5.Anatomic Characteristics Of The Fetal Head And MaternalPresentation Transcript
Anatomic Characteristics of the Fetal Head and Maternal Pelvis
Du Xue , PHD
Department of Obstetrics
General Hospital of TianJin
The fetal skull
Base: to protect the vital structures
Vault(cranium): to overlap under pressure and to change shape to confirm to the maternal pelvis (molding)
---interconnected by membranes
Fetal Head--- the cranium
Occipital bone (posteriorly)
Two parietal bone (bilaterally)
Two frontal bone (anteriorly)
Two temporal bone (anteriorly)
the membrane-occupied space between the cranial bone
Sagittal: Between the parietal bones; anteroposterior direction Divide Right and left Lambdoid: Extends from the posterior fontanelle laterally Separate occipital from parietal bones Coronal : Extends from the anterior fontanelle laterally Separate parietal from frontal bones Frontal : Between the frontal bones Extends from the anterior fontanelle to the glabella
Fetal Head--- Fontanelles
Defininion:the membrane-filled space located at the point where the sutures intersect.
The most important of which are the anterior and posterior fontanelles.
More useful in diagnosing the fetal head position than sutures
intersection of sagittal, frontal, coronal sutures
closes at 18 months after birth
The anterior fontanelle
The posterior fontanelle
Junction of the sagittal and lambdoid sutures
Y or T shaped
closes at 6-8 weeks after birth.
Fetal Head--- landmarks
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 shaped Vertex:between the fontanelles and bounded laterally by the eminences Posterior fontanelle:Y or T shaped Occiput:behind and inferior to the posterior fontanelle and lambdoid sutures
Fetal Head--- Diameters
The anteroposterior diameters
---Suboccipitobregmntic diameter (9.5cm)
---Occipital frontal diameter (11cm)
---Supraoccipitomental diameter (13.5cm)
---Submentobregmatic diameter (9.5cm)
---Biparietal diameter (9.5cm)
---Bitemporal diameter (8 cm)
Average circumference of the term fetal head(in the occipitofrontal plane)-----------------------(34.5cm)
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
Biparietal (9.5cm) largest Extend between the parietal bones Bitempotal (8cm) shortest Extend between the temporal bones
Have A Break
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)
Inlet plane: The obstetric conjugate
The diagonal conjugate :is approximated 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)
Inlet plane :the sacrum
The anterior surface of the sacrum is palpated to assess its curvature.
The usual shape is concave. A flat or convex shape may indicate anteroposterior constriction throughout the pelvis.
Inlet plane--- transverse diameter
interspinal diameter ( 髂前上棘间径， IS)
Intercristal diameter ( 髂嵴间径， IC)
Mid plane: cannot be accurately measured clinically
Bispinous diameter(10.5cm) （坐骨棘间径）
---The ischial spines are palpated carefully to assess their prominence.
the sacrospinous ligament （骶棘韧带）
---assessed by placing one finger of the ischial spine and 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
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 and then estimating the angle at which they meet
---<90º is associated with a contracted transverse diameter in the midplane and outlet.
Engagement occurs when the 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.
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.
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.