Pelvic and Perineal Anatomy

Bony Pelvis

Pelvic Types (Caldwell-Moloy Classification)

• Gynaecoid (50%): round inlet, most favourable for vaginal delivery
• Android (20%): heart-shaped inlet, narrow subpubic arch, associated with deep transverse arrest
• Anthropoid (25%): AP diameter > transverse, associated with occipitoposterior position
• Platypelloid (5%): flat, wide, transverse diameter > AP, may cause engagement difficulty

Pelvic Diameters

| Level | AP Diameter | Transverse | Oblique | |-------|-----------|------------|---------| | Inlet | 11cm (true conjugate), 10cm (obstetric conjugate) | 13cm (widest) | 12cm | | Cavity | 12cm | 12cm | 12cm | | Outlet | 13cm (AP — increases with sacrum movement) | 11cm (between ischial tuberosities) | — |

• The fetal head enters the pelvis in the transverse diameter and exits in the AP diameter
• Obstetric conjugate (shortest AP diameter of inlet) = from sacral promontory to closest point on pubic symphysis
• Diagonal conjugate (measured vaginally) = obstetric conjugate + 1.5cm

Pelvic Floor Muscles

Levator Ani (Primary Support)

• Pubococcygeus: from pubic bone to coccyx; forms the main hammock of support
• Puborectalis: sling around the anorectal junction; maintains the anorectal angle (important for continence)
• Iliococcygeus: from tendinous arch to coccyx; forms the lateral part
• Innervation: direct branches from S3-S4 (NOT pudendal nerve)
• Levator ani supports pelvic organs and maintains the urogenital hiatus

Coccygeus (Ischiococcygeus)

• From ischial spine to coccyx/sacrum; overlies sacrospinous ligament

Perineum

• Diamond-shaped area between pubic symphysis, ischial tuberosities, and coccyx
• Divided by a transverse line between ischial tuberosities into:
  • Urogenital triangle (anterior): contains superficial and deep perineal pouches
  • Anal triangle (posterior): contains ischioanal fossae
• Perineal body: central tendinous point between vagina and anus; attachment for perineal muscles; critical for pelvic floor integrity; damaged in perineal tears

Blood Supply

Internal Iliac Artery Branches

Anterior division:

• Uterine artery (key surgical landmark)
• Vaginal artery
• Obturator artery
• Internal pudendal artery
• Inferior gluteal artery
• Inferior vesical artery (equivalent to vaginal artery in females)
• Middle rectal artery
• Umbilical artery (obliterated distally in adults — gives rise to superior vesical artery)

Posterior division:

• Superior gluteal artery
• Iliolumbar artery
• Lateral sacral artery

Nerve Supply

Pudendal Nerve (S2, S3, S4)

• Leaves pelvis through greater sciatic foramen, hooks around ischial spine and sacrospinous ligament
• Enters perineum through lesser sciatic foramen via Alcock's (pudendal) canal on medial wall of ischial tuberosity
• Branches: inferior rectal nerve, perineal nerve, dorsal nerve of clitoris
• Pudendal nerve block: performed at ischial spine (landmark for injection)

Lymphatic Drainage

| Structure | Primary Drainage | |-----------|-----------------| | Vulva | Superficial inguinal nodes → deep inguinal → external iliac | | Lower vagina | Superficial inguinal nodes | | Upper vagina | Internal and external iliac nodes | | Cervix | External iliac, internal iliac, obturator, presacral nodes | | Uterine body | External iliac and para-aortic nodes | | Ovary | Para-aortic nodes (drainage follows ovarian vessels to aorta level) | | Fundus | Can drain along round ligament to superficial inguinal nodes |

Important Facts for MRCOG

• Gynaecoid pelvis is most favourable; android most unfavourable for vaginal delivery
• Obstetric conjugate (~10cm) is the narrowest AP diameter at the inlet
• Levator ani: innervated by S3-S4 directly (NOT pudendal nerve)
• Pudendal nerve: S2-S4, passes around ischial spine — landmark for pudendal block
• Internal iliac artery ligation (anterior division) used for uncontrollable PPH
• Ovarian lymph drains to para-aortic nodes (important for staging ovarian cancer)
• Perineal body is the central anchor point damaged in obstetric perineal tears

Clinically reviewed by NHS obstetricians and gynaecologists

MRCOG AI is an independent educational tool. It is not affiliated with, endorsed by, or connected to the Royal College of Obstetricians and Gynaecologists (RCOG).