Defining the LH surge in natural cycle frozen-thawed embryo transfer: the role of LH, estradiol, and progesterone

Patients undergoing NC FET should be monitored by LH, estradiol and progesterone levels. We suggest that the LH surge should be determined by an increase in LH, concomitant to a drop in estradiol and a threefold increase in progesterone levels between D- 2 to D- 1. Further large prospective studies are needed to elucidate the aforementioned recommendation prior to its routine implementation.