The TLI system can help day-3 single cleavage embryo transfer to obtain comparative clinical outcomes to day-4 or day-5

Summary The aim was to explore whether the time-lapse imaging system can help day-3 single cleavage embryo transfer to obtain comparative clinical outcomes to day-4 or 5. The data of 1237 patients who underwent single embryo transfer from January 1, 2018, to September 30, 2020, in our reproductive medicine centre were retrospectively analysed. They were divided into the day-3 single cleavage-stage embryo transfer (SCT) group (n = 357), day-4 single morula transfer (SMT) group (n = 129) and day-5 single blastocyst transfer (SBT) group (n = 751) according to the different embryo transfer stage. The clinical and perinatal outcomes of the three groups were analysed and compared. The clinical pregnancy rates of the patients in the day-3 SCT group, day-4 SMT group and day-5 SBT group were 68.07, 70.54 and 72.04%, respectively. The live birth rates were 56.86, 61.24 and 60.99%, respectively. The monozygotic twin (MZT) rate in the day-3 SCT group was significantly lower than that in the day-5 SBT group (P = 0.049). Regarding perinatal outcomes, only the secondary sex ratio had a significant difference (P < 0.05). After age stratification, no improvement was found in the pregnancy outcomes of patients >35 years of age receiving blastocyst transfer. Our findings suggest that for patients with multiple high-quality embryos on day-3, prolonging the culture time can improve the pregnancy outcome to some extent, but it will bring risks. For centres that have established morphodynamic models, day-3 SCT can also achieve an ideal pregnancy outcome and reduce the rate of monozygotic twins and sex ratio.