MRCOG Part 1 · Anatomy · Pelvic Anatomy

Bony Pelvis and Ligaments: MRCOG Part 1 questions and key facts

The bony pelvis and pelvic floor form the fundamental anatomical framework governing the mechanics of human parturition. Understanding pelvic architecture requires knowledge of the osseous boundaries, pelvic brim inclination, and changing internal pelvic dimensions between the inlet and outlet. The pelvic brim is tilted significantly in the upright posture, defining the curved pelvic axis through which the fetus descends. Below the osseous ring, muscular structures including the levator ani complex and piriformis stabilise the pelvic contents and maintain physiological continence mechanisms. For the MRCOG Part 1 examination, candidates must master the precise conjugate and transverse pelvic measurements, the muscular slings supporting the pelvic viscera, and the anatomical relations of pelvic wall musculature to adjacent somatic nerves.

The Uterio bank has 22 questions on this topic. 3 are below in full; the syllabus detail is in the Anatomy study guide.

How the exam tests bony pelvis and ligaments

  • Pelvic inlet and outlet diameters, identifying which dimension is widest at each plane to explain fetal rotation.
  • Distinguishing the anatomical landmarks of the obstetric conjugate from those of the diagonal conjugate.
  • Levator ani anatomy, specifically the role of the puborectalis sling in preserving the anorectal angle for continence.
  • Piriformis anatomy, including its sacral origin, femoral insertion, S1 to S2 innervation, and relationship to the sciatic nerve.

High-yield facts

  1. The transverse diameter of the pelvic inlet measures approximately 13.5 centimetres between the furthest points on the iliopectineal lines.
  2. The transverse diameter is the widest dimension of the pelvic inlet, whereas the anteroposterior diameter is the widest dimension of the pelvic outlet.
  3. The anteroposterior diameter of the pelvic outlet measures approximately 13 centimetres, facilitating delivery of the fetal head in an occipito-anterior position.
  4. The obstetric conjugate spans from the sacral promontory to the most prominent point on the posterior surface of the symphysis pubis.
  5. The diagonal conjugate is measured from the sacral promontory to the lower border of the symphysis pubis.
  6. In an upright posture, the plane of the pelvic brim is inclined at approximately 55 degrees to the horizontal plane.
  7. The plane of the pelvic outlet is inclined at approximately 15 degrees to the horizontal plane, shaping the curved pelvic axis.
  8. Puborectalis arises from the pubic bone and forms a U-shaped sling around the junction of the rectum and anus.
  9. Contraction of the puborectalis maintains the anorectal angle for faecal continence, whilst its relaxation straightens the angle during defaecation.
  10. The piriformis muscle originates from the anterior surface of the sacrum and inserts onto the greater trochanter of the femur.
  11. The piriformis muscle acts to laterally rotate and abduct the thigh at the hip joint when the hip is flexed.
  12. The piriformis muscle is innervated by nerve roots S1 and S2, with the sciatic nerve passing immediately below it.

Common pitfalls

  • Confusing the widest diameter at the inlet, which is transverse, with that of the outlet, which is anteroposterior.
  • Assuming the obstetric conjugate extends to the inferior pubic border rather than to the posterior aspect of the symphysis pubis.
  • Believing piriformis is supplied by the pudendal nerve roots S2 to S4, when it is innervated by S1 and S2.

Practice questions: bony pelvis and ligaments

Single best answer, exactly as the exam asks. Commit to an option before opening the explanation.

Question 1

Anatomy

In a typical gynaecoid pelvis, which dimension is the widest at the pelvic inlet?

  1. ATransverse diameter between the furthest points on each iliopectineal line
  2. BFrom sacral promontory to the midpoint of the pubic symphysis
  3. CDiagonal conjugate between the sacral promontory and the inferior pubic border
  4. DOblique line from sacroiliac joint to the iliopubic eminence
  5. EVertical axis from the sacral promontory to the ischial spine
Show answer and explanation

Correct answer: A. Transverse diameter between the furthest points on each iliopectineal line

Key takeaway

The widest transverse diameter of the pelvic inlet is measured between the furthest points on the iliopectineal lines on each side, which is important for assessing the adequacy of the pelvis for childbirth.

