Fetal head & maternal bony pelvis diameters. Clinical pelvimetry.

1. Prepared by:
Amaal Atta El-Seqali
Rasha Khamis El-Dabbagh

2. P’S IN LABOR & DELIVERY
1.Passenger= The fetus
2. Passageway= The birth canal
3. Power of labor= Force of uterine contractions
4. Placenta
5. Psyche

3. Anatomic characteristic of the
fetal head & maternal pelvis

4. Objectes
• Fetal head
• Pelvic anatomy
• Pelvic shapes
• Pelvimetry
• Cephalopelvic
disproportion

6. Fetal head
• Sutures
• Fontanelles
• Landmarks
• Diameters

7. Fetal head
From an obstetrical point of view it’s the most
important part:
• largest
• least compressible part of the fetus.
• most frequent presenting part

8. consists of:
1-Base:
large, ossified, firmly united, and noncompressible
2-vault (cranium) consists of:
-occipital bone posteriorly
- 2 parietal bones bilaterally
-2 frontal and temporal bones
anteriorly

9. Cranial bones at birth:
• Thin
• weakly ossified
• easily compressible
• interconnected only by membranes
>>>
allow them to overlap under pressure & to
change shape to conform to the maternal
pelvis, a process known as (molding).

10. Sutures
• Membrane-occupied spaces between the cranial bones
1-Sagittal suture:
- lies btw the parietal bones
-extends in an AP direction btw the fontanelles
-divides the head into right and left sides

11. 2-lambdoid suture:
• extends from the posterior fontanelle laterally
• separate the occipital from the parietal bones.

12. 3-coronal suture:
• extends from the anterior fontanelle laterally
• separate the parietal and frontal bones.

13. 4- frontal suture:
• lies between the frontal bones
• extends from the anterior fontanelle to the
glabella (the prominence between the eyebrows).

14. Clinical importance of sutures
• molding of the head in the vertex presentation
• Position of fontanelle & sagittal suture can identify attitude and
position of vertex.
• By plapating the sagittal suture during labour, degree of internal
rotation & molding of the head can be noticed.
• In deep transverse arrest, this sagittal suture lies transversely at the
level of the ischial spines.

15. Fontanelles
• membrane-filled spaces located at the point where
the sutures intersect
• more useful in diagnosing the fetal head position
than the sutures.

16. The anterior fontanelle (bregma) :
 diamond shaped area(2 × 3 cm) of unossified membrane formed by the
junction of 4 suture.
The suture are:-
Anteriorly: frontal suture
Posteriorly: sagittal suture
Laterally: on both side:-coronal suture.
 It is felt on fetal head surface as a soft shallow depression.
 It ossifies by 18 months after birth.>>>allows the skull to
accommodate the tremendous growth of the infant's brain after birth

17. Clinical importance:
1. Degree of flexion can be assessed from its position. If on vaginal
examination it is felt easily, it indicates the head is not well
flexed.
2. Helps in the molding of head.
3. Internal rotation of the head can be assessed from it’s position.
4. ICP can be roughly assessed from its condition after birth.
Depression in dehydration and bulging in raised ICP.
5. CSF can be collected from its lateral angles from the lateral
ventricles.

18. The posterior fontanelle:
 It is the triangular depressed area at the junction
of 3 suture:
Anteriorly: sagittal suture
Posteriorly: 2 lambdoid sutures at both side.
• closes at 6 to 8 weeks of life
• Y- or T-shaped

19. Clinical importance:
1. From its relation of the maternal pelvis, position
of vertex is determined.
2. Internal rotation can be assessed from its location.
3. Degree of flexion can be assessed from its
position. On vaginal examination if it is felt easily
and anterior fontanelle is not felt, this indicates
good flexion of the fetal head.

