Female Pelvis

Female Pelvis & Fetal Skull

Pelvic Diameters

• Measured at 3 levels: Brim (Inlet), Cavity (Mid-pelvis), and Outlet.

Brim (Inlet)

• Anteroposterior (True conjugate): $11$ cm
• Transverse: $\sim13$ cm
• Oblique (R & L): $\sim12$ cm
• The brim is widest transversely.

Cavity (Mid-pelvis)

• $\sim12$ cm all around (roughly circular).
• Uniform shape is important for descent.

Outlet

• Anteroposterior: $\sim13$ cm
• Transverse (bituberous): $11$ cm
• Outlet is widest anteroposteriorly.

Pelvic Shapes (Caldwell-Moloy Classification)

1. Gynaecoid Pelvis (Ideal for vaginal birth)

• Shape: Rounded brim
• AP:Transverse Ratio: Balanced (AP $\sim11$ cm, Transverse $\sim13$ cm)
• Cavity: Roomy, rounded walls
• Outlet: Wide
• Fetal Positioning: Favors occipito-anterior position (OA)
• Prevalence: $\sim50\%$ of women

2. Android Pelvis (Male-type)

• Shape: Heart-shaped brim
• AP:Transverse Ratio: Narrow AP ($\sim10$ cm), narrow transverse
• Cavity: Funnel-shaped, narrow
• Outlet: Reduced risk of arrest in descent
• Fetal Positioning: OP common, deep transverse arrest
• Prevalence: $\sim20\%$

3. Anthropoid Pelvis

• Shape: Oval brim - long AP
• AP:Transverse Ratio: AP longer ($\sim13$ cm), transverse shorter ($\sim11$ cm)
• Cavity: Deep
• Outlet: Adequate
• Fetal Positioning: OP common but birth often possible
• Prevalence: $\sim25\%$

4. Platypelloid Pelvis

• Shape: Flat pelvis - wide transverse brim
• AP:Transverse Ratio: Short AP ($\sim10$ cm), very wide transverse ($\sim14$ cm)
• Cavity: Shallow
• Outlet: Narrow AP diameter
• Fetal Positioning: Head may not engage
• Prevalence: Rare (\sim5\%$)

Clinical Relevance for Labour

• Gynaecoid: Most favorable, spontaneous vaginal birth likely
• Android: May require operative delivery (forceps/CS)
• Anthropoid: OP more likely, but vaginal birth often successful
• Platypelloid: Difficult engagement – may need C-section

FETAL SKULL

• The fetal skull is adaptable for birth due to its flexible sutures and fontanelles, allowing moulding during labour.

1. Sutures – These are fibrous joints between skull bones:

• Frontal/Metopic suture: Between the two halves of the frontal bone (may fuse by 6-8 years)
• Sagittal suture: Between the two parietal bones
• Coronal suture: Between frontal and parietal bones
• Lambdoid suture: Between parietal and occipital bones
• Squamosal suture: Between parietal and temporal bones
• These sutures allow skull bones to overlap during birth (moulding) and facilitate brain growth.

2. Fontanelles - Soft spots where 2 or more sutures meet:

• Anterior fontanelle (Bregma): Junction of sagittal, coronal, and frontal sutures; Diamond-shaped, \sim2.5 cm; Closes by 12-18 months
• Posterior fontanelle (Lambda): Junction of sagittal and lambdoid sutures; Triangular, small; Closes by 6-8 weeks
• Anterolateral (Sphenoidal): At the side of skull - near temple (frontal + parietal + sphenoid + temporal bones); Irregular shape; \sim6 months
• Posterolateral (Mastoid): Behind ear-parietal + occipital + temporal bones; Irregular shape; 6-18 months
• Fontanelles help determine fetal head position in labour and allow flexibility during birth.

3. Important Regions and Landmarks of Fetal Skull

• Vertex: Area between anterior and posterior fontanelles - presenting part in normal birth
• Sinciput: Forehead region - above eyes
• Occiput: Back of the head - important in fetal position (e.g., occiput anterior)
• Parietal eminences: Bulging areas of parietal bones - widest transverse diameter
• Mentum: Chin
• Bregma: Clinical name for anterior fontanelle
• Lambda: Clinical name for posterior fontanelle

4. Fetal Skull Diameters (important in labour)

• Suboccipitobregmatic: 9.5 cm; Head flexed - ideal for vaginal birth
• Occipitofrontal: 11.5 cm; Partially flexed head
• Mentovertical: 13.5 cm; Brow presentation - large diameter
• Submentobregmatic: 9.5$$ cm; Face presentation (fully extended)