Prenatal imaging of anorectal malformations - 10-year experience at a tertiary center in Switzerland.

BACKGROUND: Anorectal malformation is a spectrum of congenital defects of the distal bowel, mostly diagnosed at birth. OBJECTIVE: To describe the prenatal imaging findings of anorectal malformations, explore the causes of the low rates of prenatal diagnosis, compare the accuracy of prenatal ultrasound (US) and magnetic resonnance imaging [MRI] and evaluate the relevance of information obtained at MRI. MATERIALS AND METHODS: Children treated for anorectal malformation at our hospital and with available prenatal studies were retrospectively identified and included in the study. We reviewed prenatal imaging exams, listed findings suggestive of the diagnosis, and compared results with the final classification. RESULTS: Fourteen fetuses and neonates - eight with intermediate-high type anorectal malformation and six with cloacae - fulfilled the inclusion criteria. All had associated congenital anomalies. Prenatal exams included 13 US and 8 MRI exams, with 7 children having both exams. Suggestive findings for anorectal malformation were detected in 50% of the cases prenatally and in 85% upon review. They were prospectively detected in 31% and 50% of the cases at US and MRI and retrospectively in 62% and 100% at US and MRI, respectively. MRI was superior to US because it improved the diagnosis, especially in cloacae, and provided relevant additional information that changed management in two cases. CONCLUSION: The most important signs suggesting anorectal malformation are an absent target sign and anomalous distal bowel wall and rectal fluid. Complementary prenatal MRI improves the diagnosis of anorectal malformation.

Prenatal Evaluation for Detection of Anorectal Atresia: Value of Ultrasound.

OBJECTIVES: To investigate the applicability and value of ultrasound (US) in the diagnosis of anorectal atresia. METHODS: Between January 2008 and January 2016, we prospectively evaluated 63,101 fetuses (gestational age, 20-38 weeks), including low- and high-risk populations using 2-dimensional US scans. An abnormal imaging finding was defined as an anal canal diameter of less than the 95% confidence interval (small anal canal) of the normal range or the absence of an anal canal and rectum. Imaging findings were considered normal on detection of an anal canal with a normal width and the absence of abnormalities. Prenatal imaging findings were confirmed by a postnatal or postmortem examination. RESULTS: Among the investigated fetuses, 28 showed evidence of anorectal atresia on US scans, and 22 of those with anorectal atresia had additional anomalies. Six cases of isolated anorectal atresia were successfully detected during the preclusive prenatal US scans. Four cases of a low imperforate anus (including 2 covered anuses) yielded false-negative results, indicating a diagnostic rate of 87.5% (28 of 32). The normal appearance of the fetal rectum and anal canal ruled out anorectal atresia in 30 fetuses with a dilated colon. Additionally, there were 3 false-positive cases, in which a narrow anal canal was observed. CONCLUSIONS: Identifying the abnormal appearance or absence of the fetal anal canal and rectum on preclusive US anomaly scans is useful for prenatal diagnosis or exclusion of anorectal atresia, which may help improve the detection of isolated anorectal atresia. Furthermore, a combined evaluation of the longitudinal and axial appearances of the fetal anal canal and rectum can improve diagnostic accuracy.

First trimester expression of anorectal malformation: Case report and review of the literature.

A 26-years-old woman, underwent an ultrasound examination at 13.4 weeks. A cystic structure was identified in the right lower abdomen. Gradually, the cystic mass was replaced by echogenic content and eventually attained the appearance of hyperechoic bowel. At 21.2 weeks, the anal sphincter could not be demonstrated which was consistent with the diagnosis of isolated anal agenesis. Amniocentesis revealed 46XY karyotype with normal comparative genomic hybridization. After termination of pregnancy at 23 weeks, an autopsy revealed an isolated high type anorectal malformation (ARM) without fistula. We reviewed all 14 cases reported in the literature of first trimester sonographic expression of ARM.

Clarification of mammalian cloacal morphogenesis using high-resolution episcopic microscopy.

