly-anterior abdominal wall. TRUE PELVIS This part of the pelvis is chiefly of concern to the obstetricians, as it forms the canal through which the fetus has to pass, It is shallow in front, formed by symphysis pubis and measures 4 cm (1½") and deep posteriorly, formed by the sacrum and coccyx and measures 11.5 cm (4%). For descriptive purpose, it is divided into inlet, cavity and outlet. The pelvic measurements given in the text are average when measured radiologically and vary within a limited degree in different countries. The conversion of centimeters into inches is approximate. INLET As the inlet is the brim of the pelvis, the circumference of the inlet is formed by the bony landmarks mentioned previously. Shape:

for the inclination, thickness and height of the symphysis pubis. measurement is inferred by subtracting 1.2 cm (4") from Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis (Fig. 9.9). The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4"). It cannot be clinically estimated but is to be inferred from the diagonal conjugate-1.5-2 cm (3½") to be deducted or by lateral radiopelvimetry. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4%") (Fig. 9.9). osterior gatel f lesser to the wnward throughmeasured clinically during pelvic assessment in late pregnancy or in labor. Obstetric conjugate is computed by subtracting 1.5-2 cm from the diagonal conjugate depending upon the height, thickness and inclination of the symphysis pubis. How to measure? The patient is placed in dorsal position. Two fingers are introduced into the vagina taking aseptic precautions. The fingers are to follow the anterior sacral curvature. In normal pelvis, it is difficult to feel the sacral promontory or at best can be felt with difficulty. However, in order to reach the promontory, the elbow and the wrist are to be depressed sufficiently while the fingers are mobilized in upward direction. The point at which the bone recedes from the fingers is the sacral promontory. The fingers are then mobilized under the symphysis pubis and a marking is placed over the gloved index finger by the index finger of the left hand (Fig. 9.10). diagonal conjugate. For practical purpose, if the middle finger The internal fingers are removed and the distance between the marking and the tip of the middle finger gives the measurement of fails to reach the promontory or touches it with difficulty, it is likely that the conjugate is adequate for an average size head to pass through. Transverse diameter: It is the distance between the two farthest points on the pelvic brim over the iliopectineal lines. It measures 13 cm (5%") (Figs. 9.11 and 9.12). The diameter usually lies slightly closer to sacral promontory and divides the brim into anterior and posterior segment. The head negotiates the brim through a diameter, called available or obstetrical transverse. This is described as a diameter which bisects the anteroposterior diameter in the midpoint. Thus, the obstetrical transverse is either equal or less than the anatomical transverse. Fig. 9.10: Measurement of diagonal conjugate. the diagonal conjugate thus allowing for the inclination, thickness and height of the symphysis pubis. measurement is inferred by subtracting 1.2 cm (4") from Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis (Fig. 9.9). The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4"). It cannot be clinically estimated but is to be inferred from the diagonal conjugate-1.5-2 cm (3½") to be deducted or by lateral radiopelvimetry. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4%") (Fig. 9.9). osterior gatel f lesser to the wnward through uterine so that spread ough the rs are all her than atomica ween the er margi 9.9). diametr actice, wever, inlet wis Obstetric transverse Sacrocotyloid Anteroposterior, Left obliqueOblique diameters: There are two oblique diameters-right and left. Each one extends from one sacroiliac joint to the opposite iliopubic eminence and measures 12 cm (4%). Right or left denotes the sacroiliac joint from which it starts (Figs. 9.11 and 9.12). Sacrocotyloid-9.5 cm (3%"): It is the distance between the midpoint of the sacral promontory to iliopubic eminence (Fig. 9.11). It represents the space occupied by the biparietal diameter of the head while negotiating the brim in flat pelvis. CAVITY Cavity is the segment of the pelvis bounded above by the inlet and below by plane of least pelvic dimensions. Shape: It is almost round. Plane: The plane extends from the midpoint of posterior surface of symphysis pubis to the junction of second and third sacral vertebrae (Fig. 9.17). It is called plane of greatest pelvic dimensions. It is the most roomy plane of the pelvis and is almost round in shape. Axis: It is the mid-perpendicular line drawn to the plane of the cavity. Its direction is almost downward (Fig. 9.17). Diameters: Anteroposterior (12 cm or 4%"): It measures from the midpoint on the posterior surface of the symphysis pubis to the junction of second and third sacral vertebrae (Fig. 9.9). Transverse (12 cm or 4%"): It cannot be precisely measured as the points lie over the soft tissues covering the sacrosciatic notches and obturator foramina. OUTLET Obstetrical outlet: It is the segment of the pelvis bounded above by the plane of least pelvic dimensions and below by the anatomical outlet (Fig. 9.13 and Box 9.1). th fo Fig. 9.13: Obstetrical outlet (shaded area). Box 9.1: Obstetric significance of plane of least pelvic dimension It is the narrowest plane in the pelvis. This plane corresponds roughly to the origin of levator ani muscles (Fig. 9.16). It is at this plane that the internal rotation of the fetal head occum during labor. It marks the beginning of the forward curve of the pelvic a (Fig. 9.17). It is a landmark used for pudendal nerve block analgesia, This level of ischial spines indicate station 'O'. When the fetal head station is at 'O, head is considered to be engaged. DTA usually occurs at this plane. Its anterior wall is deficient at the pubic arch; its lateni walls are formed by ischial bones and the posterior wall includes whole of the coccyx. Shape: It is anteroposteriorly oval. Plane: The plane is otherwise known as plane of least pelvic dimensions or narrow pelvic plane. The plane the tip of ischial spines and posteriorly to meet the tip extends from the lower border of the symphysis pubis the fifth sacral vertebra. Diameters: Transverse-Syn: Bispinous (10.5 cm of 4%"): It is the distance between the tip of two ischial spins Anteroposterior (11 cm or 4½"); It extends from the inferior border of the symphysis pubis to the tip of the sacrum (Fig. 9.9). Posterior sagittal (5 cm or 2"): It is the distanc between the tip of the sacrum and the midpoint bispinous diameter. Axis: It is represented by a line joining the center of the plane with the sacral promontory. Its direction is alm vertical. outlet. It is bounded in front by thesymphysis pubis; laterally by the ischiopubic rami, ischial tuberosity and sacrotuberous ligament and posteriorly by the tip of coccyx (Figs. 9.14A to C). Thus, it consists of two triangular planes with a common base formed by a line joining the ischial tuberosities. The apex of the anterior triangle is formed by the inferior border of the pubic arch and that of the posterior triangle by the tip of the coccyx. Shape: It is diamond-shaped. Plane: It is formed by a line joining the lower border of the symphysis pubis to the tip of the coccyx (Fig. 9.17). It forms an angulation of 10° with the horizontal. Azis: It is a mid-perpendicular line drawn to the plane of the outlet. Its direction is downward and forward (Fig. 9.17). Diameters. Anteroposterior: It extends from the lower border of the symphysis pubis to the tip of the coccyx. It measures 13 cm or 54" with the coccyx pushed back by the head when passing through the introitus in the second stage of labor, with the coccyx in normal position, the measurement will be 2.5 cm less (Fig. 9.9). Transverse Diameter of the Outlet (TDO)-Syn: Intertuberous (11 cm or 4½"); It measures between inner borders of ischial tuberosities. Posterior sagittal diameter (8.5 cm or 3½"): It is the anteroposterior distance between the sacrococcygeal joint and the midpoint of transverse Diameter of Outlet (IDOL. It is clinically measured by the distance between the sacrococcygeal joint and anterior margin of the anus. int of It measures 85°. Subpubic angle: It is formed by the approximation of the two descending pubic rami. In normal female pelvis, of the Imost hony both the sides is of obstetric importance. Normally, it Pubic arch: Arch formed by the descending rami of C (B) Dummy showing false pelvis, true pelvis and measures 6 cm in between the pubic rami at a level of 2 cm below the apex of the subpubic arch. Clinically, it is assessed by placing 3 fingers side by side. head displaced backward and the less the room available The narrower the pubic arch, the more is the fetal for it. Normally, the subpubic arch is rounded and less space is wasted under the symphysis pubis. When a fetal head) is placed under the arch, the distance round disk of 9.3 cm diameter (diameter of well-flexed between the symphysis pubis and the circumference of space of Morris and should not exceed 1 cm in a normal pelvis. the disk is measured. This measurement is the waste space of Morris is more than 1 cm, the anterior point of Available anteroposterior diameter: When the waste the anteroposterior diameter of the outlet extends below the symphysis pubis on the pubic rami for a distance equivalent to the waste space of Morris (Figs. 9.15A to C). The distance between the said point and the tip of the sacrum is called available anteroposterior diameter of the outlet. It is through this diameter that the head escapes out of the bony outlet. MIDPELVIS Midpelvis is the segment of the pelvis bounded above by the plane of the greatest pelvic dimensions and below by a plane known as midpelvic plane. Stance Midpelvic plane: The midpelvic plane extends from the lower margin of the symphysis pubis through the level of ischial spines to meer either the junction of S, and S coincides with the plane of least pelvic dimensions. If the the sacrum. If the plane meets the tip of the fifth sacrum,plane becomes a wedge posteriorly; (B) Midpelvic plane coincides with the plane of least pelvic dimensions. plane meets the junction of S, and S, the plane becomes a wedge posteriorly (Figs. 9.16A and B). Diameters: Transverse diameter-Syn: bispinous (10.5 cm). It measures between the two ischial spines. Anteroposterior diameter (11.5 cm): It extends from the lower border of the symphysis pubis to the point on the sacrum at which the midpelvic plane meets. Posterior sagittal diameter (4.5 cm): It extends from the midpoint of the bispinous diameter to the point on the sacrum at which the midpelvic plane meets. PELVIC AXIS: Anatomical (curve of Carus): Anatomical pelvic axis is formed by joining the axes of inlet, cavity and outlet. It is uniformly curved with the convexity fitting with the concavity of the sacrum. The fetus does not, however transverse the uniform curved path (Figs. 9.17A and B). Obstetrical: It is through this axis that the fet negotiates the pelvis. It is not uniformly curved. Is direction is first downward and backward up to the leu of ischial spines and then directed abruptly forward (Figs. 9.17A and B). PELVIC JOINTS: Symphysis pubis: It is a secondar fibrocartilaginous joint. It has got no capsule and ot synovial cavity. The articular surfaces are covered hyaline cartilage. Due to softening of the ligaments during pregnancy, there is considerable amount of gliding movement.Figs. 9.17A and B: Pelvic axis: (A) Axis of inlet, cavity and outlet are shown by arrows drawn perpendicular to the planes. The shaded area over the axis is the anatomical pelvic axis (curve of Carus); (B) Obstetric pelvic axis-as shown by the shaded area is directed downward and backward up to ischial spines and then directed forward. Sacroiliac articulation: It is a synovial joint and is an articulation between the articular

for the inclination, thickness and height of the symphysis pubis. measurement is inferred by subtracting 1.2 cm (4") from Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis (Fig. 9.9). The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4"). It cannot be clinically estimated but is to be inferred from the diagonal conjugate-1.5-2 cm (3½") to be deducted or by lateral radiopelvimetry. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4%") (Fig. 9.9). osterior gatel f lesser to the wnward throughmeasured clinically during pelvic assessment in late pregnancy or in labor. Obstetric conjugate is computed by subtracting 1.5-2 cm from the diagonal conjugate depending upon the height, thickness and inclination of the symphysis pubis. How to measure? The patient is placed in dorsal position. Two fingers are introduced into the vagina taking aseptic precautions. The fingers are to follow the anterior sacral curvature. In normal pelvis, it is difficult to feel the sacral promontory or at best can be felt with difficulty. However, in order to reach the promontory, the elbow and the wrist are to be depressed sufficiently while the fingers are mobilized in upward direction. The point at which the bone recedes from the fingers is the sacral promontory. The fingers are then mobilized under the symphysis pubis and a marking is placed over the gloved index finger by the index finger of the left hand (Fig. 9.10). diagonal conjugate. For practical purpose, if the middle finger The internal fingers are removed and the distance between the marking and the tip of the middle finger gives the measurement of fails to reach the promontory or touches it with difficulty, it is likely that the conjugate is adequate for an average size head to pass through. Transverse diameter: It is the distance between the two farthest points on the pelvic brim over the iliopectineal lines. It measures 13 cm (5%") (Figs. 9.11 and 9.12). The diameter usually lies slightly closer to sacral promontory and divides the brim into anterior and posterior segment. The head negotiates the brim through a diameter, called available or obstetrical transverse. This is described as a diameter which bisects the anteroposterior diameter in the midpoint. Thus, the obstetrical transverse is either equal or less than the anatomical transverse. Fig. 9.10: Measurement of diagonal conjugate. the diagonal conjugate thus allowing for the inclination, thickness and height of the symphysis pubis. measurement is inferred by subtracting 1.2 cm (4") from Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis (Fig. 9.9). The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4"). It cannot be clinically estimated but is to be inferred from the diagonal conjugate-1.5-2 cm (3½") to be deducted or by lateral radiopelvimetry. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4%") (Fig. 9.9). osterior gatel f lesser to the wnward through uterine so that spread ough the rs are all her than atomica ween the er margi 9.9). diametr actice, wever, inlet wis Obstetric transverse Sacrocotyloid Anteroposterior, Left obliqueOblique diameters: There are two oblique diameters-right and left. Each one extends from one sacroiliac joint to the opposite iliopubic eminence and measures 12 cm (4%). Right or left denotes the sacroiliac joint from which it starts (Figs. 9.11 and 9.12). Sacrocotyloid-9.5 cm (3%"): It is the distance between the midpoint of the sacral promontory to iliopubic eminence (Fig. 9.11). It represents the space occupied by the biparietal diameter of the head while negotiating the brim in flat pelvis. CAVITY Cavity is the segment of the pelvis bounded above by the inlet and below by plane of least pelvic dimensions. Shape: It is almost round. Plane: The plane extends from the midpoint of posterior surface of symphysis pubis to the junction of second and third sacral vertebrae (Fig. 9.17). It is called plane of greatest pelvic dimensions. It is the most roomy plane of the pelvis and is almost round in shape. Axis: It is the mid-perpendicular line drawn to the plane of the cavity. Its direction is almost downward (Fig. 9.17). Diameters: Anteroposterior (12 cm or 4%"): It measures from the midpoint on the posterior surface of the symphysis pubis to the junction of second and third sacral vertebrae (Fig. 9.9). Transverse (12 cm or 4%"): It cannot be precisely measured as the points lie over the soft tissues covering the sacrosciatic notches and obturator foramina. OUTLET Obstetrical outlet: It is the segment of the pelvis bounded above by the plane of least pelvic dimensions and below by the anatomical outlet (Fig. 9.13 and Box 9.1). th fo Fig. 9.13: Obstetrical outlet (shaded area). Box 9.1: Obstetric significance of plane of least pelvic dimension It is the narrowest plane in the pelvis. This plane corresponds roughly to the origin of levator ani muscles (Fig. 9.16). It is at this plane that the internal rotation of the fetal head occum during labor. It marks the beginning of the forward curve of the pelvic a (Fig. 9.17). It is a landmark used for pudendal nerve block analgesia, This level of ischial spines indicate station 'O'. When the fetal head station is at 'O, head is considered to be engaged. DTA usually occurs at this plane. Its anterior wall is deficient at the pubic arch; its lateni walls are formed by ischial bones and the posterior wall includes whole of the coccyx. Shape: It is anteroposteriorly oval. Plane: The plane is otherwise known as plane of least pelvic dimensions or narrow pelvic plane. The plane the tip of ischial spines and posteriorly to meet the tip extends from the lower border of the symphysis pubis the fifth sacral vertebra. Diameters: Transverse-Syn: Bispinous (10.5 cm of 4%"): It is the distance between the tip of two ischial spins Anteroposterior (11 cm or 4½"); It extends from the inferior border of the symphysis pubis to the tip of the sacrum (Fig. 9.9). Posterior sagittal (5 cm or 2"): It is the distanc between the tip of the sacrum and the midpoint bispinous diameter. Axis: It is represented by a line joining the center of the plane with the sacral promontory. Its direction is alm vertical. outlet. It is bounded in front by thesymphysis pubis; laterally by the ischiopubic rami, ischial tuberosity and sacrotuberous ligament and posteriorly by the tip of coccyx (Figs. 9.14A to C). Thus, it consists of two triangular planes with a common base formed by a line joining the ischial tuberosities. The apex of the anterior triangle is formed by the inferior border of the pubic arch and that of the posterior triangle by the tip of the coccyx. Shape: It is diamond-shaped. Plane: It is formed by a line joining the lower border of the symphysis pubis to the tip of the coccyx (Fig. 9.17). It forms an angulation of 10° with the horizontal. Azis: It is a mid-perpendicular line drawn to the plane of the outlet. Its direction is downward and forward (Fig. 9.17). Diameters. Anteroposterior: It extends from the lower border of the symphysis pubis to the tip of the coccyx. It measures 13 cm or 54" with the coccyx pushed back by the head when passing through the introitus in the second stage of labor, with the coccyx in normal position, the measurement will be 2.5 cm less (Fig. 9.9). Transverse Diameter of the Outlet (TDO)-Syn: Intertuberous (11 cm or 4½"); It measures between inner borders of ischial tuberosities. Posterior sagittal diameter (8.5 cm or 3½"): It is the anteroposterior distance between the sacrococcygeal joint and the midpoint of transverse Diameter of Outlet (IDOL. It is clinically measured by the distance between the sacrococcygeal joint and anterior margin of the anus. int of It measures 85°. Subpubic angle: It is formed by the approximation of the two descending pubic rami. In normal female pelvis, of the Imost hony both the sides is of obstetric importance. Normally, it Pubic arch: Arch formed by the descending rami of C (B) Dummy showing false pelvis, true pelvis and measures 6 cm in between the pubic rami at a level of 2 cm below the apex of the subpubic arch. Clinically, it is assessed by placing 3 fingers side by side. head displaced backward and the less the room available The narrower the pubic arch, the more is the fetal for it. Normally, the subpubic arch is rounded and less space is wasted under the symphysis pubis. When a fetal head) is placed under the arch, the distance round disk of 9.3 cm diameter (diameter of well-flexed between the symphysis pubis and the circumference of space of Morris and should not exceed 1 cm in a normal pelvis. the disk is measured. This measurement is the waste space of Morris is more than 1 cm, the anterior point of Available anteroposterior diameter: When the waste the anteroposterior diameter of the outlet extends below the symphysis pubis on the pubic rami for a distance equivalent to the waste space of Morris (Figs. 9.15A to C). The distance between the said point and the tip of the sacrum is called available anteroposterior diameter of the outlet. It is through this diameter that the head escapes out of the bony outlet. MIDPELVIS Midpelvis is the segment of the pelvis bounded above by the plane of the greatest pelvic dimensions and below by a plane known as midpelvic plane. Stance Midpelvic plane: The midpelvic plane extends from the lower margin of the symphysis pubis through the level of ischial spines to meer either the junction of S, and S coincides with the plane of least pelvic dimensions. If the the sacrum. If the plane meets the tip of the fifth sacrum,plane becomes a wedge posteriorly; (B) Midpelvic plane coincides with the plane of least pelvic dimensions. plane meets the junction of S, and S, the plane becomes a wedge posteriorly (Figs. 9.16A and B). Diameters: Transverse diameter-Syn: bispinous (10.5 cm). It measures between the two ischial spines. Anteroposterior diameter (11.5 cm): It extends from the lower border of the symphysis pubis to the point on the sacrum at which the midpelvic plane meets. Posterior sagittal diameter (4.5 cm): It extends from the midpoint of the bispinous diameter to the point on the sacrum at which the midpelvic plane meets. PELVIC AXIS: Anatomical (curve of Carus): Anatomical pelvic axis is formed by joining the axes of inlet, cavity and outlet. It is uniformly curved with the convexity fitting with the concavity of the sacrum. The fetus does not, however transverse the uniform curved path (Figs. 9.17A and B). Obstetrical: It is through this axis that the fet negotiates the pelvis. It is not uniformly curved. Is direction is first downward and backward up to the leu of ischial spines and then directed abruptly forward (Figs. 9.17A and B). PELVIC JOINTS: Symphysis pubis: It is a secondar fibrocartilaginous joint. It has got no capsule and ot synovial cavity. The articular surfaces are covered hyaline cartilage. Due to softening of the ligaments during pregnancy, there is considerable amount of gliding movement.Figs. 9.17A and B: Pelvic axis: (A) Axis of inlet, cavity and outlet are shown by arrows drawn perpendicular to the planes. The shaded area over the axis is the anatomical pelvic axis (curve of Carus); (B) Obstetric pelvic axis-as shown by the shaded area is directed downward and backward up to ischial spines and then directed forward. Sacroiliac articulation: It is a synovial joint and is an articulation between the articularFigs.Figs.

Figs. 9.17A and B: Pelvic axis: (A) Axis of inlet, cavity and outlet are shown by arrows drawn perpendicular to the planes. The shaded area over the axis is the anatomical pelvic axis (curve of Carus); (B) Obstetric pelvic axis-as shown by the shaded area is directed downward and backward up to ischial spines and then directed forward. Sacroiliac articulation: It is a synovial joint and is an articulation between the articular surface of the ilium and sacrum. The articulating surfaces are not alike. It has got a capsule and a synovial cavity. Sacrococcygeal joint is a synovial hinge joint. It allows both flexion and extension. Extension increases the anteroposterior diameter of the outlet. Physiological Enlargement of Pelvis During Pregnancy and Labor Imaging studies show an increase in width and mobility of the symphysis pubis during pregnancy which returns to normal following delivery. The pubic bones may separate by 5-10 mm. Similar changes also occur in sacroiliac joints. There is gliding movement of the symphysis pubis near term. Relaxation of the pelvic joints is due to progesterone and relaxin. There is increase of the anteroposterior diameter of the inlet during labor by the rotatory movement of the sacroiliac joints. In dorsal lithotomy position, the anteroposterior diameter of the outlet may be increased to 1.5-2 cm. Furthermore, the coccyx is pushed back while the head descends down to the perineum.