The pelvic inlet's dimensions are crucial in obstetrics, as they determine whether a foetal head can pass through during labour. The transverse diameter of the pelvic inlet, measuring approximately 13.5 cm, is the greatest distance between the iliopectineal lines on either side. This measurement is clinically significant because it represents the widest part of the pelvic inlet.

Other diameters, such as the obstetric conjugate (measured from the sacral promontory to the most prominent point on the posterior aspect of the symphysis pubis) and the diagonal conjugate (measured from the lower border of the symphysis pubis to the sacral promontory), are also important. However, they do not represent the widest transverse measurement. The obstetric conjugate is the shortest anteroposterior diameter through which the foetal head must pass, while the diagonal conjugate is clinically measurable and used to estimate the obstetric conjugate. However, these measurements are anteroposterior, not transverse.

The transverse diameter is a critical factor in determining whether a pelvis is adequate for vaginal delivery. A narrow transverse diameter can indicate a contracted pelvis, which may lead to obstructed labour.

Question 2

Anatomy

The puborectalis portion of the levator ani muscle forms a 'sling' around which structure?

  1. AVagina
  2. BUrethra
  3. CJunction of the rectum and anus
  4. DCoccyx
  5. EPerineal body
Show answer and explanation

Correct answer: C. Junction of the rectum and anus

Key takeaway

The junction of the rectum and anus is encircled by the puborectalis muscle, which forms a 'sling' that maintains the anorectal angle and contributes to faecal continence.

The levator ani muscle group is a major component of the pelvic floor, crucial for supporting pelvic organs and maintaining continence. One of its parts, the puborectalis, plays a unique role.

The puborectalis muscle originates from the pubic bone and forms a U-shaped sling around the anorectal junction. This sling creates the anorectal angle, an important factor in faecal continence. During defaecation, the puborectalis relaxes, straightening the angle and allowing passage of stool.

The other options are incorrect as they are not directly encircled by the puborectalis sling. While the vagina and urethra are supported by other parts of the levator ani, they are not encircled by the puborectalis sling. The coccyx and perineal body are anatomical landmarks in the region, but they are not the structures around which the puborectalis forms its sling.

Question 3

Anatomy

In an upright posture, what is the typical angle formed between the pelvic brim and the horizontal plane?

  1. A25 degrees
  2. B30 degrees
  3. C40 degrees
  4. D55 degrees
  5. E70 degrees
Show answer and explanation

Correct answer: D. 55 degrees

Key takeaway

In the upright (anatomical) standing position, the plane of the pelvic brim (pelvic inlet) is tilted approximately 50–60° to the horizontal — conventionally taught as 55°. The angle is clinically relevant because the inlet and outlet face different directions (the "pelvic axis"), which shapes the path a fetal head takes through the birth canal.

The pelvic brim (synonymous with the pelvic inlet) is the oval ring marking the boundary between the greater (false) pelvis and the lesser (true) pelvis. In the standard anatomical upright position, the plane of the brim is tilted forward so that its anterior border (the pubic symphysis) sits well below its posterior border (the sacral promontory).

The accepted angle between the pelvic brim and the horizontal in upright posture is approximately 55°, with textbook values ranging from ~50° to ~60° depending on the reference. 25°/30°/40° would represent a pelvis tilted nearly flat (well outside normal upright-posture values), while 70° would represent excessive anterior pelvic tilt.

This angulation, combined with the separate angle of the pelvic outlet (~15° to horizontal), defines the curve of the "pelvic axis" — the path along which the fetal presenting part descends during labour, and a core determinant of how the head rotates through the birth canal.

19 more bony pelvis and ligaments questions in the bank

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What the syllabus covers under bony pelvis and ligaments

  • Bones of the Pelvis
  • Pelvic Ligaments
  • Pelvic Floor Muscles

Read the full section in the Anatomy guide, or check the next exam dates.