The developmental process through which the cloaca transforms from one hollow structure to two separated urinary and digestive outlets remains controversial and speculative. Here, we use high-resolution episcopic microscopy to examine a comprehensive series of normal and mutant mouse cloaca in which the detailed 3-dimensional (3-D) morphological features are illuminated throughout the development. We provide evidence that the dorsal peri-cloacal mesenchyme (dPCM) remains stationary while other surrounding tissues grow towards it. This causes dramatic changes of spatial relationship among caudal structures and morphological transformation of the cloaca. The 3-D characterizations of Dkk1 mutants reveal a hyperplastic defect of dPCM, which leads to a significant anterior shift of the caudal boundary of the cloaca, premature occlusion of the cloaca and, imperforate anus phenotype. Conversely, Shh knockout causes a severe hypoplastic defect of cloaca mesenchyme including dPCM and persistent cloaca. Collectively, these findings suggest that formation of the dPCM is critical for cloacal morphogenesis and furthermore, growth and movement of the mesenchymal tissues towards the dPCM lead to the cloaca occlusion and separation of the urinary and digestive outlets.

Dkk1 in the peri-cloaca mesenchyme regulates formation of anorectal and genitourinary tracts.

Anorectal malformation (ARM) is a common birth defect but the developmental history and the underlying molecular mechanism are poorly understood. Using murine genetic models, we report here that a signaling molecule Dickkopf-1 (Dkk1) is a critical regulator. The anorectal and genitourinary tracts are major derivatives of caudal hindgut, or the cloaca.Dkk1 is highly expressed in the dorsal peri-cloacal mesenchymal (dPCM) progenitors. We show that the deletion of Dkk1 causes the imperforate anus with rectourinary fistula. Mutant genital tubercles exhibit a preputial hypospadias phenotype and premature urethral canalization.Dkk1 mutants have an ectopic expansion of the dPCM tissue, which correlates with an aberrant increase of cell proliferation and survival. This ectopic tissue is detectable before the earliest sign of the anus formation, suggesting that it is most likely the primary or early cause of the defect. Deletion of Dkk1 results in an elevation of the Wnt/ß-catenin activity. Signaling molecules Shh, Fgf8 and Bmp4 are also upregulated. Furthermore, genetic hyperactivation of Wnt/ß-catenin signal pathway in the cloacal mesenchyme partially recapitulates Dkk1 mutant phenotypes. Together, these findings underscore the importance ofDKK1 in regulating behavior of dPCM progenitors, and suggest that formation of anus and urethral depends on Dkk1-mediated dynamic inhibition of the canonical Wnt/ß-catenin signal pathway.

Denonvilliers' fascia revisited.

Although several studies have reported that the peritoneum does not contribute to the formation of a fascia between the urogenital organs and rectum, Denonvilliers' fascia (DF), a fascia between the mesorectum and prostate (or vagina) in adults, is believed to be a remnant of the peritoneum. Remnants of the peritoneum, however, were reportedly difficult to detect in other fusion fasciae of the abdominopelvic region in mid-term fetuses. To examine morphological changes of the pelvic cul-de-sac of the peritoneum, we examined 18 male and 6 female embryos and fetuses. A typical cul-de-sac was observed only at 7 weeks, whereas, at later stages, the peritoneal cavity did not extend inferiorly to the level of the prostatic colliculus or the corresponding structure in females. The cul-de-sac had completely disappeared in front of the rectum at 8 weeks and homogeneous and loose mesenchymal tissue was present in front of the rectum at the level of the colliculus at 12-16 weeks. We found no evidence that linearly arranged mesenchymal cells developed into a definite fascia. Therefore, the development of the DF in later stages of fetal development may result from the mechanical stress on the increased volumes of the mesorectum, seminal vesicle, prostate and vagina and/or enlarged rectum. Therefore, we considered the DF as a tension-induced structure rather than a fusion fascia. Fasciae around the viscera seemed to be classified into (1) a fusion fascia, (2) a migration fascia and (3) a tension-induced fascia although the second and third types are likely to be overlapped.

The great divide: septation and malformation of the cloaca, and its implications for surgeons.

The anorectal and urogenital systems arise from a common embryonic structure termed cloaca. Subsequent development leads to the division/septation of the cloaca into the urethra, urinary bladder, vagina, anal canal, and rectum. Defective cloacal development and the resulting anorectal and urogenital malformations are some of the most severe congenital anomalies encountered in children. In the most severe form in females, the rectum, vagina, and urethra fail to develop separately and drain via a single common channel known as a cloaca into the perineum. In this review, we summarize our current knowledge of embryonic cloaca development and malformation, and compare them to what has already been described in the literature. We describe the use of mouse models of cloaca malformation to understand which signaling pathways and cellular mechanisms are involved in the process of normal cloaca development. We also discuss the embryological correlation of the epithelial and stromal histology found in step sections of the common channel in 14 human cloaca malformations. Finally, we highlight the significance of these findings, compare them to prior studies, and discuss their implications for the pediatric surgeons. Understanding and identifying the molecular basis for cloaca malformation could provide foundation for tissue engineering efforts that in the future would reflect better surgical reconstruction and improved quality of life for patients.

