Fetal Skull Anatomy, Female Pelvis, and Mechanism of Labor

Overview of the Fetal Skull

• Definition: The fetal skull is a structure described as being to some extent compressible. It is primarily composed of thin, pliable, tabular (flat) bones that form the vault. This vault is anchored to rigid, incompressible bones located at the base of the skull.

• Anatomic Bones and Features:

  • Frontal bone

  • Parietal bone

  • Occipital bone

  • Temporal bone (squamous portion)

  • Ossification centers

  • Fontanels: Sphenoidal fontanel and Mastoid fontanel.

Areas of the Fetal Skull

• Vertex: A quadrangular area with the following boundaries:

  • Anterior: Bregma and coronal sutures.

  • Posterior: Lambda and lambdoid sutures.

  • Lateral: Lines passing through the parietal eminences.

• Brow: An area bounded by:

  • One side: Anterior fontanel and coronal sutures.

  • Other side: Root of the nose and supraorbital ridges on either side.

• Face: Bounded by:

  • One side: Root of the nose and supraorbital ridges.

  • Other side: Junction of the floor of the mouth with the neck.

• Sinciput: The area lying in front of the anterior fontanel, corresponding to the area of the brow.

• Occiput: An area limited strictly to the occipital bone.

Sutures of the Fetal Skull

• Types of Sutures:

  • Sagittal (Longitudinal) Suture: Located between the two parietal bones.

  • Coronal Sutures: Run between the parietal and frontal bones on either side.

  • Frontal Suture: Located between the two frontal bones.

  • Lambdoid Sutures: Separate the occipital bone from the two parietal bones.

• Obstetrical Importance of Sutures:

  • Molding: They permit gliding movements of one bone over another (molding) as the head passes through the pelvis during labor.

  • Engagement: Digital palpation of the sagittal suture during internal examination in labor helps determine the manner of engagement of the head.

Fontanels

• Anterior Fontanel (Bregma):

  • Formation: Joining of four sutures in the midplane.

  • Shape: Diamond-shaped.

  • Dimensions: Anteroposterior and transverse diameters measure approximately $3\,text{cm}$ each.

  • Closure: Closes at $18\,\text{months}$ after birth.

• Posterior Fontanel (Lambda):

  • Shape: Triangular.

  • Dimensions: Measures about $1.2\,\text{cm} \times 1.2\,\text{cm}$ ($1/2" \times 1/2"$).

  • Structure: The floor is membranous but becomes bony at term.

• Sagittal Fontanel: Inconsistent in presence. If present, it is located on the sagittal suture at the junction of the anterior two-thirds and posterior one-third. It has no clinical importance.

• Clinical Importance of Fontanels:

  • Denotes the degree of flexion of the head via internal examination.

  • Facilitates molding of the head.

  • Indicates intracranial status: Depressed in dehydration; elevated in raised intracranial tension.

Diameters of the Fetal Skull

• Longitudinal Diameters:

  • Suboccipitobregmatic: From the nape of the neck to the center of the bregma ($9.5\,\text{cm}$).

  • Suboccipitofrontal: From the nape of the neck to the center of the sinciput ($10\,\text{cm}$).

  • Occipitofrontal: From the occipital protuberance to the root of the nose ($11.5\,\text{cm}$).

  • Mentovertical: From the midpoint of the chin to the highest point on the sagittal suture ($14\,\text{cm}$).

  • Submentovertical: From the junction of the mouth and neck to the highest point on the sagittal suture ($11.5\,\text{cm}$).

  • Submentobregmatic: From the junction of the floor of the mouth and neck to the center of the bregma ($9.5\,\text{cm}$).

• Transverse Diameters:

  • Biparietal Diameter: Extends between the two parietal eminences ($9.5\,\text{cm}$ or $3\,3/4"$).

  • Super-subparietal: From a point below one parietal eminence to a point above the opposite eminence ($8.5\,\text{cm}$ or $3\,1/2"$).

  • Bitemporal Diameter: Distance between the antero-inferior ends of the coronal suture ($8\,\text{cm}$ or $3\,1/4"$).

  • Bimastoid Diameter: Distance between the tips of the mastoid processes ($7.5\,\text{cm}$ or $3"$).

Molding and Birth Swellings

• Molding: The alteration of the shape of the fore-coming head while passing through the birth passage. A normal delivery commonly results in an alteration of $4\,\text{mm}$ in skull diameter.

