EFFECTS ON LABOUR

Fetal head finds difficult in engaging in the pelvis or presents with the long diameter of the head across the transverse diameter where there is room leading to:

• With good contractions, the fetal head is pushed to and fro between the sacral promontory and symphysis pubis therefore it rocks and rolls through the brim and cavity. The skull bones overlap each other and the process known as asynclitism.
• The fetal head may be obstructed in the narrow diameter of the brim and uterine contractions cause the head to extend as it descends leading to false presentation.
• If the brim is greatly contracted the fetal head remains high above leading to caesarean section.

ANTHROPOID PELVIS

Found in very tall, long legged women, common in S. Africa.

• The brim is oval shaped having a long anteroposterior diameter but a reduced transverse, fore pelvis narrowed.
• The cavity is adequate in all diameters but rather deep.
• The outlet is adequate in all diameters with a wide pubic arch.

EFFECTS ON LABOUR

The fetus presents with a long diameter of the head in the anteroposterior diameter, the occiput lying in the hollow of the sacrum resulting in face to pubis delivery instead of face to perineum.

OTHER PELVIC VARIATIONS

JUSTO MINOR PELVIS

Is a small gynaecoid pelvis but the diameters are reduced found in small women less than 1.5 m tall. The type of delivery depends on the size of the husband and fetus she may deliver a small baby vaginally with very little trouble. On the other hand, a larger baby may require a forceps delivery or even caesarean section.

CONTRACTED PELVIS

The essential diameters are reduced by 1 cm or more i.e. anteroposterior, oblique or transverse of brim, cavity or outlet is reduced by 1 cm or more.

HIGH ASSIMILATION PELVIS

This occurs when the 5th lumbar vertebra is fused to the sacrum (made up of 6 vertebrae) and the angle of inclination of the pelvic brim is increased. Engagement of the head is difficult but once achieved labor progress normally.

LOW ASSIMILATION PELVIS This is made up of 4 sacral vertebrae.

DEFORMED PELVIS This may occur from developmental anomalies or dietary deficiencies, injury or disease.

Examples of developmental anomalies of the pelvis:

ROBERT’S PELVIS

The pelvis has no wings (ala) of the sacrum contracted in all its diameter (rarely seen) its due to congenital abnormality caesarean section is done.

NAEGELE PELVIS

Only one wing is present in the sacrum giving it an obliquity of the pelvis, may be due to congenital abnormality, sometimes caused by injury. It’s rare but can occur in a woman who has walked with a limp for many years can be delivered by caesarean section.

DIETARY DEFICIENCY PELVES INCLUDE: Deficiency of vitamin and minerals necessary for the formation of health bones may complicate labor to some extent.

OSTEOMALACIC PELVIS The disease osteomalacia is rare. It’s due to deficiency of calcium and occurs in adults. As bones of the skeleton soften due to the gross calcium deficiency, the pelvic canal is flattened together until they become Y shaped slit. Labor is impossible caesarean section would be performed in early pregnancy incarceration of the gravid uterus may occur because of the gross deformity.

RACHITIC PELVIS

Rickets in early childhood can lead to gross deformity of pelvic brim. The weight of the upper body presses downwards on to the softened pelvic bones. The sacral promontory is pushed downwards and forward, and the ilium and ischium are down outwards. These results in a flat pelvic brim like that of the platypelloid pelvis. The sacrum tends to be straight with the coccyx bending accurately downwards. The tuberosities are wide and the pubic arch is wide. (1) Bow legs (2) Spinal deformity

OUTCOME OF LABOUR

If severe contractions are present caesarean section is performed. The fetal head enters the pelvis by lateral fitting of the head known as asynclitism.

ANTERIOR ASYNCLITISM

The anterior bone moves downward behind the symphysis pubis until the parietal eminence enters the brim. The movement is then reversed with the head tilts in the opposite direction until the posterior parietal bone negotiates the sacral promontory and the head is engaged.

POSTERIOR ASYNCLITISM

The movements of anterior asynclitism are reversed; the posterior parietal bone negotiates the sacral promontory prior to the anterior parietal bone moving downwards behind the symphysis pubis. Once the fetal head has engaged, descent progresses normally accompanied by flexion and internal rotation. Labor may be completed by face, brow presentation and cord prolapse.

SUMMARY OF FEATURES OF THE FOUR TYPES OF PELVIS