20. landmarks
front to back
1. Nasion (the root of the nose)
2. Glabella (the elevated area btw the orbital ridges)
3. Sinciput (brow) (the area btw AF & glabella)

21. 4-Anterior fontanelle (bregma)
5-Vertex (the area btw the fontanelles & bounded laterally by
the parietal eminences)
6-Posterior fontanelle (lambda)
7-Occiput (the area behind & inferior to PF & lambdoid
sutures)

22. Diameters
6
• Anteroposterior diameters (4):
presenting to the maternal pelvis depends on the
degree of flexion or extension of the head
• Transverse diameters (2)

23. Anteroposterior diameters
1- Suboccipitobregmatic (9.5 cm):
Extends from the undersurface of the occipital bone at the
junction with the neck to the center of the AF.

24. • Present AP diameter when the head is well
flexed >>>> OT or OA position
LOAROT

25. ROT LOT

26. LOAROA

28. 2. Occipitofrontal (11 cm):
• extends from the external occipital protuberance to the
glabella.

29. • presenting AP diameter when the head is
deflexed >>> OP

30. LOP
ROP LOP

32. 3. Supraoccipitomental (13.5 cm):
• extends from the vertex to the chin

33. -presenting AP diameter in a brow presentation
-longest AP diameter of the head

34. 4. Submentobregmatic (9.5 cm):
• extends from the junction of the neck and lower jaw to
the center of the anterior fontanelle.

35. -presenting AP diameter in face presentations

37. Transverse diameters
1-Biparietal (9.5 cm):
• the largest transverse diameter
• extends btw the parietal bones.
2-Bitemporal (8 cm):
• the shortest transverse diameter
• extends btw the temporal bones.

40. Pelvic anatomy
• Bony pelvis
• Pelvic planes
• Pelvic diameters

41. The bony pelvis
four bones :
• two hip bones(ileum, ischium & pubis) laterally &
anteriorly
• sacrum & coccyx posteriorly
‘3 joints’
• symphysis pubis anteriorly
• sacroiliac joints posteriorly

42. Sacrum
• consists of 5 rudimentary vertebrae fused together to form
a single wedge-shaped bone with a forward concavity
• The upper border ( base) articulates with the L5
• The narrow inferior border articulates with the coccyx.
• Laterally, the sacrum articulates with the two iliac bones
• The anterior and upper margins of the first sacral vertebra
bulge forward sacral promontory

44. Coccyx
• consists of 4 vertebrae fused together to form a small
triangular bone
• articulates at its base with the lower end of the sacrum
• It’s vertebrae consist of bodies only, but the first vertebra
possesses a rudimentary transverse process and cornua.
The cornua are the remains of the pedicles and superior
articular processes and project upward to articulate with
the sacral cornua

45. Hip Bone
In children:
each hip bone consists of :
• the ilium, which lies superiorly
• the ischium, which lies posteriorly and inferiorly
• the pubis, which lies anteriorly and inferiorly
 joined by cartilage at the acetabulum
 At puberty, >>> fuse together to form one large, irregular bone.
 articulate with the sacrum at the sacroiliac joints >>>>form the
anterolateral wall of the pelvis
 articulate with one another anteriorly at the symphysis pubis.

47. The ilium,
 the upper flattened part of the hip bone
• iliac crest runs between the anterior and posterior superior
iliac spines
• Below these spines are the corresponding anterior and
posterior inferior iliac spines
• The iliopectineal line runs downward and forward around
the inner surface of the ilium and serves to divide the false
from the true pelvis.

48. The ischium
 the inferior and posterior part of the hip bone
• ischial spine
• ischial tuberosity

50. 3-The pubis
 the anterior part of the hip bone
• Body bears pubic crest & pubic tubercle and articulates
with the pubic bone of the opposite side at the symphysis
pubis
• superior and inferior pubic rami

52. Pelvic brim is formed by:
• the sacral promontory behind,
• iliopectineal lines laterally,
• symphysis pubis anteriorly.
• Above the brim >>> false pelvis, which forms part of the abdominal
cavity.
• Below the brim >>> true pelvis.

53. False Pelvis
bordered by:
• lumbar vertebrae posteriorly
• iliac fossa bilaterally
• abdominal wall anteriorly.
supports the abdominal contents
after 1st trimester helps support the gravid uterus.

54. True Pelvis
bony canal and is formed by:
• the sacrum and coccyx posteriorly
• the ischium and pubis laterally and anteriorly
 It’s internal borders are solid and relatively immobile.
 The posterior wall is twice the length of the anterior wall.
 The area of concern to the obstetrician because its
dimensions are sometimes not adequate to permit passage
of the fetus.

