Maternal Pelvis

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. 
  • <85o :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.

Exam Question 

  • Most important diameter of pelvis during labour is Interspinous diameter of outlet
  • Female pelvis as compared to the male pelvis has Shallow and wide symphysis pubis
  • Maximum diameter of pelvic inlet is Transverse diameter
  • Diagonal conjugate measurement at pelvic inlet can be assessed directly
  • To obtain true conjugate 1.5 cm should be subtracted from the diagonal conjugate
  • True pelvis refers to Lower part of pelvis
  • Shortest diameter of the pelvic outlet is Posterior sagittal diameter
  • Diagonal conjugate is defined as the distance between Lower border of symphysis pubis and the sacral Promontory
  • Interspinous diameter is the smallest diameter of the pelvis in the midplane
  • A pelvis characterized by an anteroposterior diameter of the inlet greater than the transverse diameter is classified as Anthropoid
  • The smallest anteroposterior diameter of the pelvic inlet is called the Obstetric conjugate
  • Deep transverse arrest is most commonly seen in Android pelvis
  • Gynecoid  is the most common type of female pelvis
  • Subpubic angle is 85°
  • The distance from the upper end of sacrum to lower border of pubis corresponds to Diagonal conjugate
  • Transverse diameter of the female mid-pelvic plane is 10.5
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