Maternal Pelvis Notes for NEET PG Exam

Maternal Pelvis

Maternal pelvis is a crucial anatomical structure formed by two innominate bones (hip bones), the sacrum, and the coccyx. These bones are connected anteriorly at the pubic symphysis, a cartilaginous joint, and posteriorly at the sacroiliac joints, which are strong, weight-bearing joints. The pelvis supports the spine and transmits the weight of the upper body to the lower limbs, playing a vital role in pregnancy and childbirth.

Pelvic Brim

The pelvic brim (also known as the linea terminalis) is a continuous oval ridge that separates the false pelvis from the true pelvis. It is an important landmark in obstetrics. The structures forming the pelvic brim, from anterior to posterior, include:

1. Pubic Symphysis: The cartilaginous joint where the two pubic bones meet.

2. Pubic Crest: The thickened superior border of the pubic bone, extending laterally from the pubic symphysis.

3. Pubic Tubercle: A small, rounded prominence on the lateral end of the pubic crest; serves as an attachment point for the inguinal ligament.

4. Upper Border of the Ascending Ramus of the Pubic Bone: The superior part of the pubic bone that extends upward and laterally.

5. Ileopectineal Line with the Ileopectineal Eminence:

   • Ileopectineal Line (Arcuate Line): A ridge on the inner surface of the ilium that marks the boundary between the body and the ala of the ilium.

   • Ileopectineal Eminence: A rounded elevation at the junction of the iliac and pubic bones.

6. Sacroiliac Joint: The joint between the sacrum and the ilium, reinforced by strong ligaments.

7. Ala of the Sacral Bone: The wing-like lateral masses of the sacrum that articulate with the iliac bones.

8. Sacral Promontory: The most anterior part of the sacrum, projecting forward into the pelvic cavity. It is the superior, prominent edge of the first sacral vertebra ($S1$).

False vs. True Pelvis

• False Pelvis (Pelvis Major):

  • Lies above the pelvic brim.

  • Bounded by the iliac crests laterally and the lumbar vertebrae posteriorly.

  • Supports the gravid uterus during pregnancy but does not directly participate in childbirth.

• True Pelvis (Pelvis Minor):

  • Lies at or below the pelvic brim.

  • Forms a bony canal through which the fetus must pass during birth.

  • Consists of three parts:

    • Pelvic Inlet: The entrance to the true pelvis, at the level of the pelvic brim.

    • Pelvic Outlet: The inferior opening of the true pelvis, at the level of the ischial tuberosities.

    • Pelvic Cavity: The space between the inlet and outlet.

Planes of the True Pelvis

• Plane of Inlet:

  • Located at the pelvic brim.

  • Defined by the sacral promontory, arcuate lines, and the upper margin of the pubic symphysis.

• Plane of Outlet:

  • Located at the ischial tuberosities.

  • Defined by the lower margin of the pubic symphysis, ischial tuberosities, and the tip of the coccyx.

• Pelvic Cavity:

  • The space between the inlet and outlet.

  • Contains the plane of greatest pelvic dimensions and the plane of least pelvic dimensions, which are critical for fetal passage.

Pelvic Inlet

• Anterior Boundary: Pubic symphysis (upper border).

• Posterior Boundary: Sacral promontory (the anterior edge of the first sacral vertebra).

• Lateral Boundary: Ileopectineal line (arcuate line).

AP Diameters of the Pelvic Inlet

1. True Conjugate (Anatomical Conjugate):

   • From the upper border of the pubic symphysis to the middle of the sacral promontory.

   • Measures approximately $11$ cm.

   • Represents the shortest distance between the sacral promontory and the pubic symphysis.

2. Obstetrical Conjugate:

   • From the middle of the pubic symphysis to the sacral promontory accurately (approximately 1.5 cm below the true conjugate).

   • Measures $10$ to $10.5$ cm.

   • The smallest and most important AP diameter of the inlet because it is the minimum space through which the fetal head must pass. An adequate obstetrical conjugate ensures the fetal head can engage into the pelvis.

3. Diagonal Conjugate:

   • From the lower border of the pubic symphysis to the sacral promontory.

   • Measures $12$ cm.

   • This is the AP diameter that can be measured clinically during a vaginal examination, making it a valuable tool for assessing pelvic size.

Critical Obstetrical Conjugate

The value of the obstetrical conjugate below which vaginal delivery is unlikely or not possible. A pelvis with an obstetrical conjugate less than $10$ cm is considered a contracted pelvis, potentially leading to obstructed labor.

$Obstetrical Conjugate = Diagonal Conjugate - 1.5 ",to," 2 ",cm$

Clinical Measurement of Diagonal Conjugate

During a vaginal examination, if the sacral promontory cannot be reached, the diagonal conjugate is at least $12$ cm, indicating an adequate pelvis for vaginal delivery. This assessment helps determine if the pelvic inlet is large enough for the fetal head to pass through.

