This topic includes difference between female and male pelvis, various pelvis types, general description of pelvis bones, division of pelvis, landmarks of pelvis, plane, axis, sacral angle, diameters of inlet, cavity and outlet.

1. BY, MS. PRIYANKA GOHIL
MSc (N) OBG
PhD SCHOLAR

4. From the obstetrical standpoint, it is useful to consider the bony
pelvis as a whole rather than separately.
For descriptive purpose, an articulated pelvis is composed of
four bones:
two innominate bones
sacrum
coccyx.
These are united together by four joints—two sacroiliac joints,
sacrococcygeal joint and the symphysis pubis.

6. The pelvis is anatomically divided into:
A false pelvis
A true pelvis
The boundary line being the brim of the pelvis.

7. The bony landmarks on the brim of the pelvis from anterior to
posterior on each side are:
1. Upper border of symphysis pubis
2. Pubic crest
3. Pubic tubercle
4. Pectineal line
5. Iliopubic eminence
6. Iliopectineal line
7. Sacroiliac articulation
8. Anterior border of the ala of sacrum
9. Sacral promontory

9. The false pelvis is formed by the iliac portions of the
innominate bones and is limited above by the iliac crests.
It has got little obstetric significance except that its
measurements can to a certain extent, predict the size and
configuration of the true pelvis.
Its only obstetric function is to support the enlarged uterus
during pregnancy.
Its boundaries are: posteriorly—lumbar vertebrae, laterally—
iliac fossa and anteriorly—anterior abdominal wall.

10. This part of the pelvis is chiefly of concern to the obstetricians,
as it forms the canal through which the fetus has to pass.
It is shallow in front, formed by symphysis pubis and measures
4 cm (1 ½") and deep posteriorly, formed by the sacrum and
coccyx and measures 11.5 cm (4 ½").
For descriptive purpose, it is divided into:
Inlet
Cavity
Outlet.

11. As the inlet is the brim of the pelvis, the circumference of the
inlet is formed by the bony landmarks.
Shape:
It is almost round (gynecoid) with the anteroposterior diameter
being the shortest.
Other different shapes of the inlet are anthropoid, android and
platypelloid.

12. Plane:
It is an imaginary flat surface bounded by the bony points
mentioned as those of the brim.
It is not strictly a mathematical plane and is, therefore, often
referred to as superior strait.

14. Inclination:
In the erect posture, the pelvis is tilted forward.
As such, the plane of the inlet makes an angle of about 55°
with the horizontal and is called angle of inclination.
Another way of measuring the inclination radiographically is to
take the angle between the plane of the inlet and the front of
the body of the fifth lumbar vertebra.
The angle is normally about 135°.

15. When the angle of inclination is increased due to sacralization
of fifth lumbar vertebra, it is called high inclination.
High inclination has got obstetric significances:
1. There is delay in engagement because the uterine axis fails
to coincide with that of inlet
2. It favors occipitoposterior position
3. There is difficulty in descent of the head due to long birth
canal and flat sacrum interfering with internal rotation.

16. The angle of inclination may be lessened in case of
lumbarization of first piece of sacral vertebra and is called low
inclination.
It has got no obstetric significance.
It actually facilitates early engagement.

17. Sacral angle:
It is the angle formed by the true conjugate with the first two
pieces of the sacrum.
Normally, it is greater than 90°.
A sacral angle of lesser degree suggests funnelling of the
pelvis.

18. Axis:
It is a mid-perpendicular line drawn to the plane of the inlet.
Its direction is downward and backward.
When extended, the line passes through the umbilicus to
coccyx.
It is important that the uterine axis should coincide with the
axis of the inlet so that the force of the uterine contractions will
be spread in the right direction, to force the fetus to pass
through the brim.

20. Diameters:
The measurements of the diameters are all approximate and
minor variation is the rule rather than the exception.
1. Anteroposterior (Syn: true conjugate, anatomical
conjugate, conjugate vera)
2. Obstetric conjugate
3. Diagonal conjugate
4. Transverse diameter
5. Oblique diameters

21. Anteroposterior (Syn: true conjugate, anatomical conjugate,
conjugate vera):
It is the distance between the midpoint of the sacral promontory
to the inner margin of the upper border of symphysis pubis.
It measures 11 cm (4 ¼").
 It is not the shortest diameter of the inlet in the anteroposterior
plane.
In practice, the true conjugate cannot be estimated directly.

22.  However, its measurement is inferred by subtracting 1.2 cm (½")
from the diagonal conjugate thus allowing for the inclination,
thickness and height of the symphysis pubis.

23. Obstetric conjugate:
It is the distance between the midpoint of the sacral promontory
to prominent bony projection in the midline on the inner surface of
the symphysis pubis.
The point is somewhat below its upper border.
It is the shortest anteroposterior diameter in the anteroposterior
plane of the inlet.
It measures 10 cm (4").
It cannot be clinically estimated but is to be inferred from the
diagonal conjugate—1.5–2 cm (¾") to be deducted or by lateral
radiopelvimetry.

24. Diagonal conjugate:
It is the distance between the lower border of symphysis pubis to
the midpoint on the sacral promontory.
It measures 12 cm (4 ¾").
It is measured clinically during pelvic assessment in late
pregnancy or in labor.
Obstetric conjugate is computed by subtracting 1.5–2 cm from the
diagonal conjugate depending upon the height, thickness and
inclination of the symphysis pubis.

25. How to measure?
The patient is placed in dorsal position.
Two fingers are introduced into the vagina taking aseptic
precautions.
The fingers are to follow the anterior sacral curvature.
In normal pelvis, it is difficult to feel the sacral promontory or at
best can be felt with difficulty.
However, in order to reach the promontory, the elbow and the
wrist are to be depressed sufficiently while the fingers are
mobilized in upward direction.

