Effect of melatonin treatment on embryo development in patients who underwent IVF-ET program. Nine patients undergoing IVF-ET who failed to become pregnant in the prior IVF-ET cycle were given a 3 mg tablet of melatonin (KAL, Park City, UT, USA) orally at 22:00 hr from the fifth day of the previous menstrual cycle until the day of oocyte retrieval. Embryo quality was assessed according to Veeck criteria at 2 days after insemination, and Veeck I or II embryo was defined as a good embryo. Rate of good embryos was calculated; divide the number of good embryos (VeeckI or II) by the number of oocytes retrieved. Data are shown as the mean ± SEM. *: p < 0.05 versus control cycle.