Cryopreserved zygotes and embryos and endocrinologic factors in the replacement cycle

Several parameters relating to features of replacement cycles in 88 patients were found to influence implantation of cryopreserved zygotes and early cleaved embryos. Cryopreserved embryos were replaced in 47 patients with a natural cycle, resulting in 12 (25%) clinical pregnancies. Patients who had anovulation or irregular cycles received either 50 mg clomiphene citrate on days 5 to 9 or 100 mg on days 2 to 6; the incidence of clinical pregnancy was 7 of 23 (30%) and 2 of 18 (11%), respectively. Neither a rise in follicular phase estradiol (E2) nor absolute levels of E2 predicted implantation. The length of the follicular phase during the replacement cycle correlated well with previous menstrual cycles in 43 (54%) of the patients, and 16 (37%) of these patients became pregnant. The follicular phase was either longer or shorter than anticipated in 39 patients, and only 5 (13%) became pregnant: a detrimental effect was especially apparent when the follicular phase was shortened.