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Anatomic Characteristics of the Fetal Head and Maternal Pelvis

Du Xue , PHDDepartment of Obstetrics & Gynecology General Hospital of TianJinMedical University

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Fetal head ---caranium ---sutures ---fontanelles ---landmarks ---diameters

Pelvis ---Bony pelvis ---planes ---diameters ---landmarks ---shapes

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The fetal skull Base: to protect the vital structures ---large ---ossified ---firmly united ---noncompressible Vault(cranium): to overlap under pressure and to cha

nge shape to confirm to the maternal pelvis (molding)

---thin ---weakly ossified ---easily compressible ---interconnected by membranes

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Fetal Head---the cranium

Occipital bone (posteriorly) Two parietal bone (bilaterally) Two frontal bone (anteriorly) Two temporal bone (anteriorly)

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Fetal Head---Sutures the membrane-occup

ied space between the cranial bone

Sagittal Lambdoid Coronal Frontal

Sagittal:Between the parietal bones;anteroposterior directionDivide Right and left

Lambdoid:Extends from the posterior fontanelle laterallySeparate occipital from parietal bones

Coronal :Extends from the anterior fontanelle laterallySeparate parietal from frontal bones

Frontal :Between the frontal bonesExtends from the anterior fontanelle to the glabella

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Fetal Head--- Fontanelles Defininion:the membrane-filled space located at

the point where the sutures intersect. The most important of which are the anterior an

d posterior fontanelles. More useful in diagnosing the fetal head position

than sutures

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intersection of sagittal, frontal, coronal sutures

diamond shaped about 2x3cm closes at 18 months

after birth

The anterior fontanelle

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The posterior fontanelle Junction of the s

agittal and lambdoid sutures

Y or T shaped closes at 6-8 we

eks after birth.

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Fetal Head--- landmarks Nasion Glabella Sinciput Anterior font

anelle Vertex Posterior fo

ntanelle Occiput

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Nasion:The root of the nose

Glabella:the elevated area between the orbital ridges

Sinciput: the area between the anterior fontanelle and the glabella

Anterior fontanelle:diamond shapedVertex:between the fontanel

les and bounded laterally by the eminences

Posterior fontanelle:Y or T shaped

Occiput:behind and inferior to the posterior fontanelle and lambdoid sutures

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Fetal Head--- Diameters The anteroposterior diameters ---Suboccipitobregmntic diameter (9.5cm) ---Occipital frontal diameter (11cm) ---Supraoccipitomental diameter (13.5cm) ---Submentobregmatic diameter (9.5cm) Transverse diameter ---Biparietal diameter (9.5cm) ---Bitemporal diameter (8 cm) Average circumference of the term fetal head(in the occi

pitofrontal plane)-----------------------(34.5cm)

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Suboccipitobregmatic (9.5cm)

From the undersurface of occipital to the center of the ant fontanelle

Occipitalfrontal(11cm)

From external occipital protuberance to the glabella

Supraoccipitomental (13.5cm)From the vertex to the chin

Submentobregmatic (9.5cm)From the junction of the neck and lower jaw to the center of the ant fontanelle

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Biparietal (9.5cm) largestExtend between the parietal bones

Bitempotal (8cm) shortestExtend between the temporal bones

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Maternal pelvis

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Maternal pelvis----Clinical Pelvimetry

Not all the diameters can be assesed clinically

Assess the diameters at the first prenatal visit to screen for obvious pelvic contractions or in the late pragnancy(may be less uncomfortable and more accurate)

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Inlet plane:The obstetric conjugate The diagonal conjugate:is appr

oximated by measuring from the lower border of the pubis to the sacral promontary using the tip of the second finger and the point where the index finger meets the pubis.

The obstetric conjugate=the diagonal conjugate-(1.5~2cm)

normal value:11.5cm

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Inlet plane:the sacrum The anterior surface of the sa

crum is palpated to assess its curvature.

The usual shape is concave. A flat or convex shape may indicate anteroposterior constriction throughout the pelvis.

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Inlet plane---transverse diameter interspinal diameter

( 髂前上棘间径， IS) ---23-26cm Intercristal diameter

( 髂嵴间径， IC) ---26-28cm

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Mid plane: cannot be accurately measured clinically

Bispinous diameter(10.5cm) （坐骨棘间径） ---The ischial spines are palpated carefully to assess th

eir prominence. the sacrospinous ligament （骶棘韧带） ---assessed by placing one finger of the ischial spine an

d one finger on the sacrum in the midline ---The average length is three fingerbreadths ---A short ligament suggest a forward inclination of the

sacrum and a narrowed sacrosciatic notch

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Outlet plane: Bituberous diameter and the infrapubic angle

Bituberous diameter （坐骨结节间径） ---first placing a fist between the ischial tuberosities ---Normal value:8.5cm or above The infrapubic angle （耻骨联合角度） ---placing a thumb next to each inferior pubic ramus an

d then estimating the angle at which they meet ---<90º is associated with a contracted transverse diame

ter in the midplane and outlet.

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Bituberous diameter

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Engagement Engagement occurs when th

e widest diameter of the fetal presenting part has passed through the pelvic inlet.

in cephalic presentations, the widest diameter is biparietal;intertrochanteric(breech)

When the fetal head has engaged, the bony presenting part is at the level of the ischial spine.

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The station of the presenting part in the pelvic canal is defined as it's level above or below the plane of the ischial spine.

The level of ischial spineis assigned as “zero”, each cm above or below this level is given a minus or plus designation.

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When the presenting is out of the pelvis, it is freely movable, it is considered to be floating.

When it has passed through the plane of the inlet ,but is not yet engaged, it is considered to be dipping.

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