The Female Pelvis

Functions of the female pelvis

•  Supports upper body 
•  Transmits weight to lower limbs 
• Enables movement
• takes weight of body when sitting on ischial tuberosities 
•  protects reproductive organs, bladder & rectum 
• In pregnancy - supports the gravid uterus 
• Essential in mechanism of birth 
• Sacrum allows for distribution of nerves to the pelvis

The True Pelvis

Separated from the false pelvis by the brim. 

• Consists of 3 regions – brim, cavity & outlet 
•  Fetus negotiates each simultaneously 
• Fetus follows plane of brim & cavity to ischial spines 

The False Pelvis 

• The pelvic brim divides the two parts true and false 
• The false pelvis being located above the pelvic brim 
• It has no obstetric significance

Pelvic cavity 

• Extends downwards from the brim 
• Wedge shaped –shallow at front & deep at back 
• Viewed from above it is circular shaped 
• Designed to facilitate the descent & rotation of the presenting part of fetus
• Boundaries include – curve of sacrum, sacroiliac joints, ischia, superior & inferior pubic ramus & pubis 

Pelvic outlet

• Diamond shaped
• Can be described as an anatomical structure & by its obstetric dimension ie it space value
• Boundaries include- tip of coccyx, ischial tuberosities & pubic arch
• Obstetric boundaries – inner aspect of sacral base,ischial spines, lower inner border of symphysis pubis & sacrospinous ligaments 

                Anterior/posterior                              Oblique                           Transverse

Brim              11CM                                          12CM                               13CM

Cavity            12CM                                          12CM                               12CM

Outlet            13CM                                          12CM                               11CM

Consists of 4 pelvic bones

X 2 innominate bones each side consisting of 3 parts:

• ilium
• ischium
• pubic bone

+ X1 sacrum

+ X1 coccyx 

Gynaecoid Pelvis

• Round brim
• Curved sacrum 
• Blunt spines
• Pubic angle 90 degrees small to normal sized women

Android pelvis

• Heart shaped brim
• Straight sacrum
• Prominent spines
• Pubic arch less than 90 degrees
• Favours OP position (Occiput posterior)
• And women of short stature & from african descent

Anthropoid pelvis

• Oval shaped
• Spines not prominent
• Pubic angle wide
• Favours tall women (engaging position usually continues through pelvis)

Platypelloid pelvis 

• Kidney shaped
• Sacrum flat
• Blunt spines
• Wide pubic arch
• Fetus can enter but can’t go through outlet

Pelvic joints

• Sacroiliac joint is where ilium meets with 1st 2 sacral vertebrae 
• Symphysis pubis between 2 pubic bones
• Sacrococcygeal joint between sacrum & coccyx
• Lumbar sacral joint which articulates with the lumbar vertebrae 

All are supported by Sacroiliac ligaments

Pelvic planes

There are 3 common planes of the pelvis:

• The pelvic brim
• Mid cavity
• Outlet

A pelvis with an adequate brim and cavity rarely if ever has reduced diameters of the outlet

The coccyx has mobility which also aids available space

Increasing the diameter

• Russell’s research studied pelvic X rays of 69 women in 1969 in 3rd trimester
• The women were in dorsal (lying down) and sitting positions
• As much as 28% increase in the pelvic outlet is possible (i.e 1cm in transverse & 2cm in AP diameter , if alternative positions are adopted.

Upright or squatting is ideal

Points to note

• Pelvic measurements may widen during labour due to pelvic ligament laxity

• The position of the mother in labour also affects the pelvic outlet diameter

• Moulding may reduce the absolute measurements of the fetal skull during labour

• The degree of flexion of the fetal skull at the neck (attitude) determines the diameter of the fetal skull presenting to the pelvis