LG 4

Clinical Anatomy of the Bony Pelvis in Relation to Labor

Objectives

• To remind the anatomy of the female pelvis and its boundaries.
• To be able in diagnosing abnormal pelvis.
• To understand the anatomy of fetal skull and its diameters.

What is Labor?

• Labor: A series of continuous, progressive contractions of the uterus that helps the cervix to open (dilate) and shorten (effacement), allowing the fetus to move through the birth canal and expel from the women’s body.
• Childbirth: Includes both labor (the process of birth) and delivery (birth itself).

Components of Labor

1. Passenger (Fetus)
2. Powers (Uterine contractions)
3. Passage (The pelvis and maternal soft parts)

The Bony Pelvis

• Which bones compose the bony pelvis?

  1. Innominate bones:
  • Ilium
  • Ischium
  • Pubis
  1. Sacrum
  2. Coccyx

• Hip bone: Also known as the pelvic bone, it is irregular in shape and forms part of the pelvic girdle.

Key Features of the Female Pelvis

• Divided into two parts:
  1. False pelvis
  2. True pelvis

Pelvic Inlet (Brim)

• Shape: Oval-shaped

• Boundaries:

  • Posteriorly: Sacral promontory
  • Laterally: Iliopectineal lines
  • Anteriorly: Upper border of the symphysis pubis.

• Formed by: Arcuate line of ilium and pectineal line of pubis, which marks the border between the false and true pelvis forming the pelvic brim or linea terminalis.

Measurements of the Pelvic Inlet

• Anteroposterior diameter (true conjugate): $11.5 ext{ cm}$
• Transverse diameter: $13.5 ext{ cm}$

Diameters of the Pelvic Inlet

• Anteroposterior diameters:
  1. Anatomical (true conjugate): $11.5 ext{ cm}$
  2. Obstetric conjugate: $10.5 ext{ cm}$
  3. Diagonal conjugate: $12.5 ext{ cm}$
• The diagonal conjugate is $1 ext{ cm}$ longer than the true conjugate.

The Pelvic Mid-Cavity

• Shape: Round.
• Transverse and anteroposterior diameters: Both are $12 ext{ cm}$.
• Boundaries:
  • Anteriorly: The middle of the symphysis pubis
  • Posteriorly: Junction of the 2nd and 3rd pieces of sacrum
  • Laterally: Pubic bone, obturator fascia, inner aspect of ischial bone and spines.

Importance of Ischial Spines

• Palpable vaginally and are used to determine:
  • Station of fetal head
  • Anaesthetic block to the pudendal nerve
  • Adequacy of mid cavity

The Pelvic Outlet

• Shape: Diamond-shaped
• Boundaries:
  • Lower border of the symphysis pubis
  • Ischial tuberosities
  • Tip of the coccyx
• Anteroposterior diameter: $13.5 ext{ cm}$
• Transverse diameter: $11.5 ext{ cm}$
• Angle of the subpubic arch: $ext{≥ 90°}$

Diameters of the Pelvic Outlet

• Anteroposterior: $13 ext{ cm}$
• Transverse: $11 ext{ cm}$
• Oblique: $12 ext{ cm}$

The Pelvic Floor

• Formed by levator ani muscles with their fascia; they create a musculofascial gutter during the 2nd stage of labor.
• Creates a groove or channel stretched or displaced during labor.

Classification of Female Pelvic Types

1. Gynaecoid pelvis (50%)
2. Anthropoid pelvis (25%)
3. Android pelvis (20%)
4. Platypelloid pelvis (5%)

Gynaecoid Pelvis

• Typical female pelvis found in 50 ext{%} of women.
• Features: Rounded, slightly oval inlet, straight pelvic sidewalls with roomy pelvic cavity, good sacral curve, ischial spines are not prominent, and pubic arch is wide.

Android Pelvis

• Shape: Heart-shaped pelvic brim
• Pelvis funnels from above downwards, narrow pubic arch, prominent ischial spines.
• Delivery: Frequently requires cesarean section.

Anthropoid Pelvis

• Prevalence: 25% in white women; 50% in non-white.
• Features: Anteroposterior diameter greater than transverse diameter, long and narrow pelvic canal with long sacrum, and straight pelvic sidewalls.
• Presentations: Generally requires assisted vaginal delivery.

Platypelloid Pelvis

• Present in 5% of women.
• Features: Pelvic brim has transverse diameter greater than anteroposterior diameter, kidney-shaped and sacral promontory pushed forward.

Adequacy of the Pelvis to Achieve Vaginal Delivery

• Clinically favorable pelvis features:
  • Sacral promontory cannot be felt
  • Ischial spines not prominent
  • Subpubic arch accepts two fingers
  • Intertuberous diameter accepts four knuckles during pelvic exam

Measurement Parameters

• Diagonal conjugate: Measures the distance from sacral promontory to the inferior margin of symphysis pubis.
• Obstetric conjugate: The shortest anteroposterior diameter measured between sacral promontory and symphysis pubis, measured radiologically.
• Clinical assessment: Pelvic inlet examination via PV (per vaginal) exam and measuring the diagonal conjugate which is typically 2 cm longer than the obstetric conjugate.

Indications for Investigation

1. Previous history of pelvic fractures
2. Metabolic bone disease like rickets

• Tools for diagnosis: X-ray, CT-scan, MRI.

Anatomy of the Fetal Skull

• Composition: Consists of vault, face, and base.
• Sutures: Junction lines of two bones, soft and unossified in the vault for labor facilitation, while the sutures in the face and base are firmly united.

Types of Sutures in the Vault

• Frontal suture
• Sagittal suture
• Lambdoidal suture
• Coronal suture

Important Measurements of the Fetal Skull

• Occipitofrontal: $11.5 ext{ cm}$
• Occipitomental: $9.5 ext{ cm}$
• Suboccipitobregmatic: $9.5 ext{ cm}$
• Biparietal: $9.5 ext{ cm}$

Moulding of the Fetal Skull

• Defined as the moving together or overlapping of fetal vault bones, which reduces the skull diameter and aids labor.
• Severe moulding with prolonged labor is a sign of cephalopelvic disproportion.

Fontanelles

• Anterior fontanelle (sinciput): Diamond-shaped space between coronal & sagittal suture, measuring $3 ext{ cm} imes 3 ext{ cm}$, ossifies at 18 months.
• Posterior fontanelle (lambda): Triangle-shaped space between sagittal & lambdoid suture.

Clinical Importance of Fontanelles

• Used to diagnose:
  • Vertex presentation
  • Position of the occiput
  • Degree of flexion of the head

Fetal Skull Attitude

• Refers to the degree of flexion and extension at the upper cervical spine; critical for understanding labor dynamics.

Causes of Labor Complications

• Conditions like lax uterus, multiple pregnancy, polyhydramnios, contracted pelvis, deflexed head (as seen in conditions like thyroid tumor), and shapes of the head such as anencephaly can affect labor outcomes.

Conclusion

1. The anatomy of the maternal pelvis and fetal skull is essential for understanding normal labor and identifying abnormal labor.
2. For women with prior vaginal deliveries, clinical examination of the pelvis is the most important factor to determine the adequacy for a vaginal delivery.