Christophe Blockeel continues the endometrial theme, now for frozen transfer (FRET): which endometrial preparation for transferring a thawed embryo gives the best outcome.
Three basic approaches are contrasted: the natural cycle, the artificial (HRT) cycle and the modified-natural cycle. Each has its own logic of controllability, physiology and risk.
The artificial cycle is simple and convenient for timing — estrogen prepares the endometrium, progesterone opens the implantation window, and the transfer day is easy to plan. But convenience comes at a price.
The key figure of the talk is the miscarriage rate: in artificial cycles it is higher than in natural ones, roughly 40% versus 25%. This is a fundamental difference that tilts the choice toward physiology.
The mechanism is the same as for fresh transfer: an HRT cycle has no corpus luteum and none of the vasoactive factors it produces, which affects placentation and obstetric outcomes.
Blockeel ties transfer timing to the day from progesterone start: blastocyst transfer falls on day 5, 6 or 7 of progesterone support — and the precision of this window is critical.
A one-day error in either direction shifts the synchrony of embryo and endometrium, lowering the chance of implantation; hence the importance of standardisation and, where possible, progesterone monitoring.
Natural and modified-natural cycles bring the corpus luteum back into play, which is more physiological and, by accumulated data, safer in obstetric terms — at the cost of more demanding monitoring.
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