Type of pelvis and their effects on labour
The shape and structure of pelvis varies from one woman to another, ut ideal normal pelvis favours vaginal delivery. There are four main types of pelvis are gynaecoid, android, anthropoid and platypelloid.
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GYNAECOID PELVIS
This is another name for the female pelvis designed for child bearing. It is found in about 50% of women whose hips are broader than their shoulders with a height over 1.5m and shoe size of about 4 and above.
Characteristics
- The brim is rounded except when it is encroached upon by the circular promontory.
- It is generous and has straight side walls.
- The cavity is circular and shallow with a broad well curved sacrum. The greater sciatic notch is wide.
- The outlet is diamond shaped and the public arch forms an angle of 90 degree, the ischial spine are well rounded they are neither sharp nor prominent but brunt.
- The bones of gynaecoid pelvis are lighter than the bones of the other types
Effects of gnaecoid pelvis on labour
The fetal head is always engaged in transverse diameter of the brim in an anterior position, this causes the mechanism of labour normal, therefore the gynaecoid pelvis is the best for child bearing.
ANDROID PELVIS
This is the male type of pelvis, possessing about 20% of women. The bones are heavier than the gynaecoid pelvis.
Characteristics
- The brim is heart shaped making the front pelvis narrow, the transverse diameter does not cross the center of the pelvis but is near to the sacrum.
- This provides more pain at the back of the pelvis than in the front, the sacral promontory is flattened.
- The hollow of the sacrum is straight, the greater sciatic notch is narrowed reducing the available space within the cavity.
- The outlet sub public angle is less than 90 degrees.
- The ischial spines are prominent and the sacral spinous ligament is sharp.
- The measurement of the outlet is reduced due to convergence of the sidewalls which made it funnel shaped. This type of pelvis is found in short and heavy built women who have tender and behair suit
Effects of Android pelvis on labour
The available space in this type of pelvis is restricted by the triangular shape of the brim, the head, therefore may engaged transversely in an occipito posterior position, so this type of pelvis favours:
- Occipitoposterior position
- Late engagement of the head
- Early rupture of membrane and cord prolapse.
- It favours prolonged first stage of labour.
- Genital tract infection
- They may be difficult in rotation of the head due to the narrowed cavity and prominent ischial spine, therefore they may be deep transverse arrest and the woman subjected to instrumental delivery.
- Due to the narrow sub pubic angle, the head is forced backwards, there may be prolonged second stage and tear of the perineum may follow.
- During the third stage because of the prolonged second stage of labour, there will be anticipation of postpartum haemorrhage.
ANTHROPOID PELVIS
This is an APE type of pelvis possessing about 24–25% of women who have very tall long legs with narrow shoulders. The bones are quite heavy, this type of pelvis is common in women in South Africa.
Characteristics
- The brim is oval shaped with long anterior position diameter but reduced transverse diameter and the sacral promontory is flattened. The sacrum is long and narrow and may contain 6 sacral vertebrae and deeply concave.
- The cavity, the sciatic notches are very wide and the ischial spine are not prominent and the sub pubic angle is wide and the sacrospinous ligament is long.
Effects of Anthropoid pelvis on labour
- The fetal head engages in anterior posterior diameter.
- They may be late in engagement of the head.
- The head may be seen through the pelvis in due occipito posterior position and the baby born face to pubis.
- Generally, the pub is so large and therefore labour does not present any difficulties. In order words it favours precipitate labour.
PLATYPELLOID PELVIS
This is a flat pelvis possessing about 5% of women
Characteristics
- The anterior posterior diameter of the brim is reduced
- But the transverse diameter is increased giving the brim a kidney or bean shaped appearance
- The side wall diverged and the sacrum is flat and the cavity shallow
- The ischial spines are brunt and the sciatic notch and the sub pubic angle are both wide.
Effects of Platypelloid pelvis on labour
- The fetal head finds it difficult to engage in the pelvis and is usually present with a long diameter of the head ( sagittal suture) across the transverse diameter of the brim where there is much room. Three things may happen as labour progresses
- With good uterine contraction the head is pushed to and flo between the sacral promontory and the symphysis pubis and with this rocking and rolling movement, it gets to the cavity. The skull bone overlaps itself with this pressure and the process is known as ASYNCLITISM ( overlying of moulding of fetal skull)
- The fetal head may be obstructed in the narrow diameter of the brim and uterine contraction cause it to extend as it’s decrease resulting in face presentation
- Its brim is greatly contracted the head remains floating high above it and cesarean section is considered