Introduction
The female pelvis is a complex space in which the digestive, reproductive, and urinary systems converge. The posterior pelvic gastrointestinal organs—the sigmoid colon, rectum, and anus—are commonly involved in various diseases, ranging from fistula in ano and diverticulitis to anal and colorectal cancers. Additionally, pregnancy and childbirth contribute to pathologic processes as varied as rectal prolapse, hemorrhoidal disease, and fecal incontinence. In the pelvis, the form and function of these structures are intricately intertwined. Thus, a thorough understanding of both the anatomy and physiology of the female pelvis is essential in successfully managing such surgical diseases. This chapter reviews the anatomy, embryology, and physiology of the sigmoid colon, rectum, and anus, focusing on the features unique to the female pelvis.
Pelvic Cavity
The pelvic cavity is limited by the pelvic bones and musculature ( Fig. 1-1 ). The pubis and its symphysis, the paired ischial and ileal bones, and the sacrum and coccyx form the osseous scaffold of the pelvis. The five fused vertebrae of the sacrum create a concavity against which the rectum rests. The sacrotuberous and sacrospinous ligaments—extending from the sacrum to the ischial tuberosity and from the sacrum and coccyx to the ischial spine, respectively—shape the greater and lesser sciatic foramina, through which the internal pudendal vessels and pudendal nerve travel. On its anterolateral surface, the pelvis is lined by the obturator internus and, posterolaterally, by the piriformis muscles. The floor of the pelvis is composed of the pelvic diaphragm: the levator ani and coccygeus muscles. The female pelvis is more cylindrical than the conical male pelvis, providing greater space and wider openings to allow for childbearing. The pelvic inlet is an oval aperture at the level of the sacral promontory, marked by the superior edge of the first sacral vertebra, and the linea terminalis. Although the pelvic inlet is continuous with the abdominal cavity, the narrower pelvic outlet, the dimensions of which are important in childbirth, lies inferior to the pelvic diaphragm. The pelvic outlet is bounded by the inferior margins of the pubic symphysis and pubic rami, the ischial tuberosities, and by the coccyx and sacrotuberous ligaments. The true pelvis extends from the pelvic inlet to its outlet, whereas the false pelvis, actually a part of the abdominal cavity, sits superior to the pelvic inlet, between the iliac crests, lumbar vertebrae, and anterior abdominal wall. Although the false pelvis expands to accommodate the gravid uterus, the true pelvis remains an immobile bony canal. During pregnancy, however, laxity of the ligaments of the sacroiliac joints and the pubic symphysis permits a minor increase in the dimensions of the pelvic inlet. The sigmoid colon and rectum both are situated in the true pelvis, the inflexibility of which impacts the ease of surgery as well as the treatment of rectal cancers.
Embryology
Knowledge of the embryonic origins of the pelvic gastrointestinal organs is integral to understanding their vascular, nervous, and lymphatic supplies. The primitive intestine, derived from the endoderm of the yolk sac, begins as a straight tube situated posteriorly in the embryo. By the fourth week of gestation, the gut tube differentiates into three distinct areas: the foregut, midgut, and hindgut. Ultimately, the hindgut forms the colon distal to the splenic flexure, rectum, and the anal canal above the dentate line. The union of the anteriorly placed allantois—which later becomes the median umbilical ligament—and the most inferior extent of the hindgut creates a cul-de-sac, the cloaca (“sewer”), located approximately at the level of the pubococcygeal line ( Fig. 1-2 ). At the base of the cloaca, an area known as the cloacal membrane , the endoblast of the hindgut abuts against the ectoblast; the site of this membrane is later demarcated by the dentate line. Between the fourth and sixth weeks of gestation, the cloaca is gradually divided by the urorectal septum, a mesodermic structure comprised of the superior Tourneux fold and the two lateral Rathke folds, which descends to generate the urogenital sinus anteriorly and the upper anorectal canal posteriorly. In females, the urogenital sinus produces the bladder, membranous urethra, and vestibule of the vagina. By the end of the sixth week of gestation, the urorectal septum joins with the cloacal membrane, establishing the perineum as well as, anteriorly, the urogenital membrane and, posteriorly, the anorectal membrane.
