What are the dimensions, types, and significance of the female pelvis, including the pelvic inlet, mid pelvis, and pelvic outlet, during labor?

Female Pelvis Anatomy and Labor Significance

Pelvic Types Classification

The female pelvis is classified into four main types based on shape: gynecoid (52%), platypelloid (20%), anthropoid (15%), and android (13%), with the gynecoid type being most common but not necessarily predictive of labor outcome. 1

Four Classical Pelvic Types:

• Gynecoid pelvis: Round or oval inlet, wide transverse diameter, optimal for childbirth but does not guarantee easier labor or prevent cesarean section 1
• Android pelvis: Heart-shaped inlet with narrow anterior segment, associated with male-type characteristics, present in approximately 13% of women 1
• Anthropoid pelvis: Oval inlet with larger anteroposterior (AP) diameter than transverse diameter, found in 15% of women 1
• Platypelloid pelvis: Flat inlet with wide transverse diameter but shortened AP diameter, occurs in 20% of women with statistically significant differences in bispinous diameter compared to anthropoid type 1

Pelvic Inlet Anatomy and Measurements

Boundaries of Pelvic Inlet:

The pelvic inlet is bounded by the sacral promontory posteriorly, the iliopectineal lines laterally, and the superior margin of the pubic symphysis anteriorly 2, 3

Key Diameters and Significance:

Pelvic inlet area is the most clinically significant measurement, with women requiring cesarean section having significantly smaller inlet areas (mean 126.3 cm²) compared to those with vaginal delivery (mean 134.9 cm²). 4

Anteroposterior Diameters:

• Obstetric conjugate: Most important AP diameter, measured from sacral promontory to closest point on pubic symphysis 5
• True conjugate: From sacral promontory to superior border of pubic symphysis 5
• Clinical significance: Determines whether fetal head can enter pelvis; inadequate AP diameter leads to cephalopelvic disproportion 3

Transverse Diameter:

• Widest transverse diameter: Measured at the widest point of the pelvic inlet 1, 6
• The brim index (ratio of transverse to AP diameter) is used to classify pelvic types 1

Dynamic Changes During Pregnancy:

• Pelvic inlet capacity increases from gestational week 20 to 32 in all birthing positions 6
• The supine position optimizes pelvic inlet dimensions, while semi-lithotomy and kneeling squat positions are less favorable for inlet capacity 6

Mid-Pelvis (Midplane) Anatomy

Boundaries of Mid-Pelvis:

The midplane extends from the lower border of the pubic symphysis anteriorly, through the ischial spines laterally, to the junction of S4-S5 vertebrae posteriorly 2

Key Diameters:

Transverse Diameter (Interspinous):

• Bispinous diameter: Distance between ischial spines, the narrowest transverse diameter of the pelvis 1
• Platypelloid pelvis shows statistically significant differences in bispinous length compared to anthropoid type (p < 0.05) 1

Anteroposterior Diameter:

• Measured from lower border of pubic symphysis to sacrum at level of ischial spines 2

Clinical Significance During Labor:

• The midplane is the most critical level for assessing cephalopelvic disproportion, as arrest at this level strongly predicts difficult delivery 2
• Kneeling squat position increases midplane dimensions by up to 1 cm compared to supine position (p < 0.001) 6
• Prolonged deceleration phase (8-10 cm dilation) indicates potential midplane obstruction and predicts second stage abnormalities and shoulder dystocia 2

Pelvic Outlet Anatomy

Boundaries of Pelvic Outlet:

The pelvic outlet is bounded by the lower border of the pubic symphysis anteriorly, the ischial tuberosities laterally, the sacrotuberous ligaments, and the coccyx posteriorly 2

Key Diameters:

Anteroposterior Diameter:

• Measured from lower border of pubic symphysis to tip of coccyx (or sacrococcygeal joint) 6
• Increases during pregnancy from week 20 to 32 6

Transverse Diameter (Intertuberous):

• Distance between ischial tuberosities: The narrowest transverse diameter of the entire pelvis 1, 6
• Mean pelvic outlet is 0.2 cm larger in semi-lithotomy position compared to supine position at 32 weeks gestation (p < 0.001) 6

Clinical Significance:

• Shifting from supine to kneeling squat position increases pelvic outlet dimensions by up to 1 cm (p < 0.001), making upright positions optimal for second stage labor 6
• Semi-lithotomy position also optimizes outlet capacity compared to supine 6
• Outlet adequacy is critical for preventing perineal trauma, which occurs in up to 90% of nulliparous and 70% of multiparous women 2

Clinical Application During Labor

Assessment for Cephalopelvic Disproportion:

Thorough cephalopelvimetric assessment is mandatory before oxytocin augmentation, as 40-50% of active phase arrest cases have underlying CPD. 2, 3

Warning Signs of CPD:

• Active phase arrest (no cervical change for 4+ hours with adequate contractions) 2
• Prolonged deceleration phase (>3 hours from 8 cm to complete dilation) 2
• Excessive molding of fetal skull 2
• Malposition (occiput posterior/transverse) 2
• Maternal diabetes, obesity, or fetal macrosomia 2

Labor Management Algorithm:

1. If CPD suspected or cannot be ruled out: Cesarean delivery is safer than attempting vaginal delivery 2, 3

2. For skeletal dysplasia patients: Cesarean delivery is recommended due to proportionally small pelvic dimensions and high CPD risk 2, 3

3. For active phase protraction/arrest without CPD: Oxytocin augmentation is first-line treatment per ACOG guidelines 3

4. Position optimization: Use semi-lithotomy or kneeling squat positions during second stage to maximize outlet dimensions 6

Common Pitfalls:

• Cervical dilation alone is insufficient: Must assess fetal descent via suprapubic palpation to differentiate molding from true descent 2
• Deceleration phase exists and is clinically important: Contrary to some statistical models, the deceleration phase (8-10 cm) is a real phenomenon requiring assessment for CPD 2
• External pelvimetry is unreliable: Historical reliance on external measurements (Baudelocque's method) has been discredited; clinical assessment or imaging is required 5
• Radiopelvimetry is obsolete: Due to radiation exposure concerns, dynamic assessment during labor has replaced routine pelvimetry 5

Imaging Assessment:

• Ultrasound: Combined transabdominal and transvaginal approach provides comprehensive pelvic evaluation 2, 3
• MRI pelvimetry: Can measure pelvic dimensions at weeks 20 and 32 to assess capacity changes, though not routinely indicated 6
• CT-based measurements: Accurate for research but radiation exposure limits clinical use 1, 4

Key Measurement Thresholds:

• Pelvic inlet area <126 cm²: Associated with increased cesarean section risk 4
• First/second degree stenosis: Present in approximately 13% of women (6 cases first degree, 1 case second degree in study of 54 women) 1
• Cesarean section rate: Approximately 31.82% overall, with gynecoid pelvis not protective against operative delivery 1