This document summarizes the anatomy of the maternal pelvis. It describes how the pelvis is composed of bones that fuse together, including the innominate bones, sacrum, and coccyx. It then discusses the different planes and diameters of the pelvis, including the inlet, cavity, and outlet. For each, it provides the bony landmarks and average diameters. The document emphasizes how the shape and angles of the pelvis impact labor and delivery.

1. Maternal Pelvis

2. Introduction
• The pelvis is made up of
- two innominate bones,
- Sacrum, and
- Coccyx.
• Each innominate bone is formed by the fusion of
- Ilium
- Ischium, and
- Pubis
• The innominate bones are attached
- to each other at the symphysis Pubis
- to the Sacrum by two sacroiliac joints.
• Sacroiliac joint is a synovial joint.
Normal labour
depends on safe
journey of the
fetus through the
maternal pelvis

3. Innominate
bone

4. Maternal pelvis

5. • What is Symphysis Pubis?
- secondary fibrocartilaginous joint
- without capsule or synovial cavity
- shows gliding movement during pregnancy ( due to softening of
the ligaments).
• Pelvis may be divided by linea terminalis( pelvic brim) into
- Upper: False pelvis
- Lower: True pelvis
Introduction (continued)

6. Pelvic brim

7. True pelvis
• True pelvis lies below the pelvic brim
• Anterior wall: 5 cms
• Posterior wall: 10 cms
• Divided into 3 parts
- Inlet
- Cavity
- Outlet

8. Pelvic Inlet
(Upper Pevic Strait/ Superior Strait)
• Plane of division between false pevis and true pelvis
• Shape- Transverse oval
• Bony landmarks from anterior to posterior on each side are
- upper border of symphysis pubis
- pubic crest
- pubic tubercle
- pectineal line
- illiopubic eminence
- illiopectineal line
- sacroiliac joint
- anterior border of ala of sacrum, and
- sacral promontory
• Axis of Inlet:
- perpendicular to the plane of the brim
- proceeds backwards and downwards

11. Pelvic Inclination
• Pelvic Inlet inclined at an acute angle to horizontal ( due to oblique
articulation of pelvis with femur bones).
• Angle of inclination: 55°
• Obstetric significance:
1. Increased angle of inclination, seen in sacralization of lumbar
vertebra - delayed engagement
- favours occipito-posterior position
2. Reduced angle of inclination, seen in lumbarization of 1st sacral
vertebra – favours engagement and easy delivery

12. Angle of inclination

15. Axis of Inlet
• Imaginary straight line
• From the centre of inlet
• Perpendicular to the plane of
brim
• Proceeds backwards and
downwards
• Indicates the direction in which
fetus travels in the pelvis

16. Diameters of Pelvic Inlet
1. Antero-posterior diameters
Conjugate Extension
Measur
ement
True conjugate
(Anatomical conjugate/
Conjugate vera)
From upper border of symphysis Pubis – midpoint of sacral
promontory
11 cms
Obstetric conjugate
From centre of the pubic symphysis – midpoint of Sacral
promontory
10 cms
Diagonal conjugate
Lower border of symphysis pubis – midpoint of sacral
promontory
12 cms

17. Antero-posterior diameters of Pelvic inlet

18. Measurement of Diagonal conjugate

19. 2. Oblique diameters (12- 12.5 cms)
- Extends from one sacroiliac joint to opposite illiopectineal
eminence
3. Transverse Diameter (13 cms)
- distance between two farthest points on the pelvic brim
- Being widest, most fetuses engage in this diameter.
4. Posterior sagittal diameter (4 cms)
- Extends from the point of intersection of the obstetric conjugate
and the transverse diameter to the middle of the sacral promontory.
5. Sacrocotyloid diameter (9.5cms)
- extends from sacroiliac joint to the ipsilateral illiopectineal eminence
Diameters of Pelvic Inlet

20. Diameters of Pelvic Inlet

21. Pelvic cavity
• Space between the plane of the inlet above and plane of the outlet
below
• It is from -5 station to 0 station.
• Curved canal with
- shallow anterior wall, measuring 4 cms
- deep lateral wall, 7.5 cms
- deep posterior wall, 11.5 cms
• It has got 2 planes
- Plane of greatest pelvic dimensions
- Plane of least pelvic dimensions

22. Plane of Greatest pelvic dimensions
• Midplane
• Bounded
- Anteriorly: centre of the symphysis Pubis
- Posteriorly: junction of 2nd and 3rd sacral vertebrae
- Laterally: obturator foramina and ischial bone.
• Roomiest part of the pelvis
• Diameters:
- Antero-posterior diameter: 12 cms
- Transverse Diameter: 12 cms
- Oblique diameter: 12 cms

