[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Digestive system](https://durgical.com/en/embryology/#a-digestive) › Anorectal malformations (imperforate anus) [Embryology](https://durgical.com/en/embryology/) · [Digestive system](https://durgical.com/en/embryology/#a-digestive) # Anorectal malformations (imperforate anus) - Malformation The rectum does not open to the outside, or opens in the wrong place such as the urethra or vestibule; it is found on the newborn exam. Updated 2026-11-01 **See it in 3D:** [Anal canal](https://durgical.com/en/anatomy/anal-canal/) · [Rectum](https://durgical.com/en/anatomy/rectum/) · [External anal sphincter](https://durgical.com/en/anatomy/external-anal-sphincter/) · [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/) ## Key points - Cause: abnormal development of the urorectal septum, which leaves the cloaca incompletely divided. - High (above the puborectalis): anorectal agenesis, often with a rectourethral fistula in boys or a rectovestibular fistula in girls. - Low (below it): anal agenesis with a perineal fistula, anal stenosis or an anal membrane. - Sign: no anal opening or no meconium within 24 to 48 hours; meconium in the urine, on the perineum or from the vagina. - VACTERL association: vertebral, anorectal, cardiac, tracheoesophageal, renal and limb defects. - In girls, a persistent cloaca joins rectum, vagina and urethra into a single opening. ## What it is They are a spectrum of defects in which the rectum ends blindly, opens through a fistula to the urethra, bladder, vagina or perineum, or the anus is narrow or absent. They arise from abnormal development of the urorectal septum and cloaca in the embryonic period. They are classed as high or low by the level of the blind pouch relative to the puborectalis muscle and are often part of the VACTERL association. ## Embryological cause - Weeks 4 to 7 approx.: the urorectal septum must grow down to the cloacal membrane and separate the anorectal canal from the urogenital sinus. - If the septum falls short or deviates, the rectum stays connected to the urogenital sinus: rectourethral, rectovesical or rectovestibular fistulas. - If the anal membrane does not rupture or the anal pit forms poorly, a membrane or a low stenosis remains. - A persistent cloaca in girls reflects complete failure of division. ## How it presents - No anal opening on the newborn exam, or an anus in an abnormal position. - No passage of meconium, followed by distension and vomiting. - Meconium in the urine (urinary fistula in boys) or from the vestibule (girls). - Associations: VACTERL, tethered cord, sacral anomalies and Down syndrome. ## Diagnosis - Careful perineal exam in the first hours, looking for a fistula. - Prone cross-table lateral X-ray, after about 24 hours, to estimate the distance from the pouch to the skin. - Renal and spinal ultrasound, echocardiogram and spine X-rays to look for VACTERL. ## Memory aid The urorectal septum is the wall between two drainpipes: build it too short and the rectum's pipe ends up plumbed into the urinary one. ## Test yourself Think of the answer, then tap the question to see it. ## Which embryonic structure fails in anorectal malformations? The urorectal septum, which does not completely divide the cloaca into urogenital sinus and anorectal canal. ## Which finding suggests a rectourinary fistula in a boy? Meconium coming from the urethra or mixed in the urine. ## Which muscle defines a malformation as high or low? The puborectalis: a pouch ending above it is high and one ending below it is low. ## Which association of malformations is screened for in these newborns? VACTERL: vertebral, anorectal, cardiac, tracheoesophageal, renal and limb defects. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/anorectal-malformations/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Anal canal](https://durgical.com/en/anatomy/anal-canal/) - [Rectum](https://durgical.com/en/anatomy/rectum/) - [External anal sphincter](https://durgical.com/en/anatomy/external-anal-sphincter/) - [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/) ## Related - [Hindgut and cloaca: the urorectal septum and the pectinate line](https://durgical.com/en/embryology/hindgut-and-cloaca-the-urorectal-septum-and-the-pectinate-line/) - [Esophageal atresia and tracheoesophageal fistula](https://durgical.com/en/embryology/esophageal-atresia-and-tracheoesophageal-fistula/) - [Hemivertebra, congenital scoliosis and Klippel–Feil](https://durgical.com/en/embryology/hemivertebra-congenital-scoliosis-and-klippel-feil/) - [Renal agenesis and the Potter sequence](https://durgical.com/en/embryology/renal-agenesis-and-the-potter-sequence/) - [Hirschsprung disease (congenital aganglionic megacolon)](https://durgical.com/en/embryology/hirschsprung-disease/) - [Bladder exstrophy](https://durgical.com/en/embryology/bladder-exstrophy/) - [Bladder, urachus and urethra](https://durgical.com/en/embryology/bladder-urachus-and-urethra/) ## Related conditions - [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: [Embryology, Rectum and Anal Canal](https://www.ncbi.nlm.nih.gov/books/NBK551682/) (StatPearls Publishing (NCBI Bookshelf), 2023); [Imperforate anus](https://medlineplus.gov/ency/article/001147.htm) (MedlinePlus, U.S. National Library of Medicine, 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/anorectal-malformations/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/anorectal-malformations/