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).