Abnormal neural crest innervation in Sox10-Venus mice with all-trans retinoic acid-induced anorectal malformations.

BACKGROUND/PURPOSE: Despite technical advances in the surgical/medical care of anorectal malformation (ARM), persistent unsatisfactory postoperative bowel habit has been attributed to histopathologic abnormalities of the distal rectum/pouch (DRP) and hypoplasia of anal sphincter muscles (ASM). We used Sox10-Venus mice with ARM induced by all-trans retinoic acid (ATRA) to investigate neural crest cell (NCC) innervation in the DRP and ASM. METHOD: Pregnant Sox10-Venus mice were administered single doses of 50, 70, or 100 mg/kg of ATRA on embryonic day 8.5 (E8.5) then sacrificed on either E16.5 or E19.5. Bowel specimens comprising the anorectum were examined using fluorescence microscopy without immunohistochemical staining (FMIS). Anti-PGP9.5 was used to delineate ganglion cells and anti-SMA for smooth muscles. RESULTS: The appropriate dose of ATRA for inducing ARM was 50 mg/kg. Under FMIS, all ARM embryos (n = 5; all high type; 3 male:2 female) had less NCC innervation with thick Venus-positive nerve fibers in the DRP compared with normal embryos (n = 8); there was abnormal NCC innervation in the DRP and absent ASM in ARM mice. CONCLUSION: We are the first to delineate abnormal enteric nervous system innervation in the DRP of ARM mice without using immunohistochemical staining techniques thus allowing specimens to be examined without any distortion.

Biomechanical origin of the Denonvilliers' fascia.

INTRODUCTION: Since 1836 and the first description of the recto-genital fascia by Charles Denonvilliers, many anatomists have shown interest in this subject. Recently, pelvic surgeons have in turn shown similar interest, for they consider that perfect knowledge of this anatomical domain is crucial for optimal nerve conservation during surgery. Thanks to new anatomical description techniques, fascia location and relationships with pelvic nerves now appear clearer. OBJECTIVES: To describe and represent Denonvilliers' fascia and its relationships in the female foetus at different stages of gestation and in three-dimensional space (3D). MATERIEL/PATIENTS AND METHODS: Computer-assisted anatomical dissection technique was used. Serial histological sections were made from four human female foetuses. Sections were treated with conventional staining, as well as with nerve and smooth muscle immunostaining. Finally, the sections were digitalized and reconstructed in 3D. RESULTS: Denonvilliers' fascia was clearly located and visualized in three dimensions. It was present in the female foetus, being distinct from the fascia propria of the rectum. It appeared to be composed of multiple parallel layers situated between the vagina and the rectum. From a lateral view, it had an asymmetrical "Y-shaped" aspect that seemed to play the role of a protective sheet for the neurovascular bundles. CONCLUSION: This study betters our comprehension of the Denonvilliers' fascia in the female foetus and of its connections with pelvic nerves. It also provides a better understanding of safe planes during pelvic dissection. These findings also suggest a biomechanical theory for embryological origin of the Denonvilliers' fascia.

Spatiotemporal expression of fibroblast growth factor 10 in human hindgut and anorectal development.

As fibroblast growth factor 10 (FGF-10) gene expression may have a role in anorectal duct formation, this study aimed to assess the spatiotemporal expression pattern of FGF-10 during development of the rectum and hindgut in human embryos. FGF-10 expression was evaluated in human embryos (n = 85) at 3-8 weeks of gestation after immunohistochemical evaluation using antibodies specific for FGF-10. From weeks 4 to 7 of gestation, FGF-10 expression was observed primarily in the apical epithelium of the dorsal urorectal septum, the cloacal membrane (CM) and the hindgut. Following CM rupture (week 7), the epithelium of the anal canal was negative for FGF-10; however, it was present within the urothelium through week 7. FGF-10 expression during the development of the human hindgut and anorectum suggests that it may play a role in hindgut and anorectal morphogenesis.

Anorectal malformation associated with a mutation in the P63 gene in a family with split hand-foot malformation.