• Degrees of Molding:

  • Grade I: Bones are touching but not overlapping.

  • Grade II: Bones are overlapped but easily separable.

  • Grade III: Bones are fixed and cannot be separated.

• Caput Succedaneum: A swelling due to stagnation of fluid in the scalp layers beneath the girdle of contact (bony pelvis, cervix, or vulval ring). Characteristics: Diffuse, boggy, and not limited by suture lines.

• Cephalhematoma: A swelling due to bleeding between the skull bone and periosteum. It is caused by friction between overriding bones and the periosteum during molding.

Anatomy and Function of the Female Pelvis

• Overview: The pelvis is a bony canal through which the fetus passes. It comprises four bones: two innominate bones (hipbones), one sacrum, and one coccyx.

• Functions:

  1. Allows body movement.

  2. Permits sitting and kneeling.

  3. Transmits trunk weight to the legs.

  4. Acts as a bridge between the two femurs.

  5. Protects pelvic and abdominal organs.

  6. The sacrum transmits the cauda equina and distributes nerves to pelvic organs.

• Innominate Bone Parts:

  • Ilium: Large flat part. Features include the iliac fossa (concave surface), iliac crest (upper border), and the anterior superior iliac spine (ASIS). It also contains the anterior inferior, posterior superior, and posterior inferior iliac spines.

  • Ischium: Thick lower part. Features include the ischial tuberosity (weight-bearing surface while sitting) and the ischial spine.

  • Pubic Bone: Consists of a body and two projections (superior and inferior rami). The two bones meet at the symphysis pubis. The two inferior rami form the pubic arch.

  • Obturator Foramen: Space enclosed by the pubic bone body, ischium, and rami.

• Acetabulum: Deep cup receiving the femur head. Composed of $\frac{2}{5}$ ilium, $\frac{2}{5}$ ischium, and $\frac{1}{5}$ pubic bone.

• Sciatic Notches:

  • Greater Sciatic Notch: From posterior inferior iliac spine to ischial spine.

  • Lesser Sciatic Notch: From ischial spine to ischial tuberosity.

• Sacrum and Coccyx:

  • Sacrum: Wedge-shaped; 5 fused vertebrae. The first vertebra projects forward as the sacral promontory. Features sacral wings (ala) and 4 pairs of holes for cauda equina nerves.

  • Coccyx: "Vestigial tail"; 4 fused vertebrae forming a triangular shape.

Pelvic Joints, Ligaments, and Divisions

• Pelvic Joints:

  • Symphysis pubis ($1$)

  • Sacro-iliac joints ($2$)

  • Sacrococcygeal joint ($1$)

• Ligaments:

  • Interpubic ligament (between pubic bones).

  • Sacro-iliac ligament (between sacrum and ilium).

  • Sacropubic ligament (between sacrum and pubic bone).

  • Sacrospinus ligament (between sacrum and ischial spine).

  • Sacrotuberous ligament (between sacrum and ischial tuberosity).

• Types of Pelvis:

  • False Pelvis: Above the pelvic brim; protects abdominal organs; little obstetrical importance.

  • True Pelvis: The bony birth canal. It is shallow in front (symphysis pubis, $4\,\text{cm}$) and deep posteriorly (sacrum/coccyx, $11.5\,\text{cm}$). It contains the brim, cavity, and outlet.

Pelvic Brim, Cavity, and Planes

• Pelvic Brim (Inlet): Round except for the sacral promontory projection. Boundaries: Anterior (pubic bone), Posterior (sacral promontory and wings), Lateral (ilium).

• Pelvic Cavity: Curved canal between inlet and outlet. Circular in shape and curved forward. Anterior wall measures $4\,\text{cm}$; posterior curve of the sacrum measures $12\,\text{cm}$.

• Pelvic Plane: Imaginary flat surface bounded by the brim points; also called the "superior strait."

• Pelvic Outlet:

  • Anatomical Outlet: Diamond-shaped (bony outlet). Bounded by lower symphysis pubis, ischiopubic rami, ischial tuberosity, sacrospinous ligament, and coccyx tip.

  • Obstetrical Outlet: Segment of pelvis bounded by the plane of least pelvic dimensions and the anatomical outlet.