23. Plane of Least Pelvic Diameters( Midpelvis)
• Imaginary flat surface
• Bounded
- Anteriorly: lower border of symphysis pubis
- Laterally: tip of ischial spines
- Posteriorly: Junction of 4th or 5th sacral vertebrae
OR
Lower border of last sacral vertebra

24. • Anteroposterior (12 cm)
- measures from the mid-point of the symphysis pubis to the
junction of 2nd and 3rd sacral vertebrae.
• Transverse (10 cm):
- interspinous diameter ( smallest diameter of the pelvis)
• Posterior sagittal diameter ( 4.5 cms)
- extends from interspinous diameter to the junction of 4th and 5th
sacral vertebrae
Diameters of Midpelvis

25. Planes of Pelvic Cavity

26. Diameters of Midpelvis

27. What is the Obstetric significance of plane of least pelvic diameters or
ischial spines?🤔
• Origin of levator ani muscles
• Cardinal movements of engagement and internal rotation
• Beginning of forward curve of pelvic axis
• Descent of the fetal head in relation to ischial spines
• Landmark for pudendal nerve block
Plane of Least Pelvic Diameters

28. Obstetric Outlet
• Shallow bony segment
• Bounded
- Above by plane of least pelvic dimensions
- below by anatomical outlet

29. Pelvic outlet
(Anatomical outlet/Lower pelvic strait/ Inferior
strait)
• Rhombus shaped space
• From 0 to +4 station
• Boundaries:Anteriorly: lower border of symphysis pubis
Laterally: ischiopubic rami, ischial tuberosities and
sacrotuberous ligaments
Posteriorly: tip of Sacrum or coccyx( if it is not pushed
back).
• These boundaries lie in two triangular planes
- common base: line joining ischial tuberosities
- Apices at the Symphysis pubis and tip of Sacrum/coccyx

30. Diameters of Pelvic outlet
• Anteroposterior (13 cm)
- It extends from the lower border of the symphysis pubis to the
tip of the sacrum
• Transverse diameter(11 cms)
- intertuberous diameter
- It is the distance between inner edges of ischial tuberosities
• Posterior sagittal diameter ( 7 cms)
- middle of the transverse diameter to the sacrococcygeal junction

31. Diameters of anatomical outlet

32. Pelvic axis( Curve of Carus)
• By uniting the axes of the three planes of brim, mid-cavity and outlet
• It forms a curve , directed
- downwards and backwards (axis of brim)
- gradually more forwards
- until it reaches axis of outlet.
• Obstetric pelvic axis (true path through which head pass)
- straight in its upper part(3/4th)
- curved only in its lower portion.

33. Pelvic axis
( Curve of Carus)

34. Subpubic angle
• Formed by
approximation of the
two descending pubic
rami
• In normal adequate
pelvis, it measures
85-90°

35. Pubic arch
• Arch formed by the
descending rami of both the
sides.
• Normally, it measures 6 cm in
between the pubic rami at a
level of 2 cm below the apex
of the subpubic arch.
• Clinically, it is assessed by
placing 3 fingers side by side.

36. Waste Space of
Morris
• Únder the subpubic arch
• Distance between the
symphysis pubis and
edge of the fetal head
• It should not be >1cm
• If more than 1 cm,
available antero-
posterior diameter of
outlet is reduced.

37. Diameters of the Pelvis
Inlet Plane of greatest
pelvic dimensions
Plane of least pelvic
dimensions
Outlet
OC 10 cms AP diameter 12 cms AP diameter 12 cms AP 13 cms
AC 11 cms
DC 12 cms
Transverse 13 cms Transverse 12 cms Interspinous 10 cms Intertuberous 12 cms
Oblique 12 cms Oblique 12 cms Subpubic angle 85°
Posterior sagittal 4cms Posterior sagittal 4.5
cms
Posterior sagittal 7cms
Heart shaped Round shaped Diamond/ Rhombus

38. Types of Pelvis
(Caldwell And Moloy Classification)

40. Physiological changes in pelvis in pregnancy
and Labour
• Increase in width and mobility of symphysis pubis and sacroiliac joints
-due to hormone Relaxin and progesterone
• During labour, AP diameter of inlet is increased
- as a consequence of rotatory movements of sacroiliac joints
- Best position for vaginal delivery is dorsal lithotomy position( AP
diameter of outlet is increased by 1.5 cms
- During labour, coccyx is pushed back

41. Thank you