PURPOSE: The aims of this study were to identify the mutation gene of a Chinese family with anorectal malformation (ARM) associated with split hand-foot malformation and to determine the spatiotemporal expression of the mutated gene during hindgut and anorectum development in human embryos. METHOD: A Chinese family with intrafamilial clinically variable manifestation was analyzed and primers were designed for exons 3-14 of P63, DLX5, DLX6, DAC, and HOXD13 as candidate genes and direct sequence analysis of the exons was performed. Immunohistochemical study of mutated gene in the hindgut and anorectum of human embryos of 4th-10th weeks was performed. RESULT: Affected individuals were found to have an Arg227Gln P63 gene mutation. From the 4th-10th weeks of gestation of the human embryo, the P63-positive cells were mainly located on the epithelium of the apical urorectal septum, hindgut, and cloacal membrane. After the anorectum ruptured during the 8th week, the P63 remained strongly immunoreactive on the epithelium of the anal canal and urethra, but the mucous membrane of the rectum exhibited no reaction. CONCLUSIONS: The mutation identified strongly suggests a causal relationship between the ARM phenotype and P63. The expression of P63 was persistently active during the dynamic and incessant septation of the cloaca and hindgut, suggesting that P63 may play a pivotal role in the morphogenesis of the hindgut and anorectum.

Effect of abnormal notochord delamination on hindgut development in the Adriamycin mouse model.

BACKGROUND: Adriamycin mouse model (AMM) is a model of VACTERL anomalies. Sonic hedgehog (Shh) pathway, sourced by the notochord, is implicated of anorectal malformations. We hypothesized hindgut anomalies observed in the AMM are the result of abnormal effect of the notochord. METHODS: Time-mated CBA/Ca mice received two intraperitoneal injections of Adriamycin (6 mg/kg) or saline as control on embryonic day (E) 7 and 8. Fetuses were harvested from E9 to E11, stained following whole mount in situ hybridization with labeled RNA probes to detect Shh and Fork head box F1(Foxf1) transcripts. Immunolocalization with endoderm marker Hnf3ß was used to visualize morphology. Embryos were scanned by OPT to obtain 3D representations of expressions. RESULTS: In AMM, the notochord was abnormally displaced ventrally with attachment to the hindgut endoderm in 71 % of the specimens. In 32 % of the treated embryos abnormal hindgut ended blindly in a cystic structure, and both of types were remarked in 29 % of treated embryos. Endodermal Shh and mesenchymal Foxf1 genes expression were preserved around the hindgut cystic malformation. CONCLUSIONS: The delamination of the developing notochord in the AMM is disrupted, which may influence signaling mechanisms from the notochord to the hindgut resulting in abnormal patterning of the hindgut.

Immunophenotypic characterization of enteric neural crest cells in the developing avian colorectum.

BACKGROUND: The enteric nervous system (ENS) develops from neural crest-derived cells that migrate along the intestine to form two plexuses of neurons and glia. While the major features of ENS development are conserved across species, minor differences exist, especially in the colorectum. Given the embryologic and disease-related importance of the distal ENS, the aim of this study was to characterize the migration and differentiation of enteric neural crest-derived cells (ENCCs) in the colorectum of avian embryos. RESULTS: Using normal chick embryos and vagal neural tube transplants from green fluorescent protein (GFP) -transgenic chick embryos, we find ENCCs entering the colon at embryonic day (E) 6.5, with colonization complete by E8. Undifferentiated ENCCs at the wavefront express HNK-1, N-cadherin, Sox10, p75, and L1CAM. By E7, differentiation begins in the proximal colon, with L1CAM and Sox10 becoming restricted to neuronal and glial lineages, respectively. By E8, multiple markers of differentiation are expressed along the entire colorectum. CONCLUSIONS: Our results establish the pattern of ENCC migration and differentiation in the chick colorectum, demonstrate the conservation of marker expression across species, highlight a range of markers, including neuronal cell adhesion molecules, which label cells at the wavefront, and provide a framework for future studies in avian ENS development.

Normal development of hindgut and anorectum in human embryo.

