Sequential progesterone monitoring: a novel protocol for transfer timing following natural cycle frozen embryo transfer

While natural cycle frozen embryo transfer (NC-FET) is increasingly common, practice variation exists in timing of embryo transfer (ET). Most protocols time transfer on the basis of an exogenous ovulation trigger or a precisely determined endogenous LH surge, resulting in cycle cancellation when ovulation occurs prior to trigger or is missed in the course of routine monitoring. Furthermore, the rate of progesterone rise following ovulation may vary among patients, such that a pre-specified time post-trigger or surge may not be optimal for all patients. With these limitations in mind, some physicians at our center have adopted a novel and more flexible NC-FET protocol with ET timing based on sequential progesterone (P4) levels.