Randomization First, ninety random numbers generated by means of computer were randomly divided into study group and control group. Then, all subjects in an enrolled date order were arranged according to the sequence of random numbers. So, ninety subjects were randomly divided into the study group and control group. The random allocation sequence was concealed in a closed and dark-colored envelope until the day of oocyte pick-up. Randomization occurred after patients agreed to participate in the study (Figure 1). All women of two groups underwent a conventional IVF or ICSI procedure, but in study group, the patients were added with a uterine cavity injection of day 2 embryo culture supernatant. The IVF or ICSI procedure was performed by same physicians and laboratory technicians. The treating sequence of subjects was as same as the enrolling sequence. The study was not blinded, because the patients as well as the clinicians were aware of the study content.