PURPOSE: The aim of the present analysis is to examine the morphological changes, the spatiotemporal distribution of apoptosis/proliferation in the human embryonic anorectum, to reveal the normal development of human anorectum, and investigate the possible roles of apoptosis/proliferation during anorectal development. MATERIALS AND METHODS: The embryos were sectioned serially and sagittally, stained with hematoxylin and eosin (H & E) between the third and eighth week of gestation, TdT-mediated dUTP-digoxigenin nick end-labeling (TUNEL) and proliferative cell-specific nuclear antigen (PCNA) immunohistochemical staining from the sixth to the eighth week. RESULTS: From the fourth to the seventh week, with the growth of the mesenchyme around the cloaca, the cloaca was remolded, subsequently, the cloacal membrane (CM) moved perpendicularly then horizontally. The dorsal cloaca gradually descended to the tail groove, the urorectal septum (URS) and the CM approximated; however, the fusion of URS with the dorsal CM was never observed. During the eighth week, the URS shifted ventrally and finally fused with the ventral CM. Moreover, from the sixth to the eighth week, the apoptotic cells were concentrated in the CM, the mesenchyme of terminal rectum, and the dorsal rectum. Meanwhile, the proliferative cells could be observed in the ventral mesenchyme around the cloaca, the CM, the fused tissue between the URS, and the ventral CM. CONCLUSIONS: During the development of human anorectum, it was intriguing to reveal that the URS never fused with the dorsal CM before dorsal CM disintegration, the normal anorectal development may depend on the dorsal cloaca and the dorsal CM; furthermore, the distribution of apoptosis and proliferation in the anorectum and ventral cloacal mesenchyme played a pivotal role in the formation of the anorectum.

Spatiotemporal expression of Wnt5a during the development of the hindgut and anorectum in human embryos.

PURPOSE: The aim of the study was to determine the spatiotemporal expression of Wnt5a during hindgut and anorectum development in human embryos and to explore the possible role of Wnt5a during the morphogenesis of the human hindgut and anorectum. MATERIALS AND METHODS: The embryos (n = 107) were sectioned serially and sagittally, using Wnt5a immunohistochemical staining on the caudal midline from the 4th-9th weeks of gestation. RESULTS: From the 4th-7th week of gestation, the Wnt5a-positive cells were mainly located on the epithelium of the apical urorectal septum, hindgut, and cloacal membrane. After the anorectum and the urogenital sinus (UGS) opened to the amniotic cavity during the 7th week, the Wnt5a-positive cells disappeared and remained negative up to the 9th week on the epithelium of the anal canal. CONCLUSIONS: The expression of Wnt5a was constantly active during human hindgut and anorectum development and disappeared after the anus formed, suggesting that Wnt5a plays an important role in human hindgut and anorectal morphogenesis.

Wnt5a expression in the hindgut of fetal rats with chemically induced anorectal malformations--studies in the ETU rat model.

PURPOSE: Wnt5a is involved in the initiating and patterning morphological adaptations of gut. However, its role remained unknown during terminal hindgut development in the normal and anorectal malformation (ARM) rat embryos. This study was designed to investigate the expression pattern of Wnt5a in the terminal hindgut in ARM rat embryos. MATERIALS AND METHODS: Ethylenethiourea-induced ARM model was introduced to investigate the expression pattern of Wnt5a during terminal hindgut development using immunohistochemical staining, reverse transcriptase polymerase chain reaction (RT-PCR), and Western blot analysis. RESULTS: Immunostaining revealed that Wnt5a expression showed space-dependent changes in the developing terminal hindgut. On embryonic day 17 (E17) in normal embryos, the Wnt5a protein was initially expressed in the mesenchyme of the terminal hindgut. From E18 to 19, the positive staining cells gradually increased. The expression was detected mainly in the circular muscle and myenteric plexus of hindgut. In the ARM embryos, on E17, the Wnt5a protein was also expressed in the hindgut. However, from E18 to 19, the positive staining cells in the middle hindgut gradually increased but in the terminal hindgut decreased. In Western blot and RT-PCR, time-dependent changes of Wnt5a protein and mRNA expression were remarkable during the terminal hindgut development in normal and ARM embryos. CONCLUSION: These data implied that the downregulation of Wnt5a at the time of hindgut neuromuscular development might partly be related to the maldevelopment of terminal hindgut in ARM.

Sonographic evaluation of the development of the fetal rectum and anal canal.