Transverse Diameter of the Inlet

The distance between the two farthest points on the ileopectineal line (arcuate line); typically measures $13$ cm. This is the widest diameter of the pelvic inlet, facilitating the fetal head's entry into the pelvis in a transverse position.

Oblique Diameter of the Inlet

The distance between the sacroiliac joint of one side to the ileopectineal eminence of the opposite side; measures approximately $12$ cm. There are two oblique diameters:

• Right Oblique: From the right sacroiliac joint to the left iliopectineal eminence.

• Left Oblique: From the left sacroiliac joint to the right iliopectineal eminence.

The fetal head typically engages into the pelvis in one of the oblique diameters, allowing it to navigate through the inlet.

Shape of Inlet

In a normal female pelvis (gynecoid pelvis), the transverse diameter is greater than the AP diameter, resulting in a transverse oval or gynecoid shape. This shape facilitates the entry of the fetal head in a transverse position, which is the most common orientation for labor.

Pelvic Cavity

The pelvic cavity is the space between the pelvic brim and the outlet. It is a critical area through which the fetus must pass during delivery. The cavity contains two important planes:

1. Plane of Greatest Pelvic Dimensions

2. Plane of Least Pelvic Dimensions

The shape of the pelvic cavity is like a truncated cylinder, with relatively consistent dimensions throughout.

Plane of Greatest Pelvic Dimensions

• Anterior Boundary: Middle of the pubic symphysis.

• Posterior Boundary: Junction of the second and third sacral vertebrae ($S2$, $S3$).

• Lateral Boundary: Obturator foramen.

All diameters at this level are approximately $12$ cm. This plane is the roomiest part of the pelvic cavity and has no obstetrical significance, as it does not typically impede fetal descent.

Plane of Least Pelvic Dimensions

• Anterior Boundary: Lower border of the pubic symphysis.

• Posterior Boundary: Junction of the fourth and fifth sacral vertebrae ($S4$, $S5$).

• Lateral Boundary: Ischial spines.

• AP Diameter: Line joining the lower border of the pubic symphysis to the $S4$-$S5$ junction; measures $11.5$ to $12$ cm.

• Transverse Diameter (Interspinous Diameter or Bispinous Diameter): Distance between the ischial spines; measures $10$ cm. It is the smallest diameter in the entire pelvis and the most important diameter during labor, as it represents the narrowest point through which the fetal head must pass. A contracted interspinous diameter can lead to obstructed labor.

Mid-Pelvis

The area between the plane of greatest pelvic dimensions and the plane of least pelvic dimensions. In other words, mid pelvis lies at the level of the plane of least pelvic dimensions.

Posterior Sagittal Diameter of Mid-Pelvis

From the posterior boundary (junction of $S4$ and $S5$) to the intersection of the AP and transverse diameters; measures $4.5$ to $5$ cm. This diameter helps to assess the available space in the posterior aspect of the mid-pelvis.

Clinical Assessment of Mid-Pelvis

Normally, both ischial spines cannot be touched simultaneously with two fingers of one hand during a vaginal examination. If they can be touched easily, the interspinous diameter may be contracted, indicating potential mid-pelvis dystocia.

Anatomical Outlet

The anatomical outlet lies at the level of the ischial tuberosities and the sacrotuberous ligament. Its shape is formed by two triangles with the base at the line joining the ischial tuberosities. It is the inferior boundary of the true pelvis.

• Anterior Boundary: Lower border of the pubic symphysis.

• Posterior Boundary: Tip of the sacrum or coccyx.

• Lateral Boundary: Ischial tuberosities and sacrotuberous ligaments.

Diameters of Anatomical Outlet

• AP Diameter: From the lower border of the pubic symphysis to the tip of the coccyx; measures approximately $13$ cm. This diameter can increase slightly during labor as the coccyx is pushed posteriorly.

• Transverse Diameter (Bituberous Diameter): Distance between the ischial tuberosities; normally $11$ cm. If less than $8$ cm, the outlet is considered contracted, potentially leading to obstructed labor.

• Posterior Sagittal Diameter: From the tip of the sacrum to the intersection of the AP and transverse diameters; approximately $7$ cm.

Clinical Assessment of Outlet

Ideally, four knuckles should fit between the ischial tuberosities during clinical examination. If not, the outlet is considered contracted, which may impede fetal descent and rotation.

Angles of the Pelvis

1. Angle of Inclination: The angle the pelvic brim makes with the horizontal when a person is standing; approximately $55$ degrees. This angle affects the direction of the birth canal.

2. Subpubic Angle: The angle between the descending rami of the pubic bones; obtuse (wide) in females (typically greater than 90 degrees) and acute (narrow) in males. A wider subpubic angle in females facilitates fetal passage.

Clinical Pelvimetry

Clinical assessment of the pelvis to determine its adequacy for delivery. It should be performed between $38$ to $39$ weeks in primigravidas (first-time mothers) and at the onset of labor in multigravidas (women who have had previous pregnancies). Clinical pelvimetry involves manual examination to assess the size and shape of the pelvic inlet, mid-pelvis, and outlet.