55. Pelvic Planes
• imaginary, flat surfaces that extend across the
pelvis at different levels.
four planes :
1. The pelvic inlet
2. The plane of greatest diameter
3. The plane of least diameter
4. The pelvic outlet

56. 1-The plane of the inlet:
bordered by:
• pubic crest anteriorly
• iliopectineal line of the innominate bones laterally
• promontory of the sacrum posteriorly.
fetal head enters the pelvis through this plane in the transverse
position.

58. 2-The plane of greatest diameter:
• largest part of the pelvic cavity
bordered by:
• the posterior midpoint of the pubis anteriorly
• the upper part of the obturator foramina laterally
• the junction of the 2nd and 3rd sacral vertebrae posteriorly.
The fetal head rotates to the anterior position in this plane

59. 3-The plane of least diameter:
the most important from a clinical standpoint, because most
instances of arrest of descent occur at this level.
bordered by:
• the lower edge of the pubis anteriorly
• the ischial spines and sacrospinous ligaments laterally
• the lower sacrum posteriorly.
Low transverse arrests generally occur in this plane.

60. 4-The plane of the pelvic outlet :
is formed by 2 triangular planes with a common base at the level of the
ischial tuberosities.
The anterior triangle is bordered by:
• the subpubic angle at the apex,
• the pubic rami on the sides,
• the bituberous diameter at the base.
The posterior triangle is bordered by:
• the sacrococcygeal joint at its apex,
• the sacrotuberous ligaments on the sides,
• and the bituberous diameter at the base.
This plane is the site of a low pelvic arrest.

62.  represent the amount of space available at each level.
 The key measurements for assessing the capacity of the
maternal pelvis include the following:
1. The obstetric conjugate of the inlet
2. The bispinous diameter
3. The bituberous diameter
4. The posterior sagittal diameter at all levels
5. The curve and length of the sacrum
6. The subpubic angle
Pelvic Diameters

64. pelvic inlet
5 important diameters:
1-The true conjugate (anatomic conjugate)
the anatomic diameter and extends from the middle of the
sacral promontory to the superior surface of the pubic
symphysis.
2-The obstetric conjugate
the actual space available to the fetus and extends from the
middle of the sacral promontory to the closest point on the
convex post. surface of the symphysis pubis.

66. 3-The transverse diameter :
the widest distance between the iliopectineal lines.
4-Each oblique diameter:
extends from the sacroiliac joint to the opposite iliopectineal
eminence.
5-The posterior sagittal diameter:
extends from the AP and transverse intersection to the middle
of the sacral promontory

68. Greatest Diameter
2 diameters:
1- The AP diameter:
extends from the midpoint of the posterior surface of the
pubis to the junction of the 2nd and 3rd sacral vertebrae
2-The transverse diameter:
The widest distance btw the lateral borders of the plane

69. Least Diameter (Midplane)
• 3 important diameters:
1-The AP diameter:
extends from the lower border of the pubis to the junction of
the 4th & 5th sacral vertebrae
2-The transverse (bispinous) diameter:
extends btw the ischial spines
3- The posterior sagittal diameter:
extends from the midpoint of the bispinous diameter to the
junction of the 4th & 5th sacral vertebrae.

71. Pelvic Outlet
• 4 important diameters:
1-The anatomic AP diameter:
extends from the inferior margin of the pubis to the tip of the
coccyx
2-obstetric AP diameter:
extends from the inferior margin of the pubis to the sacrococcygeal
joint.
3-The transverse (bituberous) diameter:
extends btw the inner surfaces of the ischial tuberosities,
4-Post. sagittal diameter:
extends from the middle of the transverse diameter to the
sacrococcygeal joint.