PURPOSE: To describe the sonographic (US) appearance of fetal anal canal and rectum and establish nomograms of their normal measurements. METHODS: This was a prospective, cross-sectional study of 524 healthy women (mean age, 27 years; range, 21-37 years) with normal singleton pregnancy between 18 and 40 weeks of gestational age (GA). High-resolution transabdominal US was used to visualize and measure the normal fetal anal canal and rectum. RESULTS: Satisfactory images and measurements of the fetal anal canal and rectum were obtained in 496 normal fetuses. The diameters of the normal anal canal and rectum were plotted as a function of GA in a sigmoid curve. The curve estimations were expressed by the following cubic regression equations with R(2) of 0.87 and 0.88, respectively (p < 0.001): anal canal diameter (mm) = 18.272 - 2.151 × GA + 0.0095 × GA(2) - 0.0011 × GA(3) , and rectal diameter (mm) = 18.545 - 2.543 × GA + 0.1237 × GA(2) - 0.0016 × GA(3) . CONCLUSIONS: The fetal anal canal and rectum are visible sonographically between 18 and 40 weeks of GA. The knowledge of their normal US appearance and size from the second trimester of pregnancy onwards may help identify developmental anomalies.

The C. elegans Hox gene egl-5 is required for correct development of the hermaphrodite hindgut and for the response to rectal infection by Microbacterium nematophilum.

Members of the Hox gene family encode transcription factors that specify positional identity along the anterior-posterior axis of nearly all metazoans. One among the Caenorhabditis elegans Hox genes is egl-5. A deletion allele of egl-5 was isolated in a screen for animals which fail to develop swollen tails when exposed to the bacterial pathogen Microbacterium nematophilum. We show that compromised rectal development, which occurs as a result of loss of egl-5 function, results in a failure of rectal epithelial cells to express the ERK MAP kinase mpk-1, which was previously shown to mediate tail-swelling in response to bacterial infection. Tissue-specific rescue experiments demonstrated that egl-5 and mpk-1 act autonomously in rectal cells in the morphological response. The weak egl-5 allele (n1439), which does not compromise rectal development, fails to affect tail-swelling. We find that this allele carries an inserted repeat element approximately 13.8 kb upstream of the egl-5 open reading frame, which specifically disrupts the cell-specific expression of this gene in HSN egg-laying neurons. Together these findings extend the complexity of regulation and function of Hox genes in C. elegans and demonstrate the importance of their tissue-specific expression for correct development and response to infection.

New insights into the development and differentiation of the human anorectal epithelia. Are there clinical consequences?

PURPOSE: The epithelial lining of the anorectum still raises discussions concerning the levels of transition between the various zones and leads to an incomplete understanding of the immmunoprofile of rectal carcinoma. Since the expression of cytokeratins depends on the epithelial cell-type and the parahox-gene CDX2 is important for the development of the colorectal epithelium, we investigated different cytokeratins and CDX2 in the anorectum of human prenatal stages and in adult normal and neoplastic anorecta. MATERIALS AND METHODS: The differentiation and spatiotemporal distribution of the epithelial zones were examined in 33 human embryos and fetuses, in a 2-year-old child and four adults. In comparison, 17 specimens of ultralow rectal adenocarcinoma and 4 specimens of anal carcinoma were investigated. Monoclonal antibodies were directed against cytokeratin (CK) 18, 20, 7 and 14 and CDX2. RESULTS: Due to the cytokeratin profile and to CDX2 expression, the different anorectal zones could already be differentiated in human prenatal life. We showed that anorectal epithelial differentiation including the squamous epithelia ran in a craniocaudal direction, and that the anorectal zone was a transitional zone between rectal zone and anal transitional zone where CK 7, 18, 20 and CDX2 are simultaneously expressed. All cases of rectal adenocarcinoma showed positivity for CK 18, 20 and CDX2, and three also labelled for CK 7, whereas CK 14 was only expressed in the cases of anal carcinoma. CONCLUSIONS: Our results elucidate the connection between the prenatal pattern and the origin of the different types of anorectal carcinoma.

Imaging of anorectal malformations in utero.

PURPOSE: To document the imaging findings suggestive of anorectal malformation (ARMs) on prenatal US and MRI. METHODS: Retrospective evaluation of the screening US and prenatal MRI exams of the rectum and ano-perineal region in normal fetuses and in patients with ARMs. RESULTS: Examples showing the normal rectal and anoperineal anatomy on prenatal US and MRI exams and the imaging findings observed in different types of confirmed ARMS. CONCLUSIONS: Prenatal diagnosis of ARMs requires both a systematic evaluation of the fetal pelvis and perineum and an appropriate knowledge of its suggestive imaging findings.