MECHANISM OF LABOUR AND ctg DR. Nael obeidat

Pelvic Anatomy

Bony pelvis The bony pelvis is made up of four bones ( Sacrum , Coccyx and 2 Innominate ) . Held together by ( Sacroiliac , Symphysis pubis , Sacrococcygeal ) joints . Sacrum

The pelvis is divided into the false pelvis above and the true pelvis below the linea terminalis ( the edge of the pelvic inlet ) .

The pelvic brim is the edge of the pelvic inlet The pelvic brim is the edge of the pelvic inlet. It is an approximately apple-shaped line passing through the prominence of the scrum, the arcuate and pectineal lines , and the upper margin of the pubic symphysis .

The pelvic floor  is composed of muscle fibers of the levator ani ,the coccygeus muscle , and associated connective tissue which span the area underneath the pelvis . Pelvic floor

Pelvic planes & diameters Pelvic planes are imaginary flat surfaces that extend across the pelvis at different levels. The diameters of the pelvic planes represent the amount of space available at each level . Pelvic planes Pelvic inlet Mid pelvis Pelvic outlet

Pelvic inlet Pubic crest anteriorly Is bordered by the Pubic crest anteriorly iliopectineal line of the innominate laterally Promontory of the sacrum posteriorly The fetal head enters the pelvis through this plane in the transverse position .

Pelvic inlet diameters - Transverse diameter of inlet: 13.5 cm - Obstetric conjugate; AP diameter of inlet: 11 cm

Is bordered by the lower edge of the pubis anteriorly and the ischial spines and sacrospinous ligaments laterally and the lower sacrum posteriorly . Diameters: - Similar in directions to pelvic inlet diameters and all measure 12 cm. Ischial spines palpable vaginally = 1. landmark for descent of the head in vaginal examination 2. landmark for anesthetic block to pudendal nerve Mid pelvis

Pelvic outlet The pelvic outlet bordered by pubic arch anteriorly. Tip of coccyx posteriorly. Lower margin of sacrotuberous ligament and ischial tuberosity laterlly. Diameters: Transverse Diameter: 11 cm. (between lowest ends of ischial tubersity). AP diameter: 13.5 cm. (from sacrococcygeal joint to lower border of pubic symphysis). Pelvic outlet

Based on the general bony architecture , the pelvis may be classified into four basic types Gynecoid Android Anthropoid Platypelloid Pelvic shapes

The gynecoid pelvis is the classic female type and is found in approximately 50% of women. Round at the inlet Sidewalls straight The fetal head generally rotates into the occipitoanterior position in th type of pelvis . Gynecoid

The android pelvis is the typcal male type of pelvis The android pelvis is the typcal male type of pelvis . It found in less than 30% of women . Funneling (Limited space at inlet and gets narrower as you go down the pelvis). Fetal head is forced to be in Occipitoanterior position. Android

The anthropoid pelvis resembles that of the anthropoid ape The anthropoid pelvis resembles that of the anthropoid ape . It found in approximately 20% of women . A much larger ap than transverse diameter , creating a long narrow oval at the inlet The fetal head can engage only in the ap diameter and usually does so in the occipitoposterior position . Anthropoid

The platypeloid pelvis is best described as being flattened gynecoid pelvis . It’s found in only 3% of women . Oval shaped inlet with wide transverse diameter Fetal head has to engage in the transverse diameter Increased risk of obstructed labor. Platypelloid

Fetal skull

Fetal Skull The fetal skull is the most difficult part of the baby to pass through the mother’s pelvic canal, due to the hard bony nature of the skull. It is the largest and least compressible part of the fetus Fetal skull = Base + vault “cranium” Base; large, ossified, firmly united, and non-compressible bones

Cranium Cranium = occipital + 2 parietal + 2 frontal + 2 temporal Thin, weakly ossified, easily compressible, interconnected by membranes >> molding.

Sutures Sutures; membrane-occupied spaces between the cranial bones. Sutures are joints between the bones of the skull. Sagittal suture joins the two parietal bones together. Lambdoid forms the junction between the occipital and the paterietal bone Coronal joins the frontal bone to the two parietal bones. Frontal joins the two frontal bones together.

Molding the natural process by which a baby's head is shaped during labor as it is squeezed into and through the birth passage by the forces of labor. The head often becomes quite elongated, and the bones of the skull may be caused to overlap slightly at the suture lines. The biparietal diameter of the head may be compressed as much as 0.5 cm without intracranial damage. Most of the changes caused by molding resolve themselves during the first few days of life.

Fontanelles A fontanel is the space created by the joining of two or more sutures. It is covered by thick membranes and the skin on the baby’s head, protecting the brain underneath the fontanel from contact with the outside world. Identification of the two large fontanels on the top of the fetal skull helps you to locate the angle at which the baby’s head is presenting during labour and delivery. Posterior fontanelle: closes at 6-8 weeks, sagittal and lambdoid, T-shaped Anterior fontanelle ( bregma ): closes at 18 months, sagittal, frontal, coronal, diamond shape, 2-3 cm

The vertex is the area of the vault bounded anteriorly by the anterior fontanelle and the coronal suture, posteriorly by the posterior fontanelle and the lambdoid suture and laterally by 2 lines passing through the parietal eminences The brow is the area of skull which extends from the anterior fontanel to the upper border of the eye. A brow presentation is a significant risk for the mother and the baby. The face extends from the upper ridge of the eye to the nose and chin (lower jaw). A face presentation is also a significant risk for the mother and baby. The occiput is the area between the base of the skull and the posterior fontanel. It is unusual and very risky for the occiput to be the presenting part.

Cephalic Presentation A cephalic presentation is a situation at childbirth where the fetus is in a longitudinal lie and the head enters the pelvis first the most common form of cephalic presentation is the vertex presentation where the occiput is the leading part (the part that first enters the birth canal)

The vertex presentations are further classified according to the position of the occiput, it being right, left, or transverse, and anterior or posterior: Left Occipito-Anterior (LOA), Left Occipito-Posterior (LOP), Left Occipito-Transverse (LOT); Right Occipito-Anterior (ROA), Right Occipito-Posterior (ROP), Right Occipito-Transverse (ROT);

Management A vertex presentation is the ideal situation for a vaginal birth, however, occiput posterior positions tend to proceed more slowly, often requiring an intervention in the form of forceps, vacuum extraction, or Cesarean section

Diameters