Unlike the rectum and upper anal canal, the anal canal distal to the dentate line evolves from the proctodeum, or anal pit. This anal pit arises from a proliferation of mesenchymal tissue surrounding the anal membrane. In the eighth week of gestation, the membrane breaks down, allowing the ectoderm to become continuous with the endoderm of the hindgut. Ultimately, the proctodeum transforms into the anal canal distal to the dentate line. The structures in this area are supplied by the internal iliac artery, whereas the hindgut derivatives obtain their blood supply from the inferior mesenteric artery. By the ninth to tenth weeks of gestation, bilateral ectodermal protuberances—the anal tubercles—adjacent to the anus unite posteriorly to fashion the external anal sphincter, which then expands cranially. The anterior fusion of these paired structures results in the perineal body. The internal anal sphincter appears between the sixth and twelfth weeks of fetal development as a caudal extension of the circular muscle layer of the rectal wall.
Layers of the Sigmoid Colon and Rectum
The wall of the gastrointestinal tract is comprised of four principal layers: the mucosa, submucosa, muscularis propria, and serosa ( Fig. 1-3 ). The colon and rectum are internally lined by the mucosa, a simple columnar epithelium that, unlike that of the small intestine, features no villi; however, the long intestinal glands of the colon contain numerous absorptive and goblet cells, active in the absorption of nutrients, water, and electrolytes and in the secretion of mucus, respectively. Separating the mucosa from the submucosa, the muscularis mucosa is formed of thin inner circular and outer longitudinal layers of smooth muscle. The submucosa is the dense connective tissue scaffold between the mucosa and muscularis propria. Like the muscularis mucosa, the muscularis propria includes an inner circular and an outer longitudinal layer of smooth muscle. In the colon, the longitudinal muscle appears as a thin layer between three thickened muscular bands, the teniae coli: the taenia libera, taenia mesocolica, and taenia omentalis. Diverticula—herniations of the mucosa and submucosa—develop between the teniae where the colonic wall is penetrated by blood vessels, the vasa recti. The teniae coli, one sixth the length of the colon, produce the characteristic haustrations of the colon. These bands coalesce into a single layer over the rectal wall. The serosa is the thin lining of loose connective tissue and squamous mesothelium over the colon, which is absent over the majority of the rectum. Numerous epiploic appendages—bodies of fat—are present over the antimesenteric surface of the colon, particularly the sigmoid colon, contained within the peritoneal lining; these appendages disappear over the rectal wall. Ultrasound imaging of the rectal wall demonstrates five layers, three of which are hyperechoic (white) and two—the muscular segments—hypoechoic (black): the mucosal surface, mucosa/muscularis mucosa, submucosa, muscularis propria, and serosa or perirectal fat ( Fig. 1-4 ).
Sigmoid and Rectum
The sigmoid colon represents the terminus of the colon. The sigmoid colon lies in the left lower quadrant, usually in the true pelvis, although it is lifted into the false pelvis by the gravid uterus ( Fig. 1-5 ). It varies in length from 15 to 50 cm. The sigmoid colon is the narrowest portion of the colon, with a diameter equivalent to that of the ileum. The sigmoid colon usually curves toward the left in an omega formation but may assume other, more tortuous, configurations due to its mobile mesentery. When maneuvering a rigid proctoscope through the rectosigmoid junction, the instrument must be turned anteriorly and to the left. As the sigmoid colon is unfixed, it may at various times abut the small intestine, bladder, uterus, ovaries, and rectum; inflammatory conditions of the sigmoid colon may result in fistulae with these organs. Additionally, a longer mesentery predisposes to volvulus. Within the sigmoid mesentery are the inferior mesenteric vessels as well as the nerves and lymphatics to the sigmoid colon. The sigmoid mesocolon forms an inverted V , with the base attached to the retroperitoneum in the left lateral pelvis; the mesentery reaches from the iliac fossa to the second or third sacral vertebrae, passing over the sacral promontory and sacroiliac joint. The upper limb of the mesentery is in close proximity to the left external and common iliac vessels. The intersigmoid fossa sits between the two limbs of the sigmoid mesocolon; this recess marks the location of the left ureter as it travels to the pelvis. As the sigmoid colon approaches the pelvis, its mesentery gradually shortens until it ultimately disappears to become the mesorectum. The sigmoid colon is an intraperitoneal structure, completely invested by a peritoneal lining. The white line of Toldt indicates the union of the visceral peritoneum of the sigmoid mesocolon with the parietal peritoneum of the left lateral abdominal wall in the paracolic gutter; mobilization of the sigmoid colon entails the incision of this peritoneal reflection.
