Unit I: Anatomy & Physiology
The Bony Pelvis
1. The Pelvic Joints:
· Symphysis Pubis: A cartilaginous joint between the two pubic bones. During pregnancy, the hormone Relaxin
softens this joint, allowing a slight separation (up to 1 cm) to widen the birth canal. This is why women
experience "waddling gait" and pelvic pain in the third trimester.
2. The Pelvic Planes and Diameters (Exam Critical):
The pelvis is not a uniform cylinder; it's a curved canal with three distinct levels. You must know the
diameters of the Inlet because the baby enters here, and the Outlet because the baby exits here.
Level Key Diameter Normal Measurement Clinical Significance
Inlet Obstetric Conjugate 10.5 cm or more The most important measurement. If this is <10 cm, the head cannot
enter (CPD).
Midpelvis Interspinous Diameter 10.5 cm Narrowest part of the pelvis. If the baby's head stops here, it's Deep
Transverse Arrest.
Outlet Intertuberous Diameter 11 cm If this is narrow (<8 cm), the mother tears severely during crowning.
The Fetal Skull
1. Regional Naming (The "Vertex" Presentation):
The fetal head is divided into regions from top to bottom:
· Vertex: The area bounded by the two parietal bones and the anterior fontanelle. This is the most common
and most favorable presentation because it allows the head to flex and present the smallest diameter.
· Sinciput: The brow area.
· Face: The chin is the presenting part. Nursing Alert: If the chin is posterior (Mento-Posterior), vaginal delivery
is usually impossible and requires C-section.
2. Important Diameters of the Fetal Skull:
The head molds to fit the pelvis. You need to know which diameter engages in which pelvic type.
· Suboccipitobregmatic (9.5 cm): The smallest anteroposterior diameter. It presents when the head is well-
flexed (Vertex presentation).
· Occipitofrontal (11.5 cm): Presents when the head is deflexed (Military attitude).
· Mentovertical (13.5 cm): Presents when the head is partially extended (Brow presentation). This is the largest
diameter; vaginal delivery is impossible.
· Submentobregmatic (9.5 cm): Presents when the head is fully extended (Face presentation).
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Unit II: Normal Pregnancy
Physiological Changes - Why It Matters for Nursing Care
The Bony Pelvis
1. The Pelvic Joints:
· Symphysis Pubis: A cartilaginous joint between the two pubic bones. During pregnancy, the hormone Relaxin
softens this joint, allowing a slight separation (up to 1 cm) to widen the birth canal. This is why women
experience "waddling gait" and pelvic pain in the third trimester.
2. The Pelvic Planes and Diameters (Exam Critical):
The pelvis is not a uniform cylinder; it's a curved canal with three distinct levels. You must know the
diameters of the Inlet because the baby enters here, and the Outlet because the baby exits here.
Level Key Diameter Normal Measurement Clinical Significance
Inlet Obstetric Conjugate 10.5 cm or more The most important measurement. If this is <10 cm, the head cannot
enter (CPD).
Midpelvis Interspinous Diameter 10.5 cm Narrowest part of the pelvis. If the baby's head stops here, it's Deep
Transverse Arrest.
Outlet Intertuberous Diameter 11 cm If this is narrow (<8 cm), the mother tears severely during crowning.
The Fetal Skull
1. Regional Naming (The "Vertex" Presentation):
The fetal head is divided into regions from top to bottom:
· Vertex: The area bounded by the two parietal bones and the anterior fontanelle. This is the most common
and most favorable presentation because it allows the head to flex and present the smallest diameter.
· Sinciput: The brow area.
· Face: The chin is the presenting part. Nursing Alert: If the chin is posterior (Mento-Posterior), vaginal delivery
is usually impossible and requires C-section.
2. Important Diameters of the Fetal Skull:
The head molds to fit the pelvis. You need to know which diameter engages in which pelvic type.
· Suboccipitobregmatic (9.5 cm): The smallest anteroposterior diameter. It presents when the head is well-
flexed (Vertex presentation).
· Occipitofrontal (11.5 cm): Presents when the head is deflexed (Military attitude).
· Mentovertical (13.5 cm): Presents when the head is partially extended (Brow presentation). This is the largest
diameter; vaginal delivery is impossible.
· Submentobregmatic (9.5 cm): Presents when the head is fully extended (Face presentation).
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Unit II: Normal Pregnancy
Physiological Changes - Why It Matters for Nursing Care
