To examine the relationship of early human embryonic development parameters to day 3 reactive oxygen species (D-3 ROS) levels in culture media.Prospective study.Tertiary care hospital.Patients were undergoing IVF (n = 92; 36 with intracytoplasmic sperm injection [ICSI]).The D-3 ROS levels in sample and control of each embryo culture dish were measured by the chemiluminescence method using a luminol probe.Embryo quality (days 3 and 5) and pregnancy rates (PR).The D-3 ROS level was significantly lower in pregnant cycles 26.8 ± 13.9 × 10 6 cpm (counted photon per minute) versus nonpregnant cycles 66.4 ± 39.4 × 10 6 cpm. This relationship was maintained when the cycles were stratified to conventional IVF (27.1 ± 14.95 vs. 67.0 ± 39.9 × 10 6 cpm) or ICSI (25.6 ± 12.75 vs. 65.5 ± 39.7 × 10 6 cpm). After controlling for all variables, D-3 ROS levels were negatively correlated with blastocyst development rate as well as PR. Odds ratio (OR) (95% confidence interval [CI]) of clinical pregnancy corresponding to a 10 × 10 6 cpm increase in D-3 ROS was 0.47 (0.30–0.74) for ICSI and 0.56 (0.37–0.85) for IVF.During extended in vitro culture, ROS generated in culture media by day 3 may be an important biochemical marker for blastulation. An increase of 10 units in D-3 ROS may decrease the clinical pregnancy by 41%.