Gavrizi S, et al. - This study was performed to attempt to characterize the timing of the luteal-placental shift utilizing pregnancies resulting from frozen embryo transfer (FET) as a model. Frozen embryo transfer provides an excellent model for the evaluation of the luteal-placental shift, as pregnant patients produce essentially no endogenous E2 or P4 until that time. By assessing both absolute E2 and P4 levels and also the rate of rise, they can surmise the timing of the actual shift. This information proposes that the actual shift occurs around 7 weeks' gestation and that patients undergoing FET ought to continue to receive estradiol replacement until at least 7 weeks' gestational age and progesterone replacement until at least 8-9 weeks' gestational age.