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Obstetrics Simplified - Diaa M. EI-Mowafi

Anatomy of the Female Pelvis

The female bony pelvis is divided into:

  • False pelvis: above the pelvic brim and has no obstetric importance.
  • True pelvis: below the pelvic brim and related to the child -birth.

THE TRUE PELVIS

It is composed of inlet, cavity, and outlet.

The Pelvic Inlet (Brim)

Boundaries

  • Sacral promontory,
  • alae of the sacrum,
  • sacroiliac joints,
  • iliopectineal lines,
  • iliopectineal eminencies,
  • upper border of the superior pubic rami,
  • pubic tubercles,
  • pubic crests and
  • upper border of symphysis pubis.

Diameters

  • Antero -posterior diameters:
    • Anatomical antero-posterior diameter (true conjugate) = 11cm
      • from the tip of the sacral promontory to the upper border of the symphysis pubis.
    • Obstetric conjugate = 10.5 cm
      • from the tip of the sacral promontory to the most bulging point on the back of symphysis pubis which is about 1 cm below its upper border. It is the shortest antero-posterior diameter.
    • Diagonal conjugate = 12.5 cm
      • i.e. 1.5 cm longer than the true conjugate. From the tip of sacral promontory to the lower border of symphysis pubis.
    • External conjugate = 20 cm
      • from the depression below the last lumbar spine to the upper anterior margin of the symphysis pubis measured from outside by the pelvimeter . It has not a true obstetric importance.
  • Transverse diameters:
    • Anatomical transverse diameter =13cm
      •  between the farthest two points on the iliopectineal lines.
      •  It lies 4 cm anterior to the promontory and 7 cm behind the symphysis.
      •  It is the largest diameter in the pelvis.
    • Obstetric transverse diameter:
      • It bisects the true conjugate and is slightly shorter than the anatomical transverse diameter.
  • (C) Oblique diameters:
    •  Right oblique diameter =12 cm
      • from the right sacroiliac joint to the left iliopectineal eminence.
    •  Left oblique diameter = 12 cm
      • from the left sacroiliac joint to the right iliopectineal eminence.
    •  Sacro-cotyloid diameters = 9-9.5 cm
      • from the promontory of the sacrum to the right and left iliopectineal eminence, so the right diameter ends at the right eminence and vice versa.

The Pelvic Cavity

It is a segment, the boundaries of which are:

  • the roof is the plane of pelvic brim,
  •  the floor is the plane of least pelvic dimension,
  •  anteriorly the shorter symphysis pubis,
  •  posteriorly the longer sacrum.

The Pelvic Outlet

Anatomical outlet

It is lozenge-shaped bounded by;

  •  the lower border of symphysis pubis,   
  •  pubic arch,
  •  ischial tuberosities,
  •  sacrotuberous and sacrospinous ligaments and,     
  •  tip of the coccyx.

Obstetric outlet

It is a segment, the boundaries of which are:

  •  the roof is the plane of least pelvic dimension,
  •  the floor is the anatomical outlet,
  •  anteriorly the lower border of symphysis pubis,
  •  posteriorly the coccyx.
  •  laterally the ischial spines.

Diameters of pelvic outlet

  • Antero - posterior diameters:
    • Anatomical antero-posterior diameter =11cm
      • from the tip of the coccyx to the lower border of symphysis pubis.
    • Obstetric antero-posterior diameter = 13 cm
      • from the tip of the sacrum to the lower border of symphysis pubis as the coccyx moves backwards during the second stage of labour.
  • Transverse diameters:
    • Bituberous diameter = 11 cm
      • between the inner aspects of the ischial tuberosities.
    • Bispinous diameter = 10.5 cm
      • between the tips of ischial spines.

Pelvic Planes

These are imaginary planes lie as follow:

Plane of pelvic inlet:

passing with the boundaries of pelvic brim and making an angle of 55o with the horizon (angle of pelvic inclination).

Plane of mid cavity (plane of greatest pelvic dimensions)

  • pass between the middle of the posterior surface of the symphysis pubis and the junction between 2nd and 3rd sacral vertebrae. Laterally, it passes to the centre of the acetabulum and the upper part of the greater sciatic notch.
  • It is a round plane with diameter of 12.5 cm.
  • Internal rotation of the head occurs when the biparietal diameter occupies this wide pelvic plane while the occiput is on the pelvic floor i.e. at the plane of the least pelvic dimensions.

Plane of obstetric outlet (plane of least pelvic dimensions):

passes from the lower border of the symphysis pubis anteriorly, to the ischial spines laterally, to the tip of the sacrum posteriorly.

Plane of anatomical outlet:

passes with the boundaries of anatomical outlet and consists of 2 triangular planes with one base which is the bituberous diameter.

  • Anterior sagittal plane: its apex at the lower border of the symphysis pubis.
  •  Posterior sagittal plane: its apex at the tip of the coccyx.
  • Anterior sagittal diameter: 6-7 cm
    • from the lower border of the symphysis pubis to the centre of the bituberous diameter.
  • Posterior sagittal diameter: 7.5-10 cm
    • from the tip of the sacrum to the centre of the bituberous diameter.

Pelvic Axes

Anatomical axis (curve of Carus)

  • It is an imaginary line joining the centre points of the planes of the inlet, cavity and outlet.
  •  It is C shaped with the concavity directed forwards.
  •  It has no obstetric importance.

Obstetric axis

  •  It is an imaginary line represents the way passed by the head during labour.
  •  It is J shaped passes downwards and backwards along the axis of the inlet till the ischial spines where it passes downwards and forwards along the axis of the pelvic outlet.

Caldwell- Moloy Classification of Pelvic Types (1933)

Four types of female pelves were described. Actually, the majority of pelves are of mixed types:

  • Gynaecoid pelvis(50%):
    • It is the normal female type.
    • Inlet is slightly transverse oval.
    • Sacrum is wide with average concavity and inclination.
    • Side walls are straight with blunt ischial spines.
    • Sacro-sciatic notch is wide.
    • Subpubic angle is 90-100o.
  • Anthropoid pelvis (25%):
    • It is ape-like type.
    • All anteroposterior diameters are long.
    • All transverse diameters are short.
    • Sacrum is long and narrow.
    • Sacro-sciatic notch is wide.
    • Subpubic angle is narrow.
  • Android pelvis (20%):
    • It is a male type.
    • Inlet is triangular or heart-shaped with anterior narrow apex.
    • Side walls are converging (funnel pelvis) with projecting ischial spines.
    • Sacro-sciatic notch is narrow.
    • Subpubic angle is narrow <90o.
  • Platypelloid pelvis (5%):
    • It is a flat female type.
    • All anteroposterior diameters are short.
    • All transverse diameters are long.
    • Sacro-sciatic notch is narrow.
    • Subpubic angle is wide.

N.B. At the Level of Ischial Spines:

  • The plane of obstetric outlet (plane of the least pelvic dimensions) is at this level.
  • The levator ani muscles are situated at this level and its ischio-coccygeous part is attached to the ischial spines.
  • The obstetric axis of the pelvis changes its direction.
  • The head is considered engaged when the vault is felt vaginally at or below this level.
  • Internal rotation of the head occurs when the occiput is at this level.
  • Forceps is applied only when the head at this level (mid forceps) or below it (low and outlet forceps).
  • Pudendal nerve block is carried out at this level.
  • The external os of the cervix is located normally.
  • The vaginal vault is located nearly.
  • The ring pessary should be applied above this level for treatment of prolapse.

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