Amniotic sheets: Imaging features, subtypes, and obstetric outcomes.

A 31-year-old G2P1001 woman with a history of polycystic ovarian syndrome (PCOS) and fibroids presented for evaluation of elevated maternal serum alpha-fetoprotein (MSAFP). Her first pregnancy resulted in a full-term SVD without obstetric or neonatal complications. She had no history of STIs or abnormal Pap smears, and no previous surgeries. At 17w0d GA during her second pregnancy, routine maternal screening identified elevated MSAFP of 3.16. Amniocentesis was performed at 20w2d, showing no chromosomal abnormalities, positive acetylcholinesterase, and negative fetal hemoglobin. Additional US and MRI were obtained to further evaluate for fetal anomalies. Prenatal US at 20w2d GA identified a dorsal vertebral defect with preserved lower extremity motion, most consistent with myelomeningocele ( Fig. 4A ). The fetal head was lemon-shaped with a cerebellum that tightly wrapped around the midbrain (banana sign), suggesting downward herniation of posterior fossa contents in the setting of Chiari II malformation ( Fig. 4B ). Two amniotic sheets were identified adjacent to the placental cord insertion, without fetal entanglement or entrapment ( Fig. 4C ). The amniotic sheets were incomplete, orthogonal to the placenta, and attached to the placental surface. Fetal MRI was performed at 21w0d GA to better characterize the neural tube defect. Since the MRI was protocoled to evaluate for fetal anatomy, the gestational sac was not entirely imaged and the amniotic sheets were not visualized. In addition, the appearance of the neural tube defect was interpreted as myeloschisis due to the absence of an overlying sac or membrane on the MR images. MRI confirmed the presence of Chiari II malformation with supratentorial ventriculomegaly and hindbrain herniation. At 24w3d GA, the patient underwent elective fetoscopic myelomeningocele closure with external cephalic version for fetal malpresentation. Intraoperatively, direct fetoscopy confirmed the presence of myelomeningocele rather than myeloschisis. Since the surgeons operated from the anterior uterus and the amniotic sheets were located along the posterior uterine body, the amniotic sheets were not visualized intraoperatively. After the surgery, the patient underwent 5 days of twice daily non-stress tests (NSTs) with reassuring fetal heart tones and fetal movement. After discharge, the patient had weekly reactive NSTs and US imaging that showed normal amniotic fluid, fetoplacental Dopplers, and fetal movement. Postprocedural fetal MRI was obtained at 27w3d GA and showed a repaired posterior lumbosacral spinal dysraphism and persistent tethered spinal cord. MRI was not protocoled to optimize visualization of the gestational sac, and the amniotic sheets were not visualized. The pregnancy course was also complicated by FGR, with fetal abdominal circumference measuring at the 4th percentile by 28w2d GA. The patient ultimately underwent an uncomplicated, scheduled primary transverse cesarean section at 36w2d GA due to history of fundal hysterotomy. The infant had Apgar scores of 8 and 9 at 1 and 5 min, respectively, and did not require NICU admission.

First described by Mahony and colleagues in 1985 [ 7 ], amniotic sheets occur in approximately 0.45% to 1.13% of pregnancies [ 12 , 14 , 15 ] and can form when growing chorioamniotic membranes reflect against uterine synechiae. Although the terms amniotic sheets and synechiae have been used interchangeably in the literature, in this review, amniotic sheets refer to redundant chorioamniotic membranes and synechiae refer to pre-existing uterine adhesions [ 1 – 3 ]. Risk factors for amniotic sheets include previous pregnancy and curettage [ 14 ], and 32% to 78% of patients with amniotic sheets have had a documented history of prior uterine instrumentation [ 8 , 9 , 11 , 12 ]. These estimates are limited as these studies did not collect other causes of endometrial trauma which may result in synechiae, such as prior endometrial infection. In this review, all patients with amniotic sheets had at least one prior pregnancy. Cases with the most extensive amniotic sheets, such as Case 2, also had prior deliveries complicated by a retained placenta requiring manual removal. Ultrasonography remains the first-line imaging modality for identifying amniotic sheets and other intrauterine membranes, such as amniotic bands, circumvallate placenta, and uterine septa. Since intrauterine membranes each carry different fetal and obstetric risks, it is critical to distinguish these entities on prenatal imaging to guide appropriate counselling, surveillance, and delivery planning [ 6 ]. The identification of amniotic bands is particularly important, as amniotic bands are associated with fetal anomalies and still-birth, and incorrectly interpreting other types of