In frozen embryo transfers there is no estrogens level minimum cutoff to decide doing a transfer or not. If the endometrium is thick enough (at least 6 mm, but ideally over 7 mm), we can do the embryo transfer, regardless the estradiol level. In a cycle of IVF, high estrogens levels are thought as non-physiological and some recommend an embryo transfer in a different cycle. I agree although the evidence supporting it is still weak. Levels of progesterone are important both in a fresh IVF cycle and in a FET. Most studies showed that levels over 1.5 are associated to lower pregnancy rates. However, most authors recommend measuring that cutoff in each clinic as it may vary. We use 1.5 as a maximum. Although there are few studies talking about an endometrium too thick and conclusions are missing, when it is over 14 mm we prefer to cancel and start again.

There is really no minimum or maximum estrogen and progesterone. Most clinics will want the former to be above 150 and the latter 10 but it will vary based on the route (oral, patch, vagina) that you are taking your medication. Most clinics like the lining to be 7mm or greater. For women with a history of thin lining a 6 may be adequate.