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Fetal rectum

The fetal rectum is the terminal portion of the developing gastrointestinal tract. It begins to form during embryogenesis from the hindgut and cloaca, completing its separation from the urogenital sinus by the end of the 7th gestational week.

By mid-gestation, the fetal rectum is a tubular structure lying posteriorly in the pelvis, continuous proximally with the sigmoid colon and distally with the developing anal canal. It is an important structure to evaluate on prenatal imaging because abnormalities here may suggest anorectal malformations, obstruction, or cloacal anomalies.

Synonyms

  • Prenatal rectum

  • Fetal hindgut terminal segment

  • Distal fetal bowel

Structure and Development

  • Embryologic origin: Derived from the hindgut and separated from urogenital sinus by the urorectal septum

  • Layers: Mucosa, submucosa, muscularis propria, and serosa (similar to rest of bowel)

  • Canalization: Becomes patent during mid-gestation

  • Contents: May contain meconium, a mixture of swallowed amniotic fluid, intestinal secretions, bile pigments, and shed epithelial cells

Relations

  • Anteriorly: Developing bladder, prostate/urethra in males, vagina/uterus in females

  • Posteriorly: Sacrum and coccyx

  • Superiorly: Continuous with sigmoid colon

  • Inferiorly: Continuous with anal canal

Function

  • Acts as the terminal reservoir for meconium accumulation during gestation

  • Plays a role in maturation of fetal gastrointestinal motility and peristalsis

  • Serves as an imaging marker for bowel patency and anorectal development

Clinical Significance

  • Anorectal malformations: Imperforate anus, cloacal malformation, or atresia may be suspected if rectum is not visualized or dilated

  • Obstruction: May cause distal bowel dilatation and polyhydramnios

  • Meconium peritonitis: Can result from rectal or intestinal perforation in utero

  • Hydrocolpos or urogenital anomalies: May mimic or distort the fetal rectum on imaging

  • Prenatal imaging: MRI is used when ultrasound is inconclusive for pelvic or anorectal anomalies

MRI Appearance

T2 HASTE (T2 GRE):

  • Rectal wall: Hypointense rim

  • Rectal lumen (fluid-filled): Hyperintense (bright)

  • Rectal lumen (meconium-filled): Hypointense to intermediate (darker than fluid, depends on protein/bile content)

  • Rectal lumen (air-filled): Signal void (black), though air is not expected prenatally

T1 GRE:

  • Rectal wall: Low signal intensity

  • Rectal lumen (fluid-filled): Hypointense (dark)

  • Rectal lumen (meconium-filled): Hyperintense (bright, due to protein, bile, and debris)

  • Rectal lumen (air-filled): Signal void (black on all sequences; abnormal if seen prenatally)

MRI image

fetal Rectum MRI axial anatomy image-img-00000-00000

MRI image

fetal Rectum mri image