intrauterine membranes as an amniotic bands on prenatal US can lead to parental anxiety and unnecessary surveillance efforts [ 16 ]. Other intrauterine membranes, including amniotic sheets, circumvallate placenta, and uterine septa, are not associated with fetal anomalies but are linked to obstetric complications like preterm birth [ 15 , 17 ]. Intrauterine membranes can be differentiated on prenatal US based on their thickness, attachment sites, orientation, and relationship to fetal anomalies, as summarized in Table 1 . Circumvallate placenta develops when there are discrepancies between chorionic and basal plate size, leading to infoldings of amnion and chorion at the placental margin. For this reason, circumvallate placenta presents on US as a short, thick echogenic wedge that follows the placental circumference [ 6 ]. Uterine septa result from partial or failed septal resorption following Müllerian duct fusion and present on US as a flat structure isoechoic to the myometrium. Septa extend from the uterine fundus, and the external uterine fundal contour can be mildly concave, flat, or convex [ 17 ]. Since amniotic bands are formed by fibrous remnants of amnion, they present as wavy, thin echogenic strands that attach to the uterine wall and sites of fetal defects, such as asymmetric craniofacial anomalies, limb and digit amputations, and abdominal or chest wall defects [ 18 ]. On the other hand, amniotic sheets appear thicker than amniotic bands on US because amniotic sheets consist of four layers, with two layers of chorion encased between two layers of amnion ( Fig. 5 ), and amniotic sheets are not associated with fetal entrapment or anatomic anomalies. [ 16 ]. Since they may develop by folding over uterine synechiae, amniotic sheets often have a wide triangular base that encapsulates the inciting synechiae [ 6 ]. In this review, all four patients were diagnosed with amniotic sheets on second trimester ultrasound, with the amniotic sheets appearing as linear intrauterine echogenicities with a triangular base. Three-dimensional (3D) US also played an important role in distinguishing amniotic sheets from other intrauterine membranes and identifying sheet location, discontinuities, and potential attachments to the fetus and umbilical cord. In select cases, complementary MR imaging was obtained to provide a broader overview of the intrauterine contents. On T2-weighted MR imaging, amniotic sheets appeared as homogenous, linear hypointensities with isointense, triangular bases. Compared to amniotic sheets, the placenta appears homogeneously hyperintense with scattered flow voids and the myometrium is heterogeneously hyperintense. In this review, both 3D US and pelvic MRI with T2-weighted steady state free precession and turbo spin echo sequences allowed for further characterization of amniotic sheets, including the presence of discontinuities and uterine compartmentalization. Amniotic sheets can be further characterized by location [ 19 ], orientation relative to the placenta [ 12 ], presence of discontinuities [ 19 , 20 ], placental implantation, and degree of uterine compartmentalization [ 19 ]. While amniotic sheets have not been associated with fetal anomalies, some subtypes of amniotic sheets have been linked with obstetric and fetal risks. In one case series, amniotic sheets perpendicular to the placental surface were associated with fetal malpresentation, as compared to sheets that were parallel or oblique to the placental surface [ 12 ]. Cases of partial placental implantation onto amniotic sheets have been reported but have not been associated with obstetric or fetal complications [ 10 ]. Although amniotic sheets do not attach to or directly restrict fetal movement, sheets may partially compartmentalize the amniotic cavity and prevent movement of the fetus into a vertex presentation [ 19 – 21 ], Thus far, five cases of intrauterine demise have been reported in patients with amniotic sheets [ 2 , 14 , 19 ]. Postmortem exam in three cases showed evidence of fetal asphyxiation or anoxia [ 2 , 19 ], with constriction bands on the umbilical cords of two cases [ 19 ]. In the two latter cases, the authors hypothesized that umbilical cord prolapse through discontinuities in the amniotic sheet could have led to fetal asphyxiation and intrauterine demise. Patients in this review had amniotic sheets with varying characteristics, as summarized in Table 2 . Cases 1 and 2 had extensive amniotic sheets, which presented on US and MRI as thick, discontinuous membranes that partially compartmentalized the uterine cavity. Case 1 involved an incomplete, lower uterine segment (LUS) amniotic sheet that was perpendicular to the placenta. The patient’s pregnancy was complicated by preterm labor and NICU admission for respiratory distress. Case 2 included T-shaped, incomplete, LUS amniotic sheets that were