Maternal Pelvis

BONY PELVIS

• The pelvis is made up of two innominate bones, sacrum, coccyx.
• The pelvis may be divided by linea terminalis into an upper part known as pelvis major or false pelvis and a lower part known as pelvis minor or true pelvis.
• Female pelvis as compared to the male pelvis has Shallow and wide symphysis pubis

PLANES :

4 imaginary planes:

• Plane of pelvic inlet or superior strait
• Plane of greatest pelvic dimensions
• Plane of midpelvis or least pelvic dimensions
• Plane of outlet or inferior strait

TRUE PELVIS

• Anterior wall and posterior wall of the true pelvis measures 5 cm and 10 cm respectively.

In the standing position:

• Upper part of pelvic canal is directed downwards and backwards 
• Lower part curves and is directed downwards and forwards. 
• The side walls of true pelvis converge a little. 

The true pelvis is divided into 3 parts: 

• Inlet 
• Cavity
• Outlet

 

PELVIC INLET

Boundaries: 

• Posteriorly: Sacral promontory and alae of the sacrum.
• Laterally: Linea terminalis.
• Anteriorly : Horizontal rami of Pubic bones and pubic symphysis.
• Typically round than ovoid.

DIAMETERS :

• Anteroposterior diameter
  • Obstetric conjugate(10.5 cm): 
    • Smallest anterioposterior diameter
    • Shortest distance from symphysis pubis to the middle of sacral promontory.
    • Presenting part of Fetus should pass through it. 
  • True conjugate/Anatomic conjugate/Conjugate vera(11 cm): Middle of sacral promontory to superior surface of the pubic symphysis.
  • Diagonal Conjugate(12.5 cm) :Subpubic angle to middle of sacral promontory.
    • Can be measured clinically :Subtracting 1.5 cm from this gives obstetric conjugate. 
  • The inlet is adequate for a normal fetus if the diagonal conjugate is 12cm or more.
• Transverse diameter(13.5cm) 
  • Widest distance between iliopectineal lines
  • Widest diameter of the inlet 
  • Most fetuses engage in transverse or oblique diameter 
• Oblique diameters(12.5cm)
  • Sacroiliac joint of one side to opposite iliopectineal eminence 
• Posterior saggital diameters(5cm)
  • Point of intersection of obstetric conjugate and transvese diameter to middle of sacral promontory
• Sacrocotyloid diameter(9.5cm)
  • Midpoint of sacral promontory and ipsilateral iliopectineal eminence

MIDPELVIS

• Cardinal movements of engagement and internal rotation occur here.
• Bounded by 4th & 5th sacral vertebrae,white line,ischial spines,sacrospinous ligaments and pubic symphysis . 

Diameters : 

• Anteroposterior diameter(11.5cm): Junction of 4th and 5th sacral vertebrae to lower border of public symphysis. 
• Transverse diameter(10.5 cm)(smallest): Between ischial spines
• Posterior sagittal diameter(6 cm): Interspinous diameter to junction of 4th and 5th sacral vertebrae.

PELVIC OUTLET:

• Made up of 2 triangles with a common base
  • Anterior triangle has subpubic angle as apex and pubic rami and ischial tuberosities as sides.

DIAMETERS: 

•  Anteroposterior diameter(12 cm):Lower margin of pubic symphysis to sacrococcygeal junction. 
• Transverse Diameter (10.5 cm): Between inner edges of ischial tuberosity. Most important diameter of pelvis during labour.
• Posterior sagittal diameter(7 cm): Middle of transverse diameter to sacrococcygeal junction.
• Subpubic angle:Meeting of the two descending pubic rami. 
  •  In females, it measures 85-90 degrees. 
  • <85^o :transverse diameter of outlet will also be less.
  •  Waste space of Morris: Distance between pubic symphysis and edge of a round disc of diameter 9.3 cm.

 Axis Of Birth Canal:

• Obtained by joining the axes of inlet, cavity and outlet. 
• It is a curve with the convexity fitting the sacral curvature and is called anatomical pelvic axis or the curve of Carus. 
• Inclination of Pelvis(assessed in standing position): The plane of inlet makes 60 degree with the horizontal. 
• Affect engagement of the fetal head.

Obstetric axis:

• Course taken by the presenting part as it moves down through the pelvis.
• At first it goes downwards and backwards upto the level of ischial spine. 
• Then, the direction changes and at the outlet it moves downwards and forwards.

CLASSIFICATION OF PELVIS (BY CALDWELL AND MOLOY)

Based on shape of pelvis:

Gynaecoid pelvis(Commonest) : 

• Transverse diameter equal to or greater than AP diameter
•  In the midpelvis, sidewalls of pelvis are straight and the spines are not prominent. 
• At the outlet, pubic arch is wide.

Android Pelvis 

• At the inlet, posterior sagittal diameter.
• Deep transverse arrest 

Anthropoid Pelvis 

• At the inlet, AP diam.>Transverse diam.
• In the midpelvis, sidewalls are convergent, ischial spines are likely to be prominent. 
• At the outlet, subpubic arch is narrowed, but well shaped.

Platypelloid pelvis:

• Flattened gynaecoid pelvis with a short AP and a wide transverse diameter.

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