4Bamhart KT, Simhan H, Kamelle SA. Diagnostic accuracy of ultrasound above and below the beta - HCG discriminatory zone. Obstet Gynecol, 1999, 94 (4): 583 -587
5Batzer FR, Schlair S, Goldfarb AF et al. Serial beta - subunit human cho - rionie gonadotropin doubling time as a prognosticator of pregnancy outcome in an infertile population. Fertil Steril, 1981, 35 (3) : 307 -312
6Pulkkinen MO, Jaakkola UM. Low serum progesterone levels and tubal dysfunction - a possible cause of ectopic pregnancy. Am J Obstet Gynecol, 1989, 161 (4): 934-937
