Sepulveda, W.; Sebire, N. J.; Hughes, K.; Odibo, A.; Nicolaides, K. H. (1996). "The lambda sign at 10-14 weeks of gestation as a predictor of chorionicity in twin pregnancies". Ultrasound in Obstetrics and Gynecology. 7 (6): 421–423. doi:10.1046/j.1469-0705.1996.07060421.x. PMID8807758.
Sepulveda, W.; Sebire, N.; Hughes, K.; Kalogeropoulos, A.; Nicolaides, K. (1997). "Evolution of the lambda or twin-chorionic peak sign in dichorionic twin pregnancies". Obstetrics & Gynecology. 89 (3): 439–441. doi:10.1016/S0029-7844(97)85858-9. PMID9052601.
Sepulveda, W.; Sebire, N. J.; Hughes, K.; Odibo, A.; Nicolaides, K. H. (1996). "The lambda sign at 10-14 weeks of gestation as a predictor of chorionicity in twin pregnancies". Ultrasound in Obstetrics and Gynecology. 7 (6): 421–423. doi:10.1046/j.1469-0705.1996.07060421.x. PMID8807758.
Sepulveda, W.; Sebire, N.; Hughes, K.; Kalogeropoulos, A.; Nicolaides, K. (1997). "Evolution of the lambda or twin-chorionic peak sign in dichorionic twin pregnancies". Obstetrics & Gynecology. 89 (3): 439–441. doi:10.1016/S0029-7844(97)85858-9. PMID9052601.