26. The point at which the bone recedes from the fingers is the sacral
promontory.
The fingers are then mobilized under the symphysis pubis and a
marking is placed over the gloved index finger by the index finger
of the left hand.
The internal fingers are removed and the distance between the
marking and the tip of the middle finger gives the measurement of
diagonal conjugate.
For practical purpose, if the middle finger fails to reach the
promontory or touches it with difficulty, it is likely that the
conjugate is adequate for an average size head to pass through.

27. Measurement of diagonal conjugate

28. Transverse diameter:
It is the distance between the two farthest points on the pelvic
brim over the iliopectineal lines.
It measures 13 cm (5 ¼").
The diameter usually lies slightly closer to sacral promontory and
divides the brim into anterior and posterior segment.
The head negotiates the brim through a diameter, called available
or obstetrical transverse.
 This is described as a diameter which bisects the anteroposterior
diameter in the midpoint.
Thus the obstetrical transverse is either equal or less than the
anatomical transverse.

29. Oblique diameters:
There are two oblique diameters—right and left.
Each one extends from one sacroiliac joint to the opposite
iliopubic eminence and measures 12 cm (4 ¾").
Right or left denotes the sacroiliac joint from which it starts.
Sacrocotyloid—9.5 cm (3 ¾"):
It is the distance between the midpoint of the sacral promontory
to iliopubic eminence.
It represents the space occupied by the biparietal diameter of the
head while negotiating the brim in flat pelvis.

30. Sacrocotyloid—9.5 cm (3 ¾"):
It is the distance between the midpoint of the sacral promontory
to iliopubic eminence.
It represents the space occupied by the biparietal diameter of the
head while negotiating the brim in flat pelvis.

31. Cavity is the segment of the pelvis bounded above by the inlet
and below by plane of least pelvic dimensions.
Shape: It is almost round.
Plane:
The plane extends from the midpoint of posterior surface of
symphysis pubis to the junction of second and third sacral
vertebrae.
It is called plane of greatest pelvic dimensions.
It is the most roomy plane of the pelvis and is almost round in
shape.

32. Axis:
It is the mid-perpendicular line drawn to the plane of the
cavity.
Its direction is almost downward.
Diameters:
1. Anteroposterior
2. Transverse

33. Anteroposterior (12 cm or 4 ¾"):
It measures from the midpoint on the posterior surface of the
symphysis pubis to the junction of second and third sacral
vertebrae.
Transverse (12 cm or 4 ¾"):
It cannot be precisely measured as the points lie over the soft
tissues covering the sacrosciatic notches and obturator
foramina.

34. OBSTETRICAL OUTLET
It is the segment of the pelvis bounded above by the plane of
least pelvic dimensions and below by the anatomical outlet.
Its anterior wall is deficient at the pubic arch; its lateral walls
are formed by ischial bones and the posterior wall includes
whole of the coccyx.
Shape: It is anteroposteriorly oval.

35. Plane:
The plane is otherwise known as plane of least pelvic
dimensions or narrow pelvic plane.
The plane extends from the lower border of the symphysis
pubis to the tip of ischial spines and posteriorly to meet the tip
of the fifth sacral vertebra.

37. Diameters:
1. Transverse
2. Anteroposterior
3. Posterior sagittal

38.  Transverse—Syn: Bispinous (10.5 cm or 4 1/5"):
It is the distance between the tip of two ischial spines.
Anteroposterior (11 cm or 4 ¼"):
It extends from the inferior border of the symphysis pubis to
the tip of the sacrum.

39. Posterior sagittal (5 cm or 2"):
It is the distance between the tip of the sacrum and the
midpoint of bispinous diameter.
Axis:
It is represented by a line joining the center of the plane with
the sacral promontory.
Its direction is almost vertical.

40. Composite measurements of the diameters of the inlet, cavity
and outlet in centimeters

41. ANATOMICAL OUTLET
 It is otherwise known as bony outlet.
It is bounded in front by the lower border of the symphysis
pubis; laterally by the ischiopubic rami, ischial tuberosity and
sacrotuberous ligament and posteriorly by the tip of coccyx.
 Thus, it consists of two triangular planes with a common base
formed by a line joining the ischial tuberosities.
The apex of the anterior triangle is formed by the inferior
border of the pubic arch and that of the posterior triangle by
the tip of the coccyx.

42. Shape: It is diamond-shaped.
Plane:
It is formed by a line joining the lower border of the symphysis
pubis to the tip of the coccyx.
It forms an angulation of 10° with the horizontal.
Axis:
It is a mid-perpendicular line drawn to the plane of the outlet.
Its direction is downward and forward.

43. PELVIC JOINTS:
1. Symphysis pubis:
It is a secondary fibrocartilaginous joint. It has got no capsule
and no synovial cavity.
The articular surfaces are covered with hyaline cartilage.
Due to softening of the ligaments during pregnancy, there is
considerable amount of gliding movement.

44. 2. Sacroiliac articulation:
It is a synovial joint and is an articulation between the articular
surface of the ilium and sacrum.
The articulating surfaces are not alike.
It has got a capsule and a synovial cavity.
Engagement to diagnose, it is better to palpate gently with two
hands facing down over the abdomen than to prod around with
Pawlik’s grip, which in non-experienced hands is painful.

45. 3. Sacrococcygeal joint:
Sacrococcygeal joint is a synovial hinge joint.
It allows both flexion and extension.
Extension increases the anteroposterior diameter of the outlet.