Maternal pelvis

OBSTETRICS

1. MATERNAL
PELVIS

2. BONY PELVIS
The pelvis is made up of two innominatebones,
sacrum, coccyx.
The pelvis may be divided by lineaterminalis
into an upper part known as pelvis major or false
pelvis and a lower part known as pelvis minor or
true pelvis.

3. PLANES AND DIAMETERS
Pelvis has 4 imaginary planes and each plane has
its own diameters.
1)Plane of pelvic inlet or superior strait
2)Plane of greatest pelvic dimensions
3)Plane of midpelvisor least pelvic dimensions
4)Plane of outlet or inferior strait

4. 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 and
the 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

5. PELVIC INLET
Boundaries:
Posteriorlyby: Sacral promontory and alaeof the
sacrum.
Laterally by: Linea terminalis.
Anteriorlyby: Horizontal ramiof Pubic bones and
pubic symphysis.
Typically round than ovoid.

6. DIAMETERS :
1.Anteroposteriordiameter.
Obstetric conjugate(10.5 cm): shortest
distance from symphysispubis to the
middle of sacral promontory.
Most important AP diameter.
Presenting part of Fetus should pass
through it.
True conjugate/Anatomic
conjugate/Conjugate vera(11 cm):
distance from middle of sacral promontory
to the superior surface of the pubic
symphysis.

7. Diagonal Conjugate(12.5 cm) extends from
subpubicangle to the middle of sacral
promontory.
Importance:
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.

8. 2.Transverse diameter(13.5cm)
oWidest distance between iliopectineallines
oWidest diameter of the inlet
oMost fetuses engage in transverse or oblique
diameter
3.Oblique diameters(12.5cm)
oExtends from sacroiliac joint of one side to
opposite iliopectinealeminence
4.Posterior saggitaldiameters(5cm)
oExtends from point of intersection of obstetric
conjugate and transvesediameter to middle of
sacral promontory

9. 5.Sacrocotyloid diameter(9.5cm)
Distance between the midpoint of sacral
promontory and the ipsilateraliliopectineal
eminence

12. MIDPELVIS
Most important plane of the pelvis at the level of the
ischialspines.
Cardinal movements of engagement and internal
rotation occur here.
Bounded by 4
th
& 5
th
sacral vertebrae,white
line,ischialspines,sacrospinousligaments and pubic
symphysis.
•Diameters :
1.Anteroposteriordiameter(11.5cm):
extends from junction of 4
th
and 5
th
sacral vertebrae to
the lower border of public symphysis.
2.Transverse diameter(10.5 cm):
is between ischialspines.
3. Posterior sagittaldiameter(6 cm):
extends from the interspinousdiameter to the junction of
4
th
and 5
th
sacral vertebrae.

14. PELVIC OUTLET
Made up of 2 triangles with a common base(a line
drawn through 2 ischialtuberosities.)
The anterior triangle has the subpubicangle as the
apex and the pubic ramiand ischialtuberositiesas
the sides.
DIAMETERS:
1.Anteroposteriordiameter(12 cm):
extends from lower margin of pubic symphysisto the
sacrococcygealjunction.
2.Transverse Diameter (10.5 cm):
Distance between inner edges of ischialtuberosity.
3.Posterior sagittaldiameter(7 cm):
Extends from middle of transverse diameter to the
sacrococcygealjunction.

16. OTHER DEFINITIONS
Subpubicangle:
Formed by meeting of the two descending pubic rami.
In females, it measures 85-90 degrees.
If the angle is less, transverse diameter of the outlet will
also be less.
Waste space of Morris:
Distance between the pubic symphysisand the edge of a
round disc of diameter 9.3 cm.(Approximating fetal head)
placed under subpubicarch.
Normally subpubicangle is well rounded and the distance
between pubic symphysisand the edge of disc should not
be more than 1 cm.
When it is more than 1 cm, AP diameter of outlet is
reduced.

17. 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.

18. 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 ischialspine.
Then, the direction changes and at the outlet it
moves downwards and forwards.

19. CLASSIFICATION OF PELVIS
(BYCALDWELL AND MOLOY)
Based on shape of pelvis:
1.Gynaecoidpelvis
Inletis slightly oval or round(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.
2.Android Pelvis
•At the inlet, posterior sagittal
diam.<Anterior Sagittaldiam.
•In the midpelvis, sidewalls are
convergent, ischialspines are prominent.
•At the outlet, subpubicarch is narrow.

20. 5.Anthropoid Pelvis
•At the inlet, AP diam.>Transverse diam.
•In the midpelvis, sidewalls are
convergent, ischialspines are likely to be
prominent.
•At the outlet, subpubicarch is narrowed,
but well shaped.
4.Platypelloidpelvis:
•Flattened gynaecoidpelvis with a short
AP and a wide transverse diameter.