Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull.

1. Fetal Skull and
Maternal Pelvis

2. FETAL SKULL
• Fetal skull is to some extent compressible and made mainly of thin
pliable tabular (flat) bones forming
• the vault. This is anchored to the rigid and incompressible bones at
the base of the skull.

4. • AREAS OF SKULL: The skull is arbitrarily divided into several zones of
obstetrical importance.
• Vertex : It is a quadrangular area bounded anteriorly by the bregma and
coronal sutures behind by the lambda and lambdoid sutures and laterally
by lines passing through the parietal eminences.
• Brow : It is an area bounded on one side by the anterior fontanel and
coronal sutures and on the other side by the root of the nose and
supraorbital ridges of either side.
• Face : It is an area bounded on one side by root of the nose and
supraorbital ridges and on the other, by the junction of the floor of the
mouth with neck.

6. • Sinciput is the area lying in front of the anterior fontanel and
corresponds to the area of brow and the occiput is limited to the
occipital bone.
• Flat bones of the vault are united together by non-ossified
membranes attached to the margins of the bones. These are called
sutures and fontanels. Of the many sutures and fontanels, the
following are of obstetric significance.

7. SUTURES
• The sagittal or longitudinal suture lies between two parietal bones.
• The coronal sutures run between parietal and frontal bones on
either side.
• The frontal suture lies between two frontal bones.
• The lambdoid sutures separate the occipital bone and the two
parietal bones.

9. • Importance:
• (1) It permits gliding movement of one bone over the other during
molding of the head, a phenomenon of significance while the head
passes through the pelvis during labor.
• (2) Digital palpation of sagittal suture during internal examination in
labor gives an idea of the manner of engagement of the head
(asynclitism or synclitism), degree of internal rotation of the head and
degree of molding of the head.

10. FONTANELS
• Wide gap in the suture line is called fontanel. Of the many fontanels
(6 in number), two are of obstetric significance:
• (1) Anterior fontanel or bregma and (2) Posterior fontanel or lambda.
• Anterior fontanel : It is formed by joining of the four sutures in the
midplane. The sutures are anteriorly frontal, posteriorly sagittal and
on either side, coronal.
• The shape is like a diamond. Its anteroposterior and transverse
diameters measure approximately 3 cm each. The floor is formed by a
membrane and it becomes ossified 18 months after birth. It becomes
pathological, if it fails to ossify even after 24 months.

12. • Importance:
• Its palpation through internal examination denotes the degree of flexion of the
head.
• it facilitates molding of the head.
• As it remains membranous long after birth, it helps in accommodating the
marked brain growth; the brain becoming almost double its size during the first
year of life.
• Palpation of the floor reflects intracranial status—depressed in dehydration,
elevated in raised intracranial tension.
• Collection of blood and exchange transfusion, on rare occasion, can be performed
through it via the superior longitudinal sinus.
• Cerebrospinal fluid can be drawn, although rarely, through the angle of the
anterior fontanel from the lateral ventricle.

13. Posterior fontanel
• It is formed by junction of three suture lines — sagittal suture
anteriorly and lambdoid suture on either side. It is triangular in shape
and measures about 1.2 × 1.2 cm (1/2" × 1/2").
• Its floor is membranous but becomes bony at term. Thus, truly its
nomenclature as fontanel is misnomer. It denotes the position of the
head in relation to maternal pelvis.

14. Sagittal fontanel
• It is inconsistent in its presence. When present, it is situated on the
sagittal suture at the junction of anterior two-third and posterior one-
third. It has got no clinical importance.

15. DIAMETERS OF SKULL
• The engaging diameter of the fetal skull depends on the degree of flexion
present.
• Biparietal diameter—9.5 cm (3 ¾"): It extends between two parietal
eminences. Whatever may be the position of the head, this diameter
nearly always engages.
• Super-subparietal—8.5 cm (3 ½"): It extends from a point placed below
one parietal eminence to a point placed above the other parietal
• eminence of the opposite side.
• Bitemporal diameter—8 cm (3 ¼"): It is the distance between the
anteroinferior ends of the coronal suture.
• Bimastoid diameter — 7.5 cm (3"): It is the distance between the tips of
the mastoid processes. The diameter is incompressible and it is impossible
to reduce the length of the bimastoid diameter by obstetrical operation.

17. • CIRCUMFERENCES: Circumference of the plane of the diameter of
engagement differs according to the attitude of the head.
• MOLDING: It is the alteration of the shape of the fore-coming head
while passing through the resistant birth passage during labor.
There is, however, very little alteration in size of the head, as volume
of the content inside the skull is incompressible although small
amount of cerebrospinal fluid and blood escape out in the process.
During normal delivery, an alteration of 4 mm in skull diameter
commonly occurs.

18. • Mechanism: There is compression of the engaging diameter of the head
with corresponding elongation of the diameter at right angle to it.
• Thus, in well flexed head of the anterior vertex presentation, the engaging
suboccipitobregmatic diameter is compressed with elongation of the head
in mento-vertical diameter which is at right angle to suboccipitobregmatic.
• During the process, the parietal bones tend to overlap the adjacent bones,
viz. the occipital bone behind, the frontal bones in front and the temporal
bones at the sides.
• In first vertex position, the right parietal bone tends to override the left
one and this becomes reverse in second vertex position. Molding
disappears within few hours after birth.