WHO (World Health Organization) does not recommend routine clinical pelvimetry for all women in labor, as it has limited predictive value and may lead to unnecessary interventions. However, it may be useful in specific cases where there is a suspicion of pelvic contraction or cephalopelvic disproportion.

Contracted Pelvis

A contracted pelvis is defined as the decrease by more than $0.5$ cm of any of the major pelvic diameters, which can impede fetal descent and lead to obstructed labor. It can also be defined based on which part of the pelvis is contracted:

• Contracted Inlet: Obstetrical conjugate less than $10$ cm.

• Contracted Mid-Pelvis: Interspinous diameter less than or equal to $8$ cm.

• Contracted Outlet: Intertuberous diameter less than or equal to $8$ cm.

Management

Cesarean section is typically indicated for contracted pelvis, as vaginal delivery is unlikely to be successful and may pose risks to both the mother and the fetus. There is generally no role for a trial of labor in cases of significant pelvic contraction.

Recurrent cesarean sections are often necessary with subsequent pregnancies in women with a contracted pelvis.

Varieties of Contracted Pelvis

1. Nagle's Pelvis: Absence of one ala of the sacrum, resulting in an asymmetric pelvic shape.

2. Robert's Pelvis: Absence of both alae of the sacrum, leading to a severely narrowed pelvic inlet.

Management for both Nagle's and Robert's Pelvis is cesarean section due to the severe distortion of the pelvic anatomy.

Varieties of Pelvis (Cadwell and Moloy Classification)

Based on the shape of the inlet, the Cadwell and Moloy classification categorizes pelves into four main types:

1. Gynecoid Pelvis:

   • Most common (50%) in women.

   • Considered the "female-like" or typical female pelvis. It is round to slightly oval.

   • Features a round or slightly oval-shaped inlet, straight side walls, and a wide subpubic angle making it the most favorable for vaginal delivery.

2. Android Pelvis:

   • Male-like pelvis (20%) in women.

   • Characterized by a heart-shaped or triangular inlet, convergent side walls, a narrow subpubic angle, and prominent ischial spines.

   • Less favorable for vaginal delivery due to the increased risk of obstructed labor.

3. Anthropoid Pelvis:

   • Occurs in approximately 25% of women.

   • Features an oval-shaped inlet that is longer in the anteroposterior diameter than the transverse diameter.

   • Has a large pelvic outlet and a wide sacrosciatic notch, which is generally favorable for vaginal delivery, although the fetal head may engage in the oblique or posterior position.

4. Platypelloid Pelvis:

   • Least common (5%) in women.

   • Characterized by a flattened, oval-shaped inlet with a short anteroposterior diameter and a wide transverse diameter.

   • Often associated with transverse arrest of the fetal head during labor.

• Platypelloid pelvis is also known as a flat gynecoid pelvis.

Cephalopelvic Disproportion (CPD)

Cephalopelvic disproportion (CPD) occurs when the fetal head is too large to pass through the maternal pelvis, even if the pelvis is of normal size. It can be due to fetal macrosomia (excessively large fetus) or an abnormally large fetal head.

• If cesarean section was performed for CPD in one pregnancy, it is not an absolute indication for cesarean section in subsequent pregnancies. The decision for mode of delivery should be based on individual circumstances, including the estimated fetal weight and the progress of labor.

• The diagnosis of CPD is typically made during a trial of labor, where the progress of labor is monitored closely to assess whether vaginal delivery is possible. Clinical pelvimetry is useful to assess the pelvic dimensions, but it is not always accurate in predicting CPD.

Differences between Contracted Pelvis and CPD

Trial of Labor

A trial of labor is a planned attempt to achieve vaginal delivery in women with suspected CPD or other factors that may impede labor progress. It involves close monitoring of the maternal and fetal condition, as well as the progress of labor.

Trial of labor is generally contraindicated in patients with a previous classical cesarean section or other uterine surgery that increases the risk of uterine rupture. In patients with a previous low transverse cesarean section, a trial of labor after cesarean (TOLAC) may be considered, but it requires careful evaluation of the risks and benefits.

Trial of scar means when you are trying vaginal delivery in a previous cesarean section patient.

Management of CPD

The management of CPD depends on the severity of the disproportion, the progress of labor, and the presence of any other complications. Options include:

1. Trial of labor: If the CPD is mild and labor is progressing, a trial of labor may be attempted with close monitoring. If successful, this will lead to vaginal delivery.

2. Cesarean section: If the trial of labor fails, or if there is severe CPD, or if there are other complications such as fetal distress, a cesarean section is indicated.

Instrumental delivery (e.g., vacuum extraction or forceps) has a limited role in CPD management and may be contraindicated in cases of significant disproportion.

Diagnosing CPD During Labor

Signs of CPD during labor may include:

• Molding: Alteration in the shape of the fetal head due to pressure from the maternal pelvis.

• Caput succedaneum: Swelling of the fetal scalp due to pressure during labor.

• Slow progress of labor: Prolonged or arrested labor despite adequate uterine contractions.