74. Pelvic Shapes &
Pelvimetry
By: Rasha Khamis Al-Dabbagh
Amaal Atta El-Seqali

75. Pelvic Shapes

76. Gynecoid Pelvis
• The classic female type.
• Found in approximately 50% of women.
• Characteristics:
1. Round inlet, with the widest transverse diameter only
slightly greater than the AP diameter
2. Side walls straight
3. Ischial spines of average prominence .
4. Well-rounded sacrosciatic notch
5. Well-curved sacrum
6. Spacious subpubic arch, with an angle of
approximately 90 degrees

78. • These features create a cylindrical
shape that is spacious throughout.
• The fetal head generally rotates
into the occipitoanterior position
in this type of pelvis.

79. Android Pelvis
• The typical male type
• Found in less than 30% of women
• Characteristics:
1. Triangular inlet with a flat posterior segment & the
widest transverse diameter closer to the sacrum than
in the gynecoid type .
2. Convergent side walls with
prominent spines
3. Shallow sacral curve
4. Long and narrow sacrosciatic notch
5. Narrow subpubic arch

81. • Limited space at the inlet &
progressively lessens down the
pelvis, owing to the funneling effect
of the side walls, sacrum, and pubic
rami.
• Restricted space at all levels.
• The fetal head is forced to be in the
occipitoposterior position to conform
to the narrow anterior pelvis.
• Arrest of descent is common at the
midpelvis.

82. Anthropoid Pelvis
• Resembles anthropoid ape pelvis.
• Found in approximately 20% of women
• Characteristics:
1. A much larger AP than transverse diameter, creating a long
narrow oval at the inlet
2. Side walls that do not converge
3. Ischial spines that are not prominent but
are close, owing to the overall shape
4. Variable, but usually posterior, inclination
of the sacrum
5. Large sacrosciatic notch
6. Narrow, outwardly shaped subpubic arch

84. • The fetal head can engage only in
the AP diameter and usually does so
in the occipitoposterior position,
because there is more space in the
posterior pelvis.

85. Platypelloid Pelvis
• Flattened gynecoid pelvis.
• Found in only 3% of women
• Characteristics:
1. A short AP & wide transverse diameter creating an oval-
shaped inlet
2. Straight or divergent side walls
3. Posterior inclination of a flat sacrum
4. A wide bispinous diameter
5. A wide subpubic arch
• The fetal head has to engage in the
transverse diameter.

89. PELVIMETRY
• Pelvimetry is the assessment of the
dimensions & capacity of adult female pelvis
in relation to the birth of a baby.
• Pelvimetry was heavily used in leading the
decision of natural, operative vaginal delivery
or CS.

90. Types of Pelvimetry
External/indirect pelvimetry
– Measures diameters of false pelvis
– Little value, unreliable, no longer used
Internal/ direct pelvimetry
Radiographic pelvimetry

91. Internal Pelvimetry
• Through vaginal examination
• At first prenatal visit screen for obvious
contractions.
• In late pregnancy (preferred)
– After 37 weeks GA or at the onset of labour
– the soft tissues are more distensible
– more accurate
– less uncomfortable

92. Pelvic Inlet
1. Palpation of pelvic brim:
• The index & middle fingers are moved
along the pelvic brim.
• Note whether round or angulated, causing
the fingers to dip into a V-shaped
depression behind the symphysis.

93. 2) Diagonal conjugate:
• Measured from the lower border of the
pubis to the sacral promontory using the tip
of the second finger and the point where the
index finger of the other hand meets the
pubis
• Normally 12.5 cm & cannot be reached.
• If it is felt the pelvis is contracted
• True conjugate = diagonal conjugate – 1.5
• Not done if the head is engaged.

97. The Midpelvis
1) Symphysis:
– Height, thickness & curvature
2) Sacrum:
– Shape & curvature
– Concave usually.
– Flat or convex shape may indicate AP constriction
throughout the pelvis.
3) Side walls:
– Straight, convergent or divergent starting from the
pelvic brim down to the base of ischial spines.
– Normally almost parallel or divergent

100. 4) Ischial spines prominence:
– The ischial spines can be located by
following the sacrospinous ligament to its
lateral end.
– Blunt (difficult to identify at all),
– Prominent (easily felt but not large) or
– Very prominent (large and encroaching on
the mid-plane).
5) Interspinous diameter:
– If both spines can be touched
simultaneously, the interspinous diameter
is 9.5 cm i.e. inadequate for an average-
sized baby.

