Influence of number of pre-embryos transferred, progesterone level and oestradiol/progesterone ratio at thaw on pregnancy results during replacement of cryopreserved pre-embryos in natural cycles

When frozen-thawed pre-embryos were transferred in a natural menstrual cycle, a clinical pregnancy rate of 22% per transfer cycle was obtained. The pregnancy rates were higher when serum progesterone levels at thaw were less than or equal to 3.4 ng/ml than when they were greater than or equal to 3.5 ng/ml. This was reflected in a significantly higher oestradiol/progesterone ratio at thaw in the conception cycles than in the non-conception cycles.