Fetal Skull Anatomy and Dimensions

• The fetal skull consists of the occipital bone, two parietal bones, and two frontal bones.
• Sutures and fontanelles separate the fetal skull bones.
• The main sutures include the frontal, coronal, sagittal, and lambdoid sutures.
• The posterior fontanelle is triangle-shaped and located at the junction of the sagittal and lambdoid sutures.
• The anterior fontanelle is diamond-shaped and found at the intersection of the coronal, sagittal, and frontal sutures.

Dimensions of the Fetal Skull

• The fetal skull is oval-shaped.
• The presenting diameter and circumference vary based on the fetal position relative to the maternal pelvis.
• In an occiput presentation with maximum flexion, the presenting diameter is suboccipitobregmatic (9.5-10 cm) with a circumference of 32-33 cm.
• In a vertex presentation, the presenting diameter is suboccipitofrontalis (10 cm) with a circumference of 3 cm.
• In a sinciput presentation, the presenting diameter is frontooccipitalis (12 cm) with a circumference of 34-35 cm.
• In a brow presentation, the presenting diameter is mentooccipitalis (13.5-14 cm) with a circumference of 35 cm.
• In a face presentation, the presenting diameter is submentobregmatic (9.5 cm) with a circumference of 34 cm.
• The bitemporal diameter measures 8-8.5 cm.
• The biparietal diameter, the widest part of the fetal head, measures about 9.5 cm.
• Vaginal birth is likely possible once the biparietal diameter passes the pelvic inlet.

Maternal Pelvis

• The pelvic inlet is oval-shaped, with a transverse diameter (13 cm) wider than its anteroposterior diameter (11 cm).
• The pelvic outlet is anteroposterior oval-shaped, with a transverse diameter of 11 cm and an anteroposterior diameter of approximately 11.5 cm.
• The birth canal's orientation changes from cranial to caudal at the pelvic inlet to dorsal to ventral at the pelvic outlet.
• Fetal descent occurs at a 90-degree angle, following the pelvic axis, an imaginary line through the center of the pelvic inlet, cavity, and outlet.

Fetal Position

• The lie refers to the relationship between the long axis of the fetus and the mother.
• Longitudinal lies can be either cephalic or breech.
• A transverse lie is also possible.
• The presenting part is the portion of the fetus closest to the pelvic outlet.
• In an occiput anterior presentation, the occiput is the presenting part.
• In a sinciput presentation, it is the part of the sinciput near the anterior fontanel.
• In a brow presentation, the brow is the presenting part.
• In a face presentation, the face is the presenting part.
• The determining point describes the position of the presenting part relative to the maternal pelvis.
• In sinciput, occiput posterior, and occiput anterior presentations, the determining point is the occiput.
• In brow and face presentations, it is the mentum (chin).
• In a breech presentation, the determining point is the sacrum.
• The terms occiput, vertex, sinciput, and mentum (chin) divide the fetal head and are used to name the presenting part.
• Sutures and fontanelles are palpated to determine the fetal head's position.
• Fetal position is the orientation of the determining point relative to the maternal pelvis.
• The pelvis is divided into anterior, left anterior, left transverse, left posterior, posterior, right posterior, right transverse, and right anterior sections.
• Annotation for a left occipito posterior presentation is written as LOP.
• Attitude refers to the position of the head relative to the trunk, ranging from full flexion (chin to chest) to full extension (occiput to dorsum).
• In an occiput presentation, the fetal head is maximally flexed.
• In a vertex presentation, the fetal head is flexed.
• In a sinciput presentation, the head is neither flexed nor extended.
• In a brow presentation, the head is extended.
• In a face presentation, the head is maximally extended.

Key Movements During Childbirth

• Engagement is determined by the amount of the head above or below the pelvic brim.
• Engagement is complete when the widest diameter of the head or breech passes through the pelvic inlet.
• Descent is the downward movement of the head towards the pelvic outlet - occurring simultaneously with engagement.
• Flexion occurs due to resistance from the cervix, pelvic walls, and pelvic floor, causing the fetal head to bend with the chin touching the chest, creating a smaller presenting diameter.
• Internal rotation is essential for the fetal head to pass the pelvic outlet, facilitated by both maternal and fetal components.
• Fetal components include the flexibility of the head relative to the trunk and the eccentric pole.
• Maternal components are contraction strength, tissue resistance, and the shape of the birth canal.
• Extension involves a decrease in flexion due to the forward-bending curve of the birth canal; the head extends and is born by rotation around the symphysis.
• External rotation occurs with internal rotation of the shoulders to pass the pelvic outlet, seen externally as the head's rotation around the long axis of its dorsum.
• Expulsion involves the birth of the remainder of the body.

Additional Terminology

• Synclitism and asynclitism describe the position of the sagittal sutures relative to the pelvic axis.
• Synclitism: the sagittal suture is in line with the pelvic axis in the anterior plane.
• Posterior asynclitism: the sagittal suture is positioned in front of the pelvic axis in the anterior plane.
• Anterior asynclitism: the sagittal suture is positioned behind the pelvic axis in the posterior plane.
• The eccentric pole is the deepest point of the presenting part within the pelvis, not along the pelvic axis.
• In a vertex presentation, the eccentric pole is the occiput.
• In a sinciput presentation, the eccentric pole is absent.
• In brow and face presentations, the eccentric pole is the chin.
• The eccentric pole follows the path of least resistance and usually rotates anteriorly to be positioned under the maternal symphysis pubis.
• The point of rotation is the part of the fetal body that passes under the pubic symphysis by rotation during the head's birth.
• In occiput and vertex presentations with the occiput anterior, the point of rotation is the posterior hairline.
• In a vertex presentation with the occiput posterior, the point of rotation is the anterior hairline.
• In a sinciput presentation, the point of rotation is the glabella.
• In a face presentation, the point of rotation is the larynx.