Obstetrics Marrow Pg 485-494 (Labor & Puerperium)

Ischial Spine Significance

• The ischial spine is a crucial anatomical landmark during labor and delivery, playing a vital role in determining fetal head station, identifying the level of internal rotation, and serving as a reference for pudendal nerve block.
• Station 0 aligns with the ischial spine, indicating the fetal head is engaged in the pelvic inlet.
• Forceps and vacuum delivery are typically indicated at station ≥ +2.
• Internal rotation of the fetal head often occurs at the level of the ischial spine, influencing the fetus' descent through the birth canal.
• Deep transverse arrest, a potential complication in labor, can occur at the level of the ischial spines.
• The levator ani muscle, a crucial structure supporting the pelvic floor, originates from the ischial spine.
• For pudendal nerve block, the sacrospinous ligament is pierced to access the nerve.

Pelvic Dimensions & Angles

• Posterior sagittal diameter of the pelvic outlet: 7 to 7.5 cm
• Posterior sagittal diameter of the mid-pelvis: ≥ 5cm
• Sub-pubic angle is typically obtuse in females and acute in males.
• The angle between descending rami of pubic bones is a significant factor in childbirth.
• The angle of inclination, defining the angle between the pelvic inlet and the horizontal plane, is approximately 55°.

Fetal Skull Diameters

• Fetal skull diameters are crucial in understanding the relationship between the fetal head and the maternal pelvis during labor.
• The longest AP diameter of the fetal skull is the mento-vertical diameter (14 cm), typically seen in brow presentation.
• Transverse diameters range from the smallest bimastoid (7.5 cm) to the longest bitemporal (8 cm).
• The engaging diameter varies based on the fetal head position:
  • Vertex (fully flexed): sub-occipital bregmatic diameter (9.5 cm)
  • Brow (partially extended): occipito-frontal diameter (11.5 cm) or mento-vertical diameter (14 cm)
  • Face (fully extended): sub-mento-vertical diameter (11.5 cm) or sub-mento-bregmatic diameter (9.5 cm)

Fetal Skull Presenting Parts

• The presenting part defines the fetal body part that occupies the lower pole of the uterus during labor.
• Vertex presentation, the most common, involves the fetal head presenting.
• Breech presentation, the most common malpresentation, involves the buttocks presenting.
• Shoulder presentation, common with transverse lie, involves the shoulder presenting.

Labor & Puerperium

• The active space, the space available for fetal descent, becomes available around 38 weeks in primigravida and at the onset of labor in multigravida.
• Molding describes the alteration in the fetal head shape as it passes through the birth canal.
• Caput succedaneum is an edematous swelling above the periosteum, occurring when the fetal head remains stationary for extended periods during labor.
• Cephalhematoma is a collection of blood beneath the periosteum, typically occurring due to traumatic instrumental delivery.

Pelvic Outlet & Mid-pelvis

• The pelvic outlet lies at the level of the ischial tuberosities.
• The AP diameter of the pelvic outlet is approximately 13 cm.
• The transverse diameter, also known as the bituberous diameter, is the distance between the ischial tuberosities (typically around 11 cm).
• A contracted outlet, defined by a bituberous diameter less than 8 cm, can make vaginal delivery challenging.

Contracted Pelvis & Cephalopelvic Disproportion (CPD)

• Contracted inlet is defined as an obstetric conjugate less than 10 cm or a diagonal conjugate less than 11.5 cm.
• Contracted mid-pelvis exists when the interischial diameter is less than 8 cm.
• A contracted outlet is diagnosed when the bituberous diameter is less than 8 cm.
• CPD is a relative finding where the fetal head is too large for the maternal pelvis, or the pelvis is too small for the fetal head, indicating the need for a C-section.
• Diagnosis of CPD is typically made during a trial of labor (vaginal delivery).

Types of Normal Pelvis (Caldwell-Moloy Classification)

• The Caldwell-Moloy classification categorizes pelvic types based on the shape of the pelvic inlet.
• Gynaecoid pelvis, the most common female pelvis, has a transverse oval inlet.
• Android pelvis, resembling a male pelvis, has a heart-shaped inlet.
• Anthropoid pelvis, resembling an ape pelvis, has a vertically oval inlet.
• Platypeloid pelvis, the least common, has a flat bowl-shaped inlet.

Note:

• There is no role for instrumental delivery in CPD.
• CPD is determined relative to the current fetus and should not be considered an indication for recurrent C-sections.
• CPD in the current pregnancy, regardless of severity, is a contraindication for vaginal birth after C-section (VBAC).

Lie, Presentation & Presenting Parts

• Lie describes the relationship between the fetal long axis and the maternal uterus' long axis.
• Longitudinal lie, the most common, aligns the fetal axis parallel to the maternal uterus.
• Oblique lie occurs when the fetal axis is at an angle to the maternal uterine axis.
• Transverse lie, when the fetal axis is perpendicular to the maternal uterine axis, increases the risk of hand and cord prolapse.
• Presentation defines the fetal part occupying the lower pole of the uterus.
• Cephalic presentation, the most common, describes the fetal head presenting, while breech presentation has the buttocks presenting.

Terminology of Labor

• The presenting part refers to the part of the presentation that lies over the internal os.
• Denominator is a reference point on the presenting part, like the occiput for vertex presentation.
• Attitude describes the relationship between fetal parts, with flexion being normal and extension leading to face or brow presentations.
• The image illustrates various fetal positions during labor.
• Hand prolapse, a common occurrence in transverse lie, necessitates a C-section, while head and hand prolapse during longitudinal lie can be managed vaginally