oblique to the placenta with mild placental extension along the sheets. During the pregnancy, there was intermittent oligohydramnios as amniotic fluid shifted between the fetal and other compartments, as well as intermittent herniation of the umbilical cord through discontinuities in the amniotic sheet. This pregnancy was complicated by previable PPROM with fetal demise. Cases 3 and 4 had thin, less extensive amniotic sheets that were non-compartmentalizing. Case 3 had a thin amniotic sheet that was parallel to the placenta and partially adherent to the umbilical cord. The pregnancy was complicated by fetal growth restriction. Case 4 had two thin amniotic sheets that were perpendicular to the placenta. These amniotic sheets were visible on US but not on MRI, likely because the sheets were thinner and non-compartmentalizing and the MRI was protocolized for fetal rather than gestational sac imaging. Overall, cases with extensive and compartmentalizing amniotic sheets faced obstetric and fetal complications. For example, in Case 2, the compartmentalizing amniotic sheet occurred alongside the development of intermittent oligohydramnios in the fetal locule. Herniation of the umbilical cord through amniotic sheet discontinuities may have intermittently constricted cord flow and increased risk for FGR and asphyxia. In this review, obstetric risk profiles varied with amniotic sheet subtype, especially in the presence of discontinuities and uterine compartmentalization on US and MRI. Limitations of this study include its small sample size and observational nature. As a result, this review only intends to summarize imaging features of amniotic sheet subtypes and to describe patient outcomes in recent cases. Additional research is ultimately required to determine the true relationship between certain amniotic sheet imaging features and obstetric risks. When reviewing imaging studies of the gravid uterus, practical pearls to keep in mind include the distinguishing features among various intrauterine membranes of pregnancy, the appropriate MRI sequences for gestational sac imaging, and the importance of characterizing certain amniotic sheet subtypes. Since the differential diagnoses for intrauterine membranes of pregnancy remain broad, identifying membrane attachment sites, thickness, and associated fetal anomalies on US can help distinguish each entity and guide accurate counseling and surveillance. While prenatal US is the first-line modality for identifying amniotic sheets, MRI can play a complementary role in providing a larger field of view and better soft tissue characterization. Pelvic MRI with T2-weighted steady state free precession and turbo spin echo are the recommended sequences for thorough characterization of the gestational sac. Finally, although amniotic sheets are not associated with fetal malformations, rare cases of obstetric complications and fetal demise have been reported. If amniotic sheets are present on an imaging study, attention should be paid to reporting signs of uterine compartmentalization and umbilical cord herniation. Ultimately, the thorough characterization of amniotic sheet features with prenatal US may be important to guide follow-up surveillance imaging, prenatal management, and patient counseling. 3D US and MRI are complementary modalities that may help further characterize amniotic sheet characteristics, especially sheet location, discontinuities, and uterine compartmentalization.

Amniotic sheets, also termed amniotic shelves, are intrauterine membranes that form when the amnion and chorion grow and fold against preexisting uterine adhesions, or synechiae [ 1 – 3 ]. Uterine synechiae typically develop following endometrial trauma, such as prior curettage, myomectomy, endometrial infection, pregnancy, or caesarean section [ 4 , 5 ]. Routine prenatal ultrasonography (US) can identify various intrauterine membranes, including amniotic sheets, amniotic bands, and circumvallate placenta. Early differentiation of these intrauterine findings is critical to guiding prenatal counselling and surveillance, as each entity is linked with varying fetal and obstetric risk profiles. [ 6 ]. Although amniotic sheets have not been associated with fetal anomalies [ 2 , 3 , 7 – 11 ], growing evidence suggests that amniotic sheets are linked with an increased risk of fetal malpresentation, cesarean delivery [ 10 ], preterm birth [ 12 – 14 ], low birth weight [ 12 , 14 ], and neonatal intensive care unit (NICU) admission [ 13 , 14 ]. Accurate and thorough prenatal characterization of amniotic sheets is essential to guide patient counseling, provide appropriate prenatal screenings, and anticipate obstetric and neonatal needs or complications. In this review, we present four patients with amniotic sheets diagnosed via US with or without accompanying magnetic resonance imaging (MRI).