Pelvic Anatomy in Obstetrics

Pelvic Anatomy in Obstetrics

• Key concern: The true pelvis's role in forming the birth canal is crucial for obstetrics.

• Pelvic Divisions: The true pelvis is divided into inlet, cavity, and outlet.

• Posterior Depth of Cavity: The average posterior depth of the true pelvic cavity is approximately 11.5 cm.

• Common Pelvic Shape: Gynecoid is the most prevalent pelvic shape.

• Pelvic Inclination: Normal pelvic inclination is around 55 degrees.

• High Inclination Implications: Increased pelvic inclination, such as with sacralization, can result in challenging head descent during labor.

• Sacral Angle Definition: The angle formed by the true conjugate and the first two sacral segments is termed the conjugal angle.

• True Conjugate Measurement: The true conjugate, a crucial anteroposterior diameter at the inlet, measures approximately 11 cm.

• Obstetric Conjugate: The shortest anteroposterior diameter of the pelvic inlet, inferred from the diagonal conjugate.

• Diagonal Conjugate Measurement: Measured by inserting two fingers into the vagina.

• Transverse Diameter of Pelvic Inlet: A crucial transverse diameter measurement of the inlet is around 13 cm.

• Obstetric vs Anatomical Transverse: The obstetric transverse diameter is smaller than the anatomical transverse diameter.

• Oblique Diameters: Pelvic oblique diameters measure approximately 12 cm.

• Sacrocotyloid Diameter: This diameter determines the space available for the fetal head at the brim.

• Plane of Least Dimensions: The inlet and outlet define the pelvic cavity. The plane of least pelvic dimensions is the narrowest section.

• Plane of Greatest Dimensions: Located at the junction of the second and third sacral vertebrae.

• Pelvic Outlet Shape: The outlet exhibits a diamond shape.

• Outlet Narrowness: The pelvic outlet is the narrowest plane in the pelvis.

• Bispinous Diameter: The bispinous outlet diameter is approximately 10.5 cm.

• Anteroposterior Outlet Diameter: The distance between the lower border of the symphysis pubis to the coccyx.

• Subpubic Arch: A crucial outlet measurement, the subpubic arch is normally about 6 cm.

• Morris' Waste Space: Measured by inserting a disc beneath the pubic arch.

• Outlet Anteroposterior Diameter Increase: Increased by pushing the coccyx backward.

• Midpelvic Plane Location: The midpelvic plane is between the pelvic inlet/outlet and the junction of the symphysis pubis and sacral spines.

• Midpelvic Anteroposterior Diameter: About 11.5 cm.

• Pelvic Axis: The pelvic axis follows a curved path.

• Sacrococcygeal Joint Mobility: Allows for flexion and extension.

• Pelvic Joint Flexibility in Pregnancy: Increased flexibility in pelvic joints due to relaxin and progesterone.

• Sacroiliac Joint Mobility in Pregnancy: The sacroiliac joint exhibits gliding motion.

• Coccyx Movement in Labor: The coccyx may be pushed backward during labor to increase outlet space.

• Sacral Angle: The sacral angle is typically normal in shape.

• Anatomical Outlet Bounding: The lower border of the symphysis pubis bounds the front of the anatomical outlet.

• Pelvic Enlargement in Pregnancy: The pubic bones separate by 5-10 mm during pregnancy.

• Anteroposterior Diameter Increase with Position: The dorsal lithotomy position often increases the anteroposterior diameter of the outlet by 1.5-2 cm.

• Sacroiliac Joint Type: The sacroiliac joint is a synovial joint.

• Fetal Head Rotation: The head rotates during the passage through the pelvic cavity.

• Sacrocotyloid Diameter Measurement: Measured from the sacral promontory midpoint to the iliopubic eminence.

• Sacral Promontory to Symphysis Pubis Distance: Approximately 10.5cm.

• Favorable Pelvic Shapes: Gynecoid pelvic shapes are favorable for childbirth.

• Android Pelvic Characteristics: Male-like characteristics.

• Android Pelvic Concerns: Narrow pelvic inlet is a significant concern.

• Favorable Pelvic Features: Wide inlet and deep cavity pelvic shapes are favorable.

• Pelvic Support in Pregnancy: The pelvis supports the maternal weight and the weight of uterus and abdominal organs.

• Sacral Promontory Use: The sacral promontory aids in locating the diagonal conjugate and determining fetal position.

• Fetal Head Accommodation During Labor: Pelvic joints widen to accommodate the fetal head.