the hormone levels are key when doing an FET - they can't transfer too early (before O)as the embryo needs hormonal support from the progesterone released after ovulation

essentially, in a "natural" or non-medicated cycle, they monitor ovulation, and then, depending on the age of the embie (day 3/day 5), they transfer the emby at that point post ovulation as the hormone levels are most appropriate for the embryo at that time

for a medicated cycle, no ovulation occurs - they essentially put you on hormone replacement therapy (like what post menopausal women take) and build up an endometrial lining - when that is sufficient, the clinic create a "dummy" ovulation day, start you on progesterone supplements, and the same number of days later (day 3/day 5) embryo is transferred

in a natural cycle the corpus luteum provides the progesterone support until the placenta takes over - for a medicated cycle, you will continue on meds (both HRT and progesterone) until placenta takes over about 12 weeks.

there are "in between cycles" where they might help to force ovulation, but they are considered a form of a natural cycle as ovulation still occurs...

HTH

BG