Single blastocyst transfer yields similar pregnancy rates compared with multiple cleavage embryo transfer, with reduced twin rate, in patients with low number of fertilized oocytes

Background In patients with low numbers of embryos, there is not yet consensus on whether to extend culture to the blastocyst stage, especially due to the risk that some or all of the embryos will not make it to the blastocyst stage. The objective of our study was to evaluate pregnancy outcomes in patients with a low number of fertilized oocytes (< 4), comparing single blastocyst transfer to one or more cleavage embryo transfer. Results We analyzed 6795 cycles from the 2014-2105 Society for Assisted Reproductive Technology (SART) registry. All patients were <= 38 years old, had less than four fertilized oocytes, and were undergoing first fresh in vitro fertilization (IVF) transfer. Primary outcomes were clinical pregnancy (CP), live birth (LB), and miscarriage rate in both cleavage stage transfer and single blastocyst transfer. A secondary outcome was the rate of twin gestation. The comparison of interest in day of transfer included (1) single blastocyst vs single cleavage and (2) single blastocyst vs multiple cleavage stage. The association between day of transfer and primary outcome was investigated using logistic regression, controlling for the age, race/ethnicity, BMI, smoking, gravidity, parity, infertility diagnoses, and assisted hatching. Single blastocyst transfer was associated with an increased odds of CP (adjusted OR 2.03) and LB (adjusted OR 1.86) when compared to single cleavage transfer, and no statistically significant association was observed when comparing single blastocyst transfer to multiple cleavage embryo transfer for CP (adjusted OR 0.94) and LB (adjusted OR = 0.88). The odds of having twins among single blastocyst transfer was significantly lower compared to those odds that among multiple cleavage stage transfer (unadjusted OR 0.09). