Anatomy of maternal pelvis and fetal head Dr Hiba Ahmed Suhail M.B. Ch. B./F.I.B.O.G. College of medicine University of Mosul

Knowlage of the anatomy of normal female plevis ,fetal skull &soft tissues is essential to understand mechanism of labour.

■ Pelvic Bones The pelvis is composed of four bones: The sacrum, The coccyx Two innominate bones . Each innominate bone is formed by the fusion of the ilium, ischium, and pubis. The innominate bones are joined to the sacrum at the sacroiliac and to one another at the symphysis pubis.

BONY PELVIS DIVIDED IN TO The false pelvis lies above the iliopectenial line and bounded posteriorly by the lumbar vertebra laterally by the iliac fossa Anteriorly the boundary is formed by the lower portion of the anterior abdominal wall. The true pelvis is the portion important in childbearing lies below iliopactenial line .It is bounded Above by the promontory and alae of the sacrum, the iliopactenial line, and the upper margins of the pubic bones Below by the pelvic outlet. Posteriorly by the anterior surface of the sacrum Laterally by the inner surface of the ischial bones and the sacrosciatic notches and ligaments. In front, by the pubic bones, the ascending superior rami of the ischial bones, and the obturator foramen

The walls of the true pelvis are partly bony and partly ligamentous. Normally, the sacrum has a marked concavity, which in abnormal pelvis may undergo important variations. The descending inferior rami of the pubic bones unite at an angle of 90 to 100 degrees to form a rounded arch under which the fetal head must pass.

■ Pelvic Joints Symphysis Pubis Anteriorly, the pelvic bones are joined together by the symphysis pubis. This structure consists of fibrocartilage and the superior and inferior pubic ligaments. Sacroiliac Joints Posteriorly, the pelvic bones are joined by articulations between the sacrum and the iliac bones to form the sacroiliac joints. Relaxation of the Pelvic Joints During pregnancy, there is remarkable relaxation of these joints, It likely results from hormonal stimulation

■ Planes and Diameters of the Pelvis The pelvis is described as having 3imaginary planes: The plane of the pelvic inlet. 2. The plane of the pelvic outlet. 3. The plane of the midpelvis.

Pelvic Inlet The superior strait or pelvic inlet is bounded posteriorly by the promontory and alae of the sacrum, laterally by the ileopectenial line . Anteriorly by the superior pubic rami and the symphysis pubis. The inlet of the female pelvis—compared with the male pelvis—typically is more nearly round than ovoid.(gynecoid pelvic)

. Diameters of the pelvic inlet are : Anteroposterior diameter is the shortest distance between the promontory of the sacrum and the symphysis pubis and is 11 cm The transverse diameter is at right angles to the anteroposterior and represents the greatest distance between the iliopactenial line on either side is 13.5 cm. Two oblique diameters extends from one of the sacroiliac to the iliopectineal eminence on the opposite side (13cm)

Mid pelvic cavity Area bounded Anteriortly by the middle of the symphesis pubis On each side by the pubic bone ,obturator fascia and inner aspect of ischial bone and spine Posteriorly by the junction of the 2nd and 3rd sacral portion , The Cavity is rounded with anterior and transverse diameter are 12cm

Pelvic Outlet This consists of two approximately triangular areas . They have a common base, which is a line drawn between the two ischial tuberosities. The posterior triangle The apex of the posterior triangle is at the tip of the sacrum, and the lateral boundaries are the sacrosciatic ligaments and the ischial tuberosities. The anterior triangle is formed by the area under the pubic arch the apex of the anterior triangle is the symphesis pubis and the lateral wall is the pubic remai . diameters of the pelvic outlet usually are described: The anteroposterior 13.5 cm The transverse 11cm

Pelvic axis Is an imaginary curve line shows the path which the centre of fetal head follows during its passage through the pelvis ,it is obtained by taking several anteroposterior diameters of the pelvis &joining their centers

It is possible to estimate The diagonal conjugate diameter vaginally which the distance between the promontory &lower margin of symphysis pubis is 12.5 cm. The true conjugate diameter between the promontory &upper margin of symphysis pubis is 11cm.

Pelvic Shapes is based on measurement of the greatest transverse diameter of the pelvic inlet and it divided the pelvis into anterior and posterior segments. (The character of the posterior segment determines the type of pelvis) as Gynecoid the gynecoid pelvis is suited for delivery of most fetuses. It is found in 45%-50% of women Anthropoid (OP) Android(DTA) Platypelloid (OBSTRUCTED LABOUR) .

Android pelvis It had many characteristics of male pelvis ,the brim is heart-shaped so the widest transverse diameter is much nearer to the sacrum,the side walls tend to converge ,the ischial spines are prominent ,the sacrum is straight&the subpubic arch is generally narrow with an angle of 70 or less. Both the anteroposterior &transverse diameters of the outlet tend to be reduced. This type of pelvis is funnel-shaped with diameters decrease from above downwards so disproportion become worse as labour proceeds.

Anthropoid pelvis The anteroposterior diameter of the brim exceeds the transverse diameter .it tends to be deep &the sacrum has six segments instead of five this is known as a high assimilation pelvis.Sacrum&axis of pelvic cavity are less curved than in gynecoid pelvis&subpubic may be little narrow,but the sacrosciatic notches are wide &anteroposterior diameter of the outlet is large

platypelloid Is described as the simple [non-rachitic] flat pelvis . The brim is elliptical with a wide transverse diameter , the subpubic arch is wide & rounded.

■ Muscular Support *The pelvic diaphragm forms a broad muscular sling and provides support to the pelvic viscera *Muscle group is comprised of the levator ani and the coccygeus muscle. *The levator ani is composed of the pubococcygeus, puborectalis, and ileococcygeus muscles.

The perineum The final obstacle to be negated by the fetus during labour is The perineum The pernial body is a condensation of muscle and fibrous tissue lying between the vagina and the anus. Its apex is at the lower end of the rectovaginal septum and its base is covered with skin and extends from the fourchette to the anus

Fetal Head the fetal head size is important because an essential feature of labor is the adaptation between the head and the maternal bony pelvis SKULL is composed of the firm bones , which is made up of two frontal, two parietal, and two temporal bones, along with the upper portion of the occipital bone and the wings of the sphenoid. Face At term ,only a small part of the head is represented by the face. Vault Base sutures These are membranous (un ossified )spaces that are separated the skull bones The most important sutures are the frontal between the two frontal bones The sagittal, between the two parietal bones the two coronal, between the frontal and parietal bones The two lambdoid, between the posterior margins of the parietal bones and upper margin of the occipital bone

fontanel an irregular space forms at the junction of the sutures , which is enclosed by a membrane and we have Greater, or anterior fontanel is a diamond -shaped space that is situated at the junction of the sagittal and the coronal sutures. The lesser, or posterior fontanel is represented by a small triangular area at the junction of the sagittal and lambdoid sutures. The localization of these fontanels gives important information concerning the presentation and position of the fetus during labor.

Vertex the area of the fetal skull that bounded by the 2 parietal eminence the anterior and the posterior fontanel

Fetal head diameters The fetal head is ovoid in shape , the attitude of the head is the degree of the flexion and extension of the head on fetal the cervical spine . to measure certain critical diameters and circumferences of the newborn head The diameters include: 1. The suboccipitobregmatic (9.5 cm), which follows a line drawn from the middle of the large fontanel to the undersurface of the occipital bone just where it joins the neck ,it is the longitudinal diameter that present in well flex head . 2.The sub occipitofrontal diameter (1o cm) from the sub occipital region to the prominence of the forehead , it presents in less well flexed head (occipit posterior position ) . 3. The occipitofrontal (11.5 cm), which follows a line extending from a point just above the root of the nose to the posterior fontanele ,a present in partially extended or deflexed head . 4.Mento vertical diameter (13 cm) from the chin to the furthest point of the vertex ,present in extended head (brow presentation).. 5. The sub mento bregmatic (9.5 cm )from below the chin to the anterior fontanelle ,present in hyper extended head (face presentation ). 6. The biparietal (9.5 cm), the greatest transverse diameter of the head, which extends between the two parietal eminences . 7. The bitemporal (8.0 cm), which is the greatest distance to the most prominent portion of the occiput.

. ■ Changes in Shape of the Fetal Head In vertex presentations, the fetal head changes shape as the result of labor forces 1-Caput Succedaneum In prolonged labors before complete cervical dilatation, the portion of the fetal scalp immediately over the cervical os becomes edematous This swelling known as the caput succedaneum It usually attains a thickness of only a few millimeters, but in prolonged labors it may be sufficiently extensive to prevent the differentiation of the various sutures and fontanels. Most cases of caput and resolve within the week following delivery

2-Moulding The bones of the cranium are normally connected only by a thin layer of fibrous tissue. this allows considerable shifting or sliding of each bone to accommodate the size and shape of the maternal pelvis. This process is termed moulding The change in fetal head shape from external compressive forces is referred to as moulding.. Moulding results in a shortened suboccipitobregmatic diameter and a lengthened mentovertical diameter. These changes are of great importance in women with contracted pelvis to make the head accommodate the pelvis Three levels of moulding are described 1 –just approximation of the sutures 2- over lapping of the sutures but it is reducible 3- non reducible over lapping