Frozen embryo transfers can be performed using different protocols to prepare the uterine lining (endometrium) and time the transfer. The main difference between these approaches is whether ovulation occurs and how timing is controlled.
Medicated FET: This approach uses estrogen and progesterone to fully control the cycle. Ovulation does not occur, so there is no corpus luteum, and all hormones are given externally. Also called hormone replacement therapy (HRT) FET, programmed FET or artificial FET.
Natural cycle FET: Uses the body’s natural hormones, with embryo transfer timed based on ovulation. This includes:
- True natural cycle: ovulation occurs spontaneously and is monitored
- Modified natural cycle: ovulation is triggered with hCG to better control timing
Mild ovarian stimulation FET: A variation of natural cycle FET where medications (eg. letrozole or gonadotropins) are used to stimulate follicle development and induce ovulation, often for patients who do not ovulate regularly.
Natural proliferative phase (NPP) FET: A newer hybrid approach where the endometrium develops naturally during the first half of a cycle, then progesterone is started once the lining is adequate (≥7 mm) and ovulation hasn’t occurred.
In general, natural-based approaches include ovulation and the presence of a corpus luteum, while medicated cycles do not, which may influence hormone exposure and pregnancy outcomes.
For more background, see my post Comparing frozen embryo transfer/FET protocols.