The rectum is the continuation of the sigmoid colon. The transition point from the sigmoid colon to the rectum is a source of controversy. Although anatomists point to the level of the third sacral vertebrae as the beginning of the rectum, surgeons advocate the sacral promontory as its origin. More likely there is a graded shift from the sigmoid colon to the rectum, the rectosigmoid junction. Some anatomists consider the rectosigmoid junction to encompass the last 5 to 8 cm of sigmoid colon and the initial 5 cm of the rectum, although others refer to the narrowest, most acutely angulated segment of the distal sigmoid colon. It is agreed that the rectum, unlike the sigmoid colon, lacks haustra, a true mesentery, teniae coli, and epiploic appendages. Also, as compared to the narrow sigmoid colon, the rectum is wide and compliant. In particular, the rectal ampulla, the lower two thirds of the rectum that rests directly above the levator ani muscle, is identified by its great distensibility. Furthermore, the rectal mucosa features a distinct submucosal vascular pattern, easily visible during endoscopy through its pink, smooth transparent lining.
Despite its name, the rectum is not a “straight” organ. The rectum travels along the concavity of the sacrum but diverges sharply posteriorly in a 90-degree angle as it passes through the levator hiatus, approximately 2 to 3 cm from the tip of the coccyx, after which it narrows to become the anus. Its length varies from 12 to 15 cm. The rectum is comprised of three lateral curves: two curves convex to the right and one to the left ( Fig. 1-6 ). Each of these three curves is reflected intraluminally by a valve of Houston, transverse folds that range in number from zero to seven. While the two left-sided folds are located at 7 to 8 cm and 12 to 13 cm, the right-sided valve of Houston is situated 9 to 11 cm from the anal verge. The middle valve (Kohlrausch’s plica) dependably marks the site of the anterior peritoneal reflection at approximately 5 to 7.5 cm from the anal verge in women, distal to which the rectum is completely extraperitoneal. Although the valves do not function as such, these sites present a low risk of perforation during rectal biopsies as several layers of the bowel wall—the mucosa, submucosa, and circular muscle—are redundant because of infolding. Following the complete mobilization of the rectum, the three curves are obliterated, resulting in a straight rectum that is 5 cm longer. Clinically, this extra length may increase the distance of a rectal lesion from the anal verge.
Although the mesosigmoid is a true mesentery, suspending the sigmoid colon, the mesorectum does not similarly serve the rectum. During embryologic development, the posterior rectum becomes completely extraperitoneal. Yet, in the case of rectal prolapse, the peritonealized mesorectum does act as a true mesentery. The mesorectum begins at the sacral promontory and ends approximately 2 cm superior to the pelvic diaphragm. The mesorectum represents the perirectal fat and areolar tissue, thicker on the posterior rectal surface. This layer holds the blood vessels and lymphatics to the rectum but no clinically important nerves. A rectal cancer may potentially extend into the mesorectum, a T3 lesion.
Unlike the sigmoid colon, the rectum is not completely invested by peritoneum. The rectum is posteriorly an extraperitoneal structure. However, the upper third of the rectum is covered by peritoneum both laterally and anteriorly. In its middle third, peritoneum lines only the anterior rectum. The lower third of the anterior rectum lies in an extraperitoneal location, where it abuts the posterior vaginal wall. The rectouterine fold of peritoneum extends from the posterior uterus and the superior vaginal wall laterally to the rectum, forming the pararectal fossae. The rectouterine pouch (of Douglas) is created by the reflection of peritoneum from the anterior rectal wall ventrally onto the posterior surface of the uterus and upper vagina, with the base of the pouch approximately 5 to 7.5 cm from the anal verge in women.
Endopelvic Fascia
The endopelvic fascia describes the dense connective tissue that extends over the pelvis and its contents. This fascia provides support to the pelvic organs. The parietal endopelvic fascia, a continuation of the abdominal transversalis fascia, internally covers the pelvic bones and musculature, including the pelvic floor muscles.
