The goal of ART labs over the last ten years was to find embryos with the best chance of implantation. In order to improve selection and increase implantation rates, blastocyst stage transfers on day 5 become more frequent considering that morphology on day 3 embryos may not be a reliable indicator of pregnancy possibility. The current study aimed to assess the clinical outcomes of Day 3 ETs and Day 5 ETs and compare the effect of the number of transferred embryos in clinical pregnancy and early abortion rates performed on D3 and D5. A randomized controlled trial was performed with 200 women under 40 years with primary infertility undergoing ICSI. On day 3 and day 5 ET, participants were divided into two groups of 100 each. After ICSI and controlled ovarian hyperstimulation, ET was carried out under ultrasound monitoring. Clinical pregnancy rates and early abortion rates were among the outcome measures. The current results indicated that there were no significant differences among the two groups regarding sperm parameters, duration of infertility, oocyte retrieval. Day 5 transfers showed the highest clinical pregnancy (71.9%) followed by day 3 (46.9% and 50.0%). Abortion rates were highest on day 3 (50.0%) and lowest on day 5 (12.1%). It was concluded that Day 5 ET yielded the best pregnancy outcomes when compared with Day 3. Further prospective researches are essential to optimize ET numbers and enhance ART outcomes.