Fetal Head & Maternal Pelvis Diameter

Subocciptobregmatic (smallest)

9.5 cm

Mentovertical/verticomental (largest)

13.5 cm

Suboccipitofrontal

10.5 cm

submentobregmatic

9.5 cm

Occipitofrontal

11.5 cm

Occipitomental

12.5 cm

brim/inlet

A/P - 11 cm

Transverse - 13 cm

Oblique - 12 cm

Cavity

All diameters are 12 cm

Outlet

A/P - 13 cm

Transverse - 11 cm

suboccipitobregmatic (for birth)

smallest diameter when head is well-flexed, optimal for birth

verticomental/mentovertical (for birth)

largest diameter, head is not flexed and is in brow presentation, making head too large for vaginal birth

upright position in labour/birth

helps gravity assist descent, more effective uterine contractions, baby aligns optimally in uterus, pelvis flexibility

supine position in labour

reduced blood flow, less effective contractions, maternal discomfort

order of fetal skull diameters (smallest to largest)

suboccipitobregmatic (9.5 cm)

submentobregmatic (9.5 cm)

suboccipitofrontal (10 cm)

occipitofrontal (11.5 cm)

mentovertical (13.5 cm)