Introduction: Spontaneous septostomy is a rare complication of multiple gestations. Related complications include cord entanglement and preterm delivery. Limited data exist to guide the management of these high-risk patients. The majority of spontaneous septostomy cases have been reported in monochorionic diamniotic twins. We present 2 cases of spontaneous septostomy occurring in a monochorionic/dichorionic hybrid twin gestation (chorionicity transitions from dichorionicity to monochorionicity within the placenta) and in a dichorionic triamniotic triplet gestation. Case Presentation: Case 1 was a monochorionic/dichorionic hybrid twin gestation with a septostomy complicated by fetal parts of one twin protruding into the co-twin’s sac as well as symptomatic polyhydramnios. Fetal magnetic resonance imaging confirmed the septostomy. Case 2 was a dichorionic triamniotic triplet gestation with septostomy and cord entanglement. Both patients were managed akin to a pseudo-monoamniotic gestation with serial ultrasound surveillance and eventual inpatient admission for heightened fetal monitoring. Case 1 underwent elective scheduled cesarean delivery at 33 weeks, and case 2 underwent emergent cesarean delivery for fetal heart rate decelerations at 28 weeks. Conclusion: With a high degree of clinical suspicion, spontaneous septostomy can be diagnosed in uncommon settings such as hybrid twin gestations and higher order multiples. Management of such patients is individualized and may include a combination of heightened outpatient and inpatient surveillance.

Adekola H, Unal ER, Thompson G, Sondgeroth K, Abrams R. Spontaneous septostomy in a monochorionic-diamniotic twin gestation. J Clin Ultrasound. 2021;49(1):66–70.
Gordon BJ, Chon AH, Korst LM, Llanes A, Miller DA, Chmait RH. Incidental septostomy after laser surgery for twin-twin transfusion syndrome: perinatal outcomes and antenatal management. Fetal Diagn Ther. 2018;44(4):285–90.
Chmait RH, Aghajanian P, Kontopoulos EV, Quintero RA. Prenatal diagnosis of spontaneous septostomy of the dividing membranes in complicated monochorionic diamniotic multiple gestations. J Ultrasound Med. 2009;28(5):663–8.
Gilbert WM, Davis SE, Kaplan C, Pretorius D, Merritt TA, Benirschke K. Morbidity associated with prenatal disruption of the dividing membrane in twin gestations. Obstet Gynecol. 1991;78(4):623–30.
Nasrallah FK, Faden YA. Antepartum rupture of the intertwin-dividing membrane in monochorionic diamniotic twins: a case report and review of the literature. Prenat Diagn. 2005;25(9):856–60.
Chadha R, Lange IR, Bratz L, Cooper SL, Roggensack A, Johnson JA. A rare case of antepartum spontaneous septostomy in a monochorionic diamniotic twin pregnancy. Case Rep Obstet Gynecol. 2012;2012:748614.
Chen SE, Trupin L, Trupin S. Antepartum rupture of diamniotic membranes separating monozygotic twins. A case report. J Reprod Med. 1994;39(1):67–70.
Jeanty C, Newman E, Jeanty P, Rodts-Palenik S. Prenatal diagnosis of spontaneous septostomy in dichorionic diamniotic twins and review of the literature. J Ultrasound Med. 2010;29(3):455–63.
Ito A, Nakata M, Oji A, Takano M, Umemura N, Nagasaki S, et al. Diagnosis of umbilical cord entanglement in a monochorionic diamniotic twin pregnancy with spontaneous septostomy of the dividing membranes using dual-gate Doppler imaging. J Med Ultrason. 2018. 45(1):189–92.
Bevilacqua E, Aliberti C, D’Ambrosio V, Giancotti A, Perrone G, La Torre R. Spontaneous septostomy in monochorionic diamniotic twins: difficult diagnosis, difficult management. J Obstet Gynaecol. 2014;34(4):359.
Sherer DM, Bitton C, Stimphil R, Dalloul M, Khoury-Collado F, Abulafia O. Cord entanglement of monochorionic diamniotic twins following spontaneous antepartum septostomy sonographically simulating a true knot of the umbilical cord. Ultrasound Obstet Gynecol. 2005;26(6):676–8.
Fleming T, Miller T. Spontaneous septostomy in monochorionic diamniotic twins resulting in cord entanglement and fetal demise. Australas J Ultrasound Med. 2012;15(3):103–6.
Reif P, Haeusler M, Gellen J. Fetal head entrapment through a spontaneous chorionic septostomy in a first trimester dichorionic diamniotic twin pregnancy: a case report. Prenat Diagn. 2013;33(3):298–300.
Chmait RH, Floyd R, Benirschke K. Duplicity: sonography suggested a twin gestation was dichorionic-and monochorionic. Am J Obstet Gynecol. 2011;205(1):87 e1–2.
Hall JG. Twinning. Lancet. 2003;362(9385):735–43.
McNamara HC, Kane SC, Craig JM, Short RV, Umstad MP. A review of the mechanisms and evidence for typical and atypical twinning. Am J Obstet Gynecol. 2016;214(2):172–91.
Corner GW. The observed embryology of human single-ovum twins and other multiple births. Am J Obstet Gynecol. 1955;70(5):933–51.
Herranz G. The timing of monozygotic twinning: a criticism of the common model. Zygote. 2015;23(1):27–40.
Neuman J, Calvo-Garcia MA, Kline-Fath BM, Bitters C, Merrow AC, Guimaraes CV, et al. Prenatal imaging of amniotic band sequence: utility and role of fetal MRI as an adjunct to prenatal US. Pediatr Radiol. 2012;42(5):544–51.
Chon AH, Korst LM, Llanes A, Miller DA, Ouzounian JG, Chmait RH. Midtrimester isolated polyhydramnios in monochorionic diamniotic multiple gestations. Am J Obstet Gynecol. 2014;211(3):303 e1–5.
You do not currently have access to this content.