The visceral endopelvic fascia rises from the parietal endopelvic fascia over the levator ani to envelop the pelvic organs ( Fig. 1-7 ). The fascia propria of the rectum specifically refers to the thin sheath of endopelvic fascia surrounding the rectum, albeit loosely to allow for distention. This fascial layer is best demonstrated over the posterior and lateral surfaces of the extraperitoneal rectum. The rectosigmoid junction marks the superior extent of the rectal fascia propria. Within the fascia propria are contained the rectum and mesorectum, along with its vasculature and lymphatics but no significant nerves. A complete mesorectal excision for rectal cancer entails removing the rectum with the fascia propria intact. For benign disease, rectal mobilization may be completed within the plane of the fascia propria, although the associated blood loss is greater.
The rectovaginal fascia (of Denonvilliers) separates the anterior surface of the extraperitoneal rectum from the posterior vaginal wall. This V -shaped fascia descends from the base of the rectovaginal pouch to the urogenital diaphragm, where it inserts into the perineal body. Mobilization of the rectum is performed anterior to the rectovaginal fascia for rectal cancer but posterior to this layer for benign disease. The plane anterior to the rectovaginal fascia places the nerves from the inferior hypogastric plexus and middle rectal artery at risk of injury. Laterally, the rectovaginal fascia continues as the lateral ligaments of the rectum.
The lateral ligaments of the rectum rest on the levator ani, situated inferior to the peritoneum. These triangular structures have a posterolateral orientation within the pelvis, extending from the lateral rectal wall to the lateral pelvic sidewall. Comprised of a dense coalescence of the parietal endopelvic fascia, the lateral ligaments primarily contain nerves and connective tissue. The middle rectal artery does not traverse the ligament, instead passing below, although small branches may be included in the structure in approximately 25% of cases.
The presacral fascia lies posterior to the rectum, where it adheres closely to the sacrum and coccyx. This layer conceals the middle sacral artery and presacral veins. The hypogastric nerves travel over this fascia. The presacral fascia continues as the lateral ligaments but also contributes to the uterosacral and cardinal ligaments as well as the anococcygeal ligament.
As with the lateral ligaments, the rectosacral fascia is an extension of the endopelvic fascia, arising from the presacral fascia. This dense condensation of connective tissue advances inferiorly from the periosteum of the S3 or S4 vertebra to the posterior rectal wall at a level 3 to 5 cm above the intersection of the anus and rectum. The fascial band secures the rectum to the sacrum. A complete mobilization of the posterior rectum requires division of the rectosacral fascia.
Anus
The gastrointestinal tract terminates as the anus ( Fig. 1-8 ). The anus is directed posteriorly and inferiorly toward the anal verge. As with the superior boundary of the rectum, the upper limit of the anus is controversial. Surgeons cite the anorectal ring, where the puborectalis and internal anal sphincter meet, while anatomists are proponents of the dentate line, which corresponds to the embryological definition of the anal canal. The length of the anal canal according to these two concepts differs by approximately 2 cm, with the surgical anal canal longer at 3 to 4 cm. The surgical anal canal allows for an accurate determination of the location of low rectal lesions by digital examination as related to the anal verge (anocutaneous line of Hilton), the most inferior extent of the anus, which lies 1 to 2 cm inferior to the dentate line.
Normally, the anus appears externally as a closed anteroposterior slit, with its lateral walls closely apposed. Damage to the surrounding sphincter mechanism, however, may result in a patulous anus. The anus is anchored anteriorly by the perineal body and, posteriorly, by the anococcygeal ligament to the coccyx. During a digital rectal examination, the rectouterine pouch, uterine cervix, and posterior wall of the vagina are palpable anteriorly. The sacrum and coccyx are similarly felt posteriorly and the ischiorectal fossae and ischial tuberosities, laterally.
The anal canal typifies the meeting of structures with an endodermal and ectodermal derivation. The undulating dentate line specifically demarcates the boundary between the inferior hindgut and the proctodeum, both with their particular vascular, lymphatic, and nervous framework. The columns of Morgagni denote the 6 to 14 longitudinal folds of rectal mucosa that end as anal valves at the level of the dentate line; these anal valves represent remnants of the obliterated proctodeal membrane. Between the valves are found the anal crypts, particularly clustered in the posterior anus. Obstruction of these crypts gives rise to infections that produce an abscess or fistula. Approximately half of the anal crypts drain an anal gland, which number between 3 and 12, with an average of 6. However, one crypt may be affiliated with more than one gland. Unlike the anal crypts, the anal glands predominate in the anterior anal canal. Their epithelium is composed of stratified columnar cells and mucous-producing goblet cells. The glands travel from the anal crypts in an inferolateral direction to terminate in the submucosa, internal anal sphincter, conjoint longitudinal muscle, intersphincteric space, or external anal sphincter; yet, the majority of these glands culminate in the intersphincteric space.