103. 6) Sacrospinous ligament:
– Its length is assessed by placing one
finger on the ischial spine & one finger
on the sacrum in the midline.
– The average length is 3 fingerbreadths.
7) Sacrosciatic notch:
– If the sacrospinous ligament is 2.5
fingers, the sacrosciatic notch is
considered adequate.
– Short ligament suggests forward
curvature of the sacrum & narrowed
sacrosciatic notch.

106. Pelvic Outlet
1) Subpubic angle:
–Assessed by placing a thumb next to each
inferior pubic ramus and then estimating the
angle at which they meet.
–Normally, it admits 2 fingers. (90o)
–Angle ≤ 90 degrees suggests contracted
transverse diameter in the midplane and
outlet.

109. 2) Mobility of the coccyx.
– by pressing firmly on it while an
external hand on it can determine its
mobility.
3) Anteroposterior diameter of the outlet:
– From the tip of the sacrum to the
inferior edge of the symphysis. (>11cm)

110. 4) Bituberous diameter:
–Done by first placing a fist between
the ischial tuberosities.
–An 8.5 cm distance (4 knuckles) is
considered to indicate an adequate
transverse diameter.

112. Adequate Pelvis
Data Finding
Forepelvis (pelvic brim) Round.
Diagonal conjugate ≥ 11.5 cm.
Symphysis Average thickness, parallel to sacrum.
Sacrum Hollow, average inclination.
Side walls Straight.
Ischial spines Blunt.
Interspinous diameter ≥ 10.0 cm.
Sacrosciatic notch 2.5 -3 finger - breadths.
Subpubic angle 2fingerbreadths (90o).
Bituberous diameter 4 knuckles (> 8.0 cm).
Coccyx Mobile.
Anterposterior diameter of outlet ≥ 11.0 cm.

113. Radiological Pelvimetry
• X-ray:
– Limited value. No role in guiding management.
• CT:
– Ease of performance, interpretation, & 10% less
radiation exposure to the fetus .
– Can evaluate fetal lie & position.
• MRI (method of choice):
– Lack of ionizing radiation, higher resolution &
contrast but also higher cost.

114. • Indications :
1. Clinical evidence or obstetric
history suggestive of pelvic
abnormalities.
2. A history of pelvic trauma.

115. • CT pelvimetry. Breech presentation.
A. Anteroposterior view is used to measure the transverse
diameter of the pelvic inlet (≥ 11.5 cm).
B. Lateral view is used to measure the anteroposterior
diameter of the inlet (≥10 cm) & midpelvis.
C. Axial view at the level of the fovea of the femoral heads
is used to measure the bi-ischial diameter (≥ 9.5 cm)

116. MRl pelvimetry with AP inlet and outlet
measurements.

117. Cephalometry
• Ultrasonography: is the safe, accurate and easy
method and can detect:
– The biparietal diameter (BPD).
– The occipito-frontal diameter.
– The circumference of the head.

118. Cephalopelvic Disproprtion
• CPD is obstructed labor resulting from disparity
between the size of the fetal head and maternal pelvis.
– E.g. small pelvis, nongynecoid pelvis, large fetus, or
more commonly a combination of these factors.
– True CPD is rare, 1 in 250 pregnancies or 0.4% of the
time
• Failure to progress : lack of progressive cervical
dilatation or lack of fetal descent.
– Mostly due to asynclitism, malpresentation or
ineffective uterine contractions.

119. • Diagnosis:
– Research indicates that pelvimetry is not
a useful diagnostic tool for CPD.
– Unless there’s obvious abnormal pelvis a
‘trial of labor’ is the only true way to
diagnose CPD
difficult to anticipate how well the fetal
head and the maternal pelvis will adjust
& mould to each other.
– Squatting will open up the pelvis at least
33% more.

120. • Treatment of CPD:
–If the surgeon is absolutely certain
that there is CPD, then a CS is the
only option for delivery.
–But if diagnosis is doubtful a ‘trial
of labour’ should always be offered
• If, after sufficient time symptoms of
prolonged labor or fetal distress
begins to develop, a CS needs to be
carried out.