Immediate Versus Delayed Natural Cycle Frozen Embryo Transfers

Cycles will be monitored from either day-10 or-12, according to patient's menstrual cycle length, with serial analysis of blood serum LH and progesterone levels and assessment of dominant follicle growth. In a spontaneous cycle, ovulation will be determined by a LH surge (\>20 IU/L) and a corresponding rise in progesterone (\>0.8 ng/ml). In a triggered cycle, ovulation will be determined by the administration of an hCG trigger when the dominate follicle reached \>16 mm (and LH was \<20 IU/L).