The mucosal lining of the anus differs along its length. The upper anal canal features a stratified columnar epithelium with a deep violaceous hue due to the underlying internal hemorrhoidal plexus, or anal cushions. These anal cushions contain valveless submucosal arterial sinusoids predominantly contributed by branches of the superior rectal artery, enmeshed in elastic and connective tissues that are supported by the smooth muscle of the mucosal suspensory ligament. The anal cushions are characteristically located in the left lateral, right anterior, and right posterior positions, although smaller cushions may be identified in between these sites. Conversely, the lining of the anus situated 1.5 cm distal to the dentate line acquires a nonkeratinized stratified squamous epithelium that lacks dermal appendages, appearing smooth, shiny, thin, and taut. This delicate anoderm is firmly attached at the anal margin to the external hemorrhoidal plexus. Between the upper and lower anal canal, superior to the dentate line, is a 6 to 12 mm anal transition zone, whose mucosa includes an amalgamation of nonkeratinized squamous, transitional, basal cell, cuboidal, and columnar epithelium. Melanin-producing cells within the anal transition zone may be a source of anal melanoma. Beyond the anal verge, the skin regains its pigmentation and characteristic appendages, including hair follicles, sebaceous glands, and apocrine sweat glands. Infection of these sweat glands results in hidradenitis suppurativa.
The sphincter apparatus—the internal and external sphincters and the conjoint longitudinal muscle—encircles the anus, where it plays a crucial role in fecal continence. The internal anal sphincter muscle is a dense continuation of the inner circular layer of the rectal wall, extending for 2.5 to 4 cm beyond the rectum. At its origin at the anorectal ring, which is at the junction of the anus and rectum, the internal anal sphincter is met by the puborectalis muscle; division of the anorectal ring invariably results in fecal incontinence. This smooth muscle ends approximately 1 to 1.5 cm distal to the dentate line; the white-appearing sphincter thus encompasses the upper two thirds of the anal canal. On a digital examination, the inferior margin of the internal anal sphincter muscle is palpable as a thickened, fibrous, rounded edge. As the internal anal sphincter muscle terminates superior to the external anal sphincter, a groove—the intersphincteric sulcus (white line of Hilton)—is evident between the two muscles. On endoanal ultrasound, the internal anal sphincter is seen as a 2- to 3-mm uniformly hypoechoic band, 2.5 to 4 cm in length, demonstrated in the upper and mid-anal canal but absent in the lower anal canal ( Fig. 1-9 ). Hypertonicity of the internal anal sphincter, creating a relative ischemia in the posterior midline, is believed to result in anal fissure.
In contrast, the larger external anal sphincter is an elliptical band of skeletal muscle circumscribing the entire length of the anal canal. In females, the external anal sphincter is invested by smooth muscle fibers emanating from the internal anal sphincter and conjoint longitudinal muscle and by connective tissue. The small striated muscle fibers, red in color, lie in a circumferential pattern. Although the external anal sphincter was once described as including subcutaneous, superficial, and deep divisions, it is now considered a single cylindrical mass. The superficial portion of the posterior external anal sphincter blends with connective tissue to form the anococcygeal ligament, which secures the anus to the coccyx. On its anterior surface, the external anal sphincter joins with the perineal body. Superiorly, the external anal sphincter continues as the puborectalis muscle. A persisting source of dispute involves whether the external anal sphincter is a component of the puborectalis muscle. However, the two muscles have separate embryologic derivations and innervations; moreover, their muscle fibers do not intermingle. On endoanal ultrasound, the external anal sphincter is demonstrated as a 5- to 8-mm hyperechoic band in the mid and lower anal canal (see Fig. 1-9 ). Occult injuries to this muscle from childbirth may produce fecal incontinence. In the upper anal canal, the puborectalis muscle is identified posteriorly as a U -shaped hyperechoic structure.
