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PELVIS OBG SYN. 7

🦴 Pelvis

📘 Important for Obstetric Anatomy, Midwifery, and Labor Management

✅ I. Introduction / Definition:

The pelvis is a bony ring-like structure at the lower end of the trunk that connects the spine to the lower limbs and supports the uterus, bladder, rectum, and reproductive organs.

✅ “The pelvis is the lower part of the trunk, made up of bones that form the bony canal through which the fetus passes during birth.”

🧱 II. Bones of the Pelvis:

Each hip bone is made up of 3 fused bones:

  • Ilium – upper, fan-shaped part
  • Ischium – lower, posterior part (forms the ischial tuberosity)
  • Pubis – anterior part; forms pubic symphysis

Other important bones:

  • Sacrum – 5 fused vertebrae forming posterior pelvic wall
  • Coccyx – terminal bone of vertebral column

🩻 III. Parts of the Pelvis:

🧩 Part📝 Description
Greater (false) pelvisAbove pelvic brim; supports abdominal organs
Lesser (true) pelvisBelow pelvic brim; forms the birth canal

📏 IV. Pelvic Planes and Diameters:

🔹 1. Pelvic Inlet (Brim):

🔸 Entry to true pelvis
🔸 Diameters:
• Anteroposterior (True conjugate): ~11 cm
• Transverse diameter: ~13 cm
• Oblique diameter: ~12 cm

🔹 2. Pelvic Cavity:

🔸 Space between inlet and outlet
🔸 Rounded and curved

🔹 3. Pelvic Outlet:

🔸 Lower boundary of the true pelvis
🔸 Diameters:
• Anteroposterior: ~11.5 cm
• Transverse: ~11 cm

✅  Diameters of Pelvic Inlet:

DiameterMeasurementDescription
True (Anatomical) Conjugate~11 cmFrom sacral promontory to upper margin of pubic symphysis
Obstetric Conjugate~10.5 cmFrom sacral promontory to inner surface of pubic symphysis (most important for labor)
Diagonal Conjugate~12.5–13 cmFrom sacral promontory to lower border of pubic symphysis; measurable per vaginally
Transverse Diameter~13 cmWidest horizontal distance across pelvic brim (between iliopectineal lines)
Oblique Diameter~12 cmFrom sacroiliac joint to opposite iliopectineal eminence

✅  Diameters of Midpelvis (Pelvic Cavity):

DiameterMeasurementDescription
Interspinous Diameter~10.5 cmBetween the two ischial spines; narrowest transverse diameter of pelvis
Anteroposterior (AP) Diameter~11–12 cmFrom posterior surface of symphysis pubis to sacrum
Plane of least pelvic dimensions–Formed by interspinous diameter and posterior sagittal diameter

✅  Diameters of Pelvic Outlet:

DiameterMeasurementDescription
Anteroposterior Outlet Diameter~11.5 cmFrom lower border of pubic symphysis to tip of sacrum
Transverse Outlet Diameter~11 cmBetween inner borders of ischial tuberosities
Posterior Sagittal Diameter~7.5 cmFrom sacrococcygeal joint to the transverse line between ischial tuberosities

🧬 V. Types of Female Pelvis (Caldwell-Moloy Classification):

1. Gynecoid Pelvis (Ideal Female Pelvis)

  • Rounded and wide pelvic inlet.
  • Subpubic angle is wide (>80°).
  • Sacrum is broad and curved backward.
  • Ischial spines are not prominent.
  • Most suitable for normal vaginal delivery (found in ~50% of women).

2. Android Pelvis (Male-Type Pelvis)

  • Heart-shaped (triangular) pelvic inlet.
  • Subpubic angle is narrow (<70°).
  • Prominent ischial spines and deep cavity.
  • Associated with obstructed labor and deep transverse arrest.
  • High risk of cesarean delivery (found in ~20% of women).

3. Anthropoid Pelvis

  • Oval-shaped pelvic inlet with long anteroposterior diameter.
  • Narrow transverse diameter.
  • Sacrum is long and narrow.
  • Often associated with occiput posterior fetal position.
  • Vaginal delivery is possible but may require interventions.

4. Platypelloid Pelvis (Flat Pelvis)

  • Flat pelvic inlet with wide transverse and short anteroposterior diameter.
  • Subpubic angle is wide, but inlet is shallow.
  • Least favorable for normal vaginal delivery.
  • High risk of prolonged labor and cesarean section (found in ~5% of women).

✅ Gynecoid pelvis is the most favorable for vaginal delivery.

🔍 VI. Obstetrical Importance of the Pelvis:

🔹 Determines ease of vaginal delivery
🔹 Narrow pelvis or abnormal shape → may cause obstructed labor
🔹 Pelvic assessment helps decide mode of delivery
🔹 CPD (Cephalopelvic Disproportion) occurs when fetal head is too large for pelvic cavity

👩‍⚕️ VII. Clinical Pelvimetry:

🩺 Pelvic assessment done during antenatal visits or labor to measure the adequacy of pelvis.

📋 Methods🛠️ Purpose
External pelvimetryUsing pelvic calipers on surface landmarks
Internal pelvimetryDone via vaginal examination
Imaging (X-ray, CT, USG)In rare cases or complex pelvis evaluation

👩‍⚕️ VIII. Nursing Responsibilities:

🟩 During Antenatal Care:
🔹 Identify signs of small or abnormal pelvis
🔹 Educate about fetal positions and posture
🔹 Refer for pelvimetry if high-risk

🟨 During Labor:
🔹 Assist obstetrician in assessing engagement
🔹 Support mother if prolonged or obstructed labor occurs
🔹 Prepare for operative delivery if pelvis is inadequate

📚 Golden One-Liners for Quick Revision:

🟡 Gynecoid is the most favorable pelvis for labor
🟡 Pelvic brim marks entry to true pelvis
🟡 Platypelloid pelvis is flat and unfavorable for vaginal delivery
🟡 Android pelvis is male-type and narrow
🟡 True conjugate is ~11 cm and important for fetal head entry

✅ Top 5 MCQs for Practice:

Q1. The most favorable pelvis type for vaginal delivery is:
🅰️ Android
🅱️ Platypelloid
✅ 🅲️ Gynecoid
🅳️ Anthropoid
Correct Answer: 🅲️ Gynecoid

Q2. Which part of the pelvis forms the birth canal?
🅰️ Greater pelvis
✅ 🅱️ Lesser pelvis
🅲️ Ilium
🅳️ Sacroiliac joint
Correct Answer: 🅱️ Lesser pelvis

Q3. The transverse diameter of the pelvic inlet is approximately:
🅰️ 9 cm
🅱️ 10 cm
✅ 🅲️ 13 cm
🅳️ 15 cm
Correct Answer: 🅲️ 13 cm

Q4. The pelvic diameter most important for engagement is:
🅰️ Oblique
🅱️ Transverse
✅ 🅲️ True conjugate
🅳️ Diagonal conjugate
Correct Answer: 🅲️ True conjugate

Q5. Which pelvis is flat and least suitable for vaginal delivery?
🅰️ Gynecoid
🅱️ Android
✅ 🅲️ Platypelloid
🅳️ Anthropoid
Correct Answer: 🅲️ Platypelloid

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Categorized as OBG-PHC-SYNOPSIS, Uncategorised