1) Chang HJ, Lee JR, Jee BC, et al : Impact of blastocyst transfer on offspring sex ratio and the monozygotic twinning rate : a systematic review and meta-analysis. Fertil Steril 91 : 2381─2390, 2009
2) Newman R, Unal ER : Multiple Gestations. In : Obstetrics : Normal and Problem Pregnancies. 6th ed. eds by Gabbe SG, Niebyl JR, Simpson JL, et al. pp673─705, Elsevier, Philadelphia, 2012
3) Murakoshi T, Ishii K, Matsushita M, et al : Monochorionic monoamniotic twin pregnancies with two yolk sacs may not be a rare finding : a report of two cases. Ultrasound Obstet Gynecol 36 : 384─386, 2010
4) Shen O, Samueloff A, Beller U, et al : Number of yolk sacs does not predict amnionicity in early first-trimester monochorionic multiple gestations. Ultrasound Obstet Gynecol 27 : 53─55, 2006
5) Corbett SL, Shmorgun D : Yolk sac number does not predict reliably amnionicity in monochorionic twin pregnancies : a case of a monochorionic monoamniotic twin pregnancy with two distinct yolk sacs on early first-trimester ultrasound. Ultrasound Obstet Gynecol 39 : 607─608, 2012
6) Lopriore E, Sueters M, Middeldorp JM, et al : Twin pregnancies with two separate placental masses can still be monochorionic and have vascular anastomoses. Am J Obstet Gynecol 194 : 804─808, 2006
7) Moore KL, Persaud TVN, Torchia MG : The Developing Human. 9th ed. Elsevier, Philadelphia, 2013
8) Spitz L, Kiely EM : Conjoined twins. JAMA 289 : 1307─1310, 2003
9) Spencer R : Theoretical and analytical embryology of conjoined twins : part I : embryogenesis. Clin Anat 13 : 36─53, 2000
10) Spencer R : Anatomic description of conjoined twins : a plea for standardized terminology. J Pediatr Surg 31 : 941─944, 1996
11) Pajkrt E, Jauniaux E : First-trimester diagnosis of conjoined twins. Prenat Diagn 25 : 820─826, 2005
12) Cunningham FG, Leveno KJ, Bloom SL, et al : Williams Obstetrics. 23 ed. McGraw-Hill, New York, 2010
13) Sibire NJ, Sepulveda W, Jeanty P, et al : Diagnostic imaging of fetal anomalies. eds by Nyberg DA, McGahan JP, Pretorius DH, et al. Lippincot Williams&Wilkins, Philadelphia, 2003
14) Doubilet PM, Benson CB : “Appearing twin” : undercounting of multiple gestations on early first trimester sonograms. J Ultrasound Med 17 : 199─203, 1998