The conjoint longitudinal muscle is comprised of fibers obtained from the outer longitudinal muscle of the rectum and the pubococcygeus muscle. Additionally, the endopelvic fascia of the levator ani contributes to this muscle. These muscle fibers are located between the internal and external anal sphincters, within the intersphincteric space. On ultrasound, this muscle appears as a narrow hyperechoic line within the intersphincteric space. The function of the muscle is uncertain. The conjoint longitudinal muscle may play a role in binding together the two sphincter muscles. Furthermore, some of these muscle fibers enter the ischiorectal fossa while others attach to the perianal skin as the corrugator cutis ani muscle. In these locations, the muscle may act to limit the spread of sepsis or of a thrombosed external hemorrhoid. The extension of this muscle medially to the smooth muscle of the submucosa, where it is fixed at a level inferior to the anal valves, is known as the mucosal suspensory ligament (ligament of Treitz); this muscle may serve as a scaffold to support the internal hemorrhoidal cushions, preventing their prolapse. Another theory supposes that the conjoint longitudinal muscle assists in defecation by everting the anus.
Perianal and Perirectal Spaces
Various potential spaces juxtapose the anus and rectum ( Fig. 1-10A and B ). These areas are clinically significant in the management of septic processes. Lateral to the anus and external anal sphincter muscles lay the ischiorectal fossae, which are divided by a narrow horizontal septum, formed by the conjoint longitudinal muscle, into the superior ischiorectal space and the inferior perianal space. The pyramidal ischiorectal space is limited superiorly by the levator ani and inferiorly by the perianal space. The space is confined anteriorly by the urogenital diaphragm and transverse perineal muscles, laterally by the pelvic side wall, and posteriorly by the gluteus maximus and sacrotuberous ligament. The inferior rectal vessels and pudendal nerve traverse the fat and connective tissue of the ischiorectal space anteriorly and medially toward the anus after exiting the pudendal (Alcock’s) canal on the superolateral wall.
The inferior third of the ischiorectal fossa is comprised of the perianal space, which circumscribes the anus at the level of the anal verge. Its borders range from the mucosal suspensory ligament at the dentate line to the corrugator cutis ani at the perianal skin. This area includes the external hemorrhoidal plexus. The perianal space communicates laterally with the subcutaneous tissue of the buttocks as well as superiorly with the intersphincteric space. This intersphincteric space—located amid the two anal sphincters—continues superiorly within the rectal wall between the inner circular and outer longitudinal muscle layers. The submucosal space between the anal mucosa and the internal anal sphincter contains the internal hemorrhoidal plexus. This space extends cranially to the rectal submucosa but ends caudally at the dentate line.
The superficial postanal space is located posterior to the anus, between the perianal skin and the anococcygeal ligament. The perianal spaces are in contact posteriorly via this space. The deep postanal space (retrosphincteric space of Courtney), situated superior to the superficial postanal space, is confined by the anococcygeal ligament and the levator ani. This space connects the ischiorectal fossae, allowing for the development of a horseshoe abscess or fistula.
The supralevator space is circumscribed by the rectosacral fascia superiorly and by the levator ani inferiorly. Its lateral boundaries are the pelvic side wall and obturator fascia. The rectum lies anterior to the horseshoe-shaped supralevator space. A septic process in the supralevator space either originates from a descending pelvic or an ascending cryptoglandular infection.
The retrorectal space rests between the fascia propria of the rectum and the presacral fascia. The posterior mobilization of the rectum ideally is performed in this avascular plane. Its lateral limits include the lateral ligaments of the rectum and the piriformis fascia. Superiorly, the space continues into the retroperitoneum, while it ends inferiorly at the rectosacral fascia. The retrorectal space only holds loose areolar connective tissue and a variable amount of fat. Nevertheless, blunt dissection should be avoided due to the possibility of injuring the presacral fascia and the underlying presacral veins.
Levator Ani
The levator ani (pelvic diaphragm) constitutes the floor of the pelvis ( Fig. 1-11 ). This structure, along with its investing fascia, is essential for support of the pelvic organs. Acute events such as pregnancy and vaginal childbirth place extreme stress upon the pelvic floor musculature. The fetal head stretches the levator muscles as it passes through the levator hiatus, with the degree of stretch directly proportional to the cranial size. In a study of the pelvic floor in women following vaginal delivery from DeLancey et al., 20% of the 160 primiparous women were found to have a defect of the levator ani on magnetic resonance imaging (MRI), as compared to none in the nulliparous group; 71% of these women with a visible injury complained of stress urinary incontinence. Additionally, cadaver studies in high-parity females have demonstrated fibrous replacement of the levator ani muscle.