19. • Grading: There are three gradings.
• Grade-1—the bones touching but not overlapping,
• Grade-2—
• overlapping but easily separated and
• Grade-3—fixed overlapping.

21. Mechanism of formation
• While the head descends to press over the dilating cervix or
vulval ring, the overlying scalp is free from pressure, but the
tissues in contact with the full circumference of the girdle of
contact is compressed. This interferes with venous return
and lymphatic drainage from the unsupported area of scalp
→ stagnation of fluid and appearance of a swelling in the
scalp. Caput usually occurs after rupture of the membranes.

22. • Importance:
• It signifies static position of the head for a long period of time.
• Location of the caput gives an idea about the position of the head
occupied in the pelvis and the degree of flexion achieved. In left
position, the caput is placed on right parietal bone and in right
position, on left parietal bone. With increasing flexion, the caput is
placed more posteriorly.

23. CAPUT SUCCEDANEUM
• It is the formation of swelling due to stagnation of
fluid in the layers of the scalp beneath the girdle of
contact.
• The girdle of contact is either bony or the dilating
cervix or vulval ring. The swelling is diffuse, boggy and
is not limited by the suture line. It may be confused
with cephalhematoma. It disappears spontaneously
within 24 hours after birth.

26. PELVIS
• 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 and coccyx. These are united
together by four joints—two sacroiliac joints, sacrococcygeal joint and
the symphysis pubis.

29. • The pelvis is anatomically divided into a false pelvis and a true pelvis,
the boundary line being the brim of the pelvis.
• The bony landmarks on the brim of the pelvis from anterior to
posterior on each side are—upper border of symphysis pubis, pubic
crest, pubic tubercle, pectineal line, iliopubic eminence, iliopectineal
line, sacroiliac articulation, anterior border of the ala of sacrum and
sacral promontory.

30. FALSE PELVIS
• 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.

31. TRUE PELVIS
• 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 and outlet.
• The pelvic measurements given in the text are average when
measured radiologically and vary within a limited degree in different
countries. The conversion of centimeters into inches is approximate.

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

35. • 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.
• 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.

36. • 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.
• 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.

37. • Diameters: The measurements of the diameters are all approximate
and minor variation is the rule rather than the exception.
• 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.
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.

38. • 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.

39. • 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.
• 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.

41. • 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.

42. • 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.

43. CAVITY
• 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.

44. • Axis: It is the mid-perpendicular line drawn to the plane of the cavity.
Its direction is almost downward.
• Diameters: 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.

45. OUTLET
• 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.
• 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.

46. • Diameters: 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.
• 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.

47. • 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.
• 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

48. • Diameters: Anteroposterior: It extends from the lower border of the symphysis pubis to the tip of
the coccyx. It measures 13 cm or 5 ¼" with the coccyx pushed back by the head when passing
through the introitus in the second stage of labor; with the coccyx in normal position, the
measurement will be 2.5 cm less.
• Transverse — Syn : Intertuberous (11 cm or 4 ¼"): It measures between inner borders of ischial
tuberosities.
• Posterior sagittal diameter (8.5 cm or 3 ½"): It is the anteroposterior distance between the
sacrococcygeal joint and the midpoint of transverse diameter of outlet (TDO). It is clinically
measured by the distance between the sacrococcygeal joint and anterior margin of the anus.
• Subpubic angle: It is formed by the approximation of the two descending pubic rami. In normal
female pelvis, it measures 85°.
• Pubic arch: Arch formed by the descending rami of both the sides is of obstetric importance.
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.

49. MIDPELVIS
• Midpelvis is the segment of the pelvis bounded above by the plane
of greatest pelvic dimensions and below by a plane known as
midpelvic plane.
• Midpelvic plane: The midpelvic plane extends from the lower margin
of the symphysis pubis through the level of ischial spines to meet
either the junction of S4 and S5 or tip of the sacrum depending upon
the configuration of the sacrum. If the plane meets the tip of the fifth
sacrum, it coincides with the plane of least pelvic dimensions. If the
plane meets the junction of S4 and S5, the plane becomes a wedge
posteriorly

50. • Diameters: Transverse diameter — Syn : bispinous (10.5 cm) : It
measures between the two ischial spines.
• Anteroposterior diameter (11.5 cm): It extends from the lower
border of the symphysis pubis to the point on the sacrum at which
the midpelvic plane meets.
• Posterior sagittal diameter (4.5 cm): It extends from the midpoint of
the bispinous diameter to the point on the sacrum at which the
midpelvic plane meets.

51. PELVIC AXIS
• Anatomical (curve of Carus): Anatomical pelvic axis is formed by
joining the axes of inlet, cavity and outlet. It is uniformly curved with
the convexity fitting with the concavity of the sacrum. The fetus does
not, however, transverse the uniform curved path.
• Obstetrical: It is through this axis that the fetus negotiates the pelvis.
It is not uniformly curved. Its direction is first downward and
backward up to the level of ischial spines and then directed abruptly
forward

52. PELVIC JOINTS
• 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.
• 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.
• Sacrococcygeal joint is a synovial hinge joint. It allows both flexion and
extension. Extension increases the anteroposterior diameter of the outlet.