Pelvic Diameters

• Brim Diameters:

  • Anteroposterior: Midpoint of sacral promontory to inner margin of upper symphysis pubis ($11\,\text{cm}$).

  • Obstetric Conjugate: Midpoint of sacral promontory to bony projection on inner symphysis pubis ($10\,\text{cm}$).

  • Diagonal Conjugate: Lower symphysis pubis to midpoint of sacral promontory ($12\,\text{cm}$).

  • Transverse Diameter: Distance between farthest points on the pelvic brim over iliopectineal lines ($13\,\text{cm}$).

  • Oblique Diameters ($2$): From one sacroiliac joint to opposite iliopubic eminence ($12\,\text{cm}$).

• Cavity Diameters:

  • Anteroposterior: Midpoint of posterior symphysis pubis to junction of $2^{nd}$ or $3^{rd}$ sacral vertebrae ($12\,\text{cm}$).

• Obstetrical Outlet Diameters:

  • Transverse (Bispinous): Between tips of ischial spines ($10.5\,\text{cm}$).

  • Anteroposterior: Inferior border of symphysis pubis to tip of sacrum ($11\,\text{cm}$).

• Anatomical Outlet Diameters:

  • Anteroposterior: Lower border of symphysis pubis to tip of coccyx ($13\,\text{cm}$).

  • Oblique: Between obturator foramen and sacrospinous ligament ($12\,\text{cm}$).

Mechanism of Labor (Left Occiput Anterior - LOA)

• Definition: The series of movements the head makes for adaptation during its journey through the pelvis. Also known as "cardinal movements."

• General Principles:

  • Descent occurs throughout labor.

  • The leading part meeting pelvic floor resistance rotates forward under the symphysis pubis.

  • The part emerging from the pelvis pivots around the pubic bone.

• Cardinal Movements in LOA:

  1. Engagement: Anteroposterior diameter is suboccipitobregmatic ($9.5\,\text{cm}$); transverse is biparietal ($9.5\,\text{cm}$). Occurs 2 weeks prior in primigravida, and at onset in multigravida.

  2. Descent: Facilitated by uterine contractions/retractions, rupture of forewaters, and maternal bearing down.

  3. Flexion: Increases throughout labor. Axis pressure is transmitted more to the occiput than sinciput because the spine is attached posteriorly. Diameter changes from suboccipitofrontal ($10\,\text{cm}$) to suboccipitobregmatic ($9.5\,\text{cm}$).

  4. Internal Rotation of Head: Occiput rotates anteriorly by $\frac{1}{8}$ of a circle to lie behind the symphysis pubis. Creates a slight twist in the neck.

  5. Crowning: Occiput slips under the sub-pubic arch; the head no longer recedes between contractions.

  6. Extension: Sinciput, face, and chin sweep the perineum and are born.

  7. Restitution: Neck twist corrects. Occiput moves $\frac{1}{8}$ circle back toward the original side.

  8. Internal Rotation of Shoulders / External Rotation of Head: Anterior shoulder rotates $\frac{1}{8}$ circle forward; head follows. Occiput points toward maternal thigh.

  9. Lateral Flexion (Expulsion): Anterior shoulder escapes under symphysis pubis; posterior shoulder sweeps the perineum.

Right Occipito Posterior (ROP) Mechanism

• Specific Criteria for ROP (Long Rotation):

  • Lie: Longitudinal.

  • Attitude: Head is deflexed.

  • Presentation: Vertex/Cephalic.

  • Position: Right occipito-posterior.

  • Denominator: Occiput.

  • Presenting Part: Middle or anterior area of the right parietal bone.

• Mechanism of ROP:

  • Engagement: Delayed due to deflexion. Occurs through right oblique diameter. Anteroposterior diameter is suboccipitofrontal ($10\,\text{cm}$) or occipitofrontal ($11.5\,\text{cm}$). Transverse is biparietal ($9.5\,\text{cm}$).

  • Rotation: The occiput reaches pelvic floor and rotates forward $\frac{3}{8}$ of a circle along the right side to lie under the symphysis pubis. Shoulders follow by turning $\frac{2}{8}$ of a circle.

  • Restitution: Occiput turns $\frac{1}{8}$ of a circle to the right to align with shoulders.

  • Lateral Flexion: Same process as LOA, utilizing lateral flexion for the expulsion of the body after shoulder rotation.