Pelvis & Perineum

The Passages (Maternal Pelvis & Perineum)

Maternal Pelvis

The pelvic can be divided into inlet, mid-cavity, and outlet.

The Pelvic Inlet (or Brim)

Boundaries:

• Anteriorly: Upper border of the symphysis pubis.

• Laterally: Upper margin of the pubic bone, Iliopectineal line, Ala of the sacrum.

• Posteriorly: Promontory of the sacrum.

• Transverse diameter is ➡️ 13.5 cm

• Anterior–posterior (A–P) diameter is ➡️ 11.0 cm

• The Fetal head typically enters the pelvis in a transverse position, in keeping with the wider transverse diameter.

The Pelvic Midcavity

The midpelvis (mid-cavity) Boundaries:

• Anteriorly: Middle of the symphysis pubis.
• Laterally: Pubic bones, Obturator fascia, Inner aspect of the ischial bone and spines.
• Posteriorly: Junction of the 2nd and 3rd sections of the sacrum.
• The midpelvis is rounded (Transverse and anterior diameters are 12 cm).

Importance of Midcavity Landmarks

The ischial spines are palpable vaginally and are used as important landmarks for:

1. To assess the descent of the presenting part on vaginal examination:
   • E.g., station zero is at the level of the ischial spines.
   • Station zero is an important landmark clinically, because instrumental delivery can only be performed if the fetal head has reached the level of the ischial spines or below.
2. To provide a local pudendal nerve block:
   • The pudendal nerve passes behind and below the ischial spine on each side.

The Pelvic Outlet

Boundaries:

• Anteriorly: The lower margin of the symphysis pubis.
• Laterally: The descending ramus of the pubic bone, ischial tuberosity, Sacrotuberous ligament.
• Posteriorly: By the last piece of the sacrum.
• The AP diameter → 13.5 cm.
• The Transverse diameter → 11 cm.
• Therefore, the AP is the wider diameter.

Summary of Pelvic Diameters

	Transverse diameter	Antero-posterior diameter
Pelvic inlet	13cm	11cm
Mid-pelvis	12cm	12cm
Pelvic outlet	11cm	13cm

Pelvic Shape

Affected by:

• Maternal stature.
• Ethnicity.
• Previous pelvic fractures.
• Metabolic bone disease, such as rickets.

It is now uncommon to perform X-rays or (CT) or (MRI) of the pelvis to measure the pelvic dimensions. Why? Because they have little clinical use in predicting the outcome of labour.

Clinical Significance of Pelvic Shapes

Pelvic shapes may contribute to difficulties in labour:

• The gynaecoid pelvis is the most favourable for labour & also the most common.
• An Android-type pelvis is associated with failure of rotation and deep transverse arrest (DTA).
• An Anthropoid shape is associated with an occipito-posterior (OP) position.
• A platypelloid pelvis is associated with an increased risk of obstructed labour due to failure of the head to engage, rotate or descend.

The Perineum

• The final obstacle to be overcome by the fetus during labour is the perineum.
• The perineum is taut & resistant in the nulliparous woman, leading to prolonged pushing. Vaginal birth may result in tearing of the perineum and pelvic floor muscles; an episiotomy (surgical cut) may be required.
• The perineum is stretchy and less resistant in multiparous women, often leading to faster labour and an intact perineum (most likely).

Episiotomy

• Midline incision
• Mediolateral incision

Perineal Tears

• First-degree tear: perineal skin and/or vaginal mucosa.
• Second-degree tear: perineal muscles but not involving the anal sphincter.
• Third-degree tear: perineum involving the anal sphincter complex.
• Fourth-degree tear: Involving the anal sphincter complex (EAS & IAS) and anorectal mucosa.