Maternal Pelvis Flashcards
What are the 3 main pelvic functions?
- Enable movement of the body
- Walking/running
- Sitting/kneeling
What is the pelvic girdle?
Basin-shaped cavity consisting of 4 bones:
- 2 innominate bones
- 1 sacrum
- 1 coccyx
What are the 2 innominate bones called?
Ilium and Ischium
How long are the walls of the cavity?
Anterior wall = 4cm
Posterior wall = 12cm
What is the true pelvis?
A bony canal that the foetus must pass through during birth, divided into 3 components:
- Pelvic brim
- Pelvic cavity
- Pelvic outlet
What is the false pelvis?
- Situated above the pelvic brim
- Formed by the upper flared-out portions of iliac bones
- Protects abdominal organs
What is the pelvic floor formed of?
Tissues which fill the outlet of the pelvis
Describe the ilium
- The upper border is the iliac crest
- The concave anterior surface is the iliac fossa
- It is the large flared-out part
Describe the ischium
- It is the thick lower part
- It has a large prominence known as the ischial tuberosity
- The pubis forms the anterior part
- The ischial spines are located here
Describe the sacrum
- Wedge-shaped bone
- Anterior surface is concave and referred to as the hollow
- Upper border of 1st vertebrae juts forward and is known as the promontory
- Consists of 5 fused vertebrae
- Posterior surface roughened to receive muscle attachments
- Laterally, bone extends into wing or ala
- Bone is pierced with 4 pairs of foramina through which nerves emerge to supply pelvic organs
Describe the coccyx
- Consists of 4 fused vertebrae
- It is a vestigial tail
What are the 4 pelvic joints?
- Symphysis pubis
- Sacroiliac joints x 2
- Sacrococcygeal joint
Describe the symphysis pubis
- Cartilage between 2 pubic bones
- Unites the rami (ramuses) on the left and right pubic bones
Describe the sacroiliac joints
- Strongest joints in the body
- Join sacrum to ilium and connect the spine to the pelvis
Describe the sacrococcygeal joint
- Formed where the base of the coccyx articulates with the tip of the sacrum
How does relaxin affect pelvic joints?
- Pelvic joints have little mobility in a non-pregnant state
- Relaxin softens joints to make room for foetal head
What is the sacrocotyloid dimension?
Passes from the sacral promontory to the iliopectineal eminence on each side and measures 9-9.5cm
What are the 3 diameters of the pelvis?
- Transverse - extends across greatest width of brim
- Oblique - extends from the iliopectineal eminence of 1 side to the sacroiliac articulation of the opposite side
- Anteroposterior (Conjugate) - extends from sacral promontory to symphysis pubis
What does the pelvic brim consist of?
- Sacral promontory
- Wings/ alae of the sacrum
- R+L sacroiliac joints
- R+L iliopectineal lines
- R+L iliopectineal eminences
- Upper inner border of superior pubic rami
- Upper inner border of body of pubis
- Upper inner border of symphysis pubis
How is the female pelvis adapted for child bearing?
- Increased width
- Rounded brim
- Concave sacrum
- Shallower than a male pelvis
What does a successful labour depend on?
The relationship between the size and shape of the maternal pelvis and foetal skull
How does the foetus negotiate the pelvis?
- Has to negotiate pelvic cavity by undergoing rotational manoeuvres
- Sling-like arrangement of pelvic floor muscles encourage rotation of presenting part
What are the 4 types of pelvis?
- Gynaecoid
- Android
- Anthropoid
- Platypelloid
What percentage of pelves are gynaecoid?
50%
What is the angle of the sub-pubic arch in a gynaecoid pelvis?
90 degrees
Describe the brim and cavity of a gynaecoid pelvis
- Brim is rounded with a generous fore pelvis; encourages round part of foetal head to present
- Cavity is round with parallel walls and a curved sacrum
What is the curve of carus?
An imaginary line made up of the axes of the pelvic canal that directs the foetal head upwards, causing lateral flexion during the 2nd stage of labour
Give some other physical features of a gynaecoid pelvis
- Shallow cavity and light in weight
- Ischial spines not prominent and blunt
Give the dimensions of the gynaecoid pelvis
Brim: AP 11cm, oblique 12cm, transverse 13cm
Cavity: AP 12cm, pblique 12cm, transverse 12cm
Outlet: AP 13cm, oblique 12cm, transverse 11cm
What percentage of pelves are android?
20%
Describe the android pelvis
- Triangular/ heart-shaped with narrow forepelvis
- Prominent ischial spines with convergent side walls
- Transverse diameter situated at bacl
- Sacrum is straight
- Deep cavity
- Sacro-sciatic notch is narrow
- Pubic arch = <90 degrees
Describe the anthropoid pelvis
- Long oval brim with narrow forepelvis
- AP diameters longer than transverse
- Side walls divergent
- Sacrum long and concave
- Ischial spines not prominent and blunt
- Sub-pubic angle = >90 degrees
- Common in women of African descent
What percentage of pelves are anthropoid?
25%
What percentage of pelves are platypelloid?
5%
Describe the platypelloid pelvis
- Flat, female-type pelvis
- Kidney-shaped brim with wide forepelvis
- Side walls divergent
- All AP diameters short
- Transverse diameters long
- Sacrosciatic notch narrow
- Sub-pubic angle = >90 degrees
- Sacrum flat and cavity shallow
- Ischial spines not prominent and blunt
What are the 3 conjugate diameters?
- True/ Anatomical Conjugate
- True/ Obstetric Conjugate
- Diagonal Conjugate
Describe the anatomical conjugate
- Sacral promontory to upper part of SP
- 12cm
Describe the obstetric conjugate
- Upper posterior border of SP to sacral promontory
- 11cm
- Represents available space for passage of foetal head through pelvis
Describe the diagonal conjugate
- Lower border of SP to sacral promontory
- 12-13cm
- Not measured in the UK
What is asynclitism?
- Lateral tilting of the head
- Allows biparietal diameter to pass narrowest AP diameter of brim
Describe anterior asynclitism
- Anterior parietal bones move down behind the SP until the parietal eminence enters the brim
- Movement is then reversed and head tilts in opposite direction until posterior parietal bone negotiates sacral promontory and head is engaged
Describe posterior asynclitism
- Anterior synclitism in reverse
- Posterior parietal bone negotiates sacral promontory prior to anterior parietal bone moving behind SP
- Once pelvic brim has been negotiated, descent progresses normally accompanied by flexion and internal rotation
Give a condition of the pelvis that arises from dietary deficiency
- Rachitic Pelvis
- Pelvis is deformed by rickets in childhood due to malnutrition; weight of upper body presses down on pelvis, causing bow legs and spinal deformity
- Sacral promontory pushed down and forward and ilium and ischium drawn outwards, resulting in flat pelvic brim
- Woman usually has CS due to asynclitism
Give a condition of the pelvis that arises from developmental anomaly
- Naegele’s Pelvis
- 1 sacral ala missing
- Sacrum is fused to the ilium, causing a grossly asymmetric brim
- Head cannot engage
Give a condition of the pelvis that arises from injury and disease
- Trauma
- Fractured pelvis will develop a callus/ fail to unite
- Reduce measurement and some degree of contraction
What happens to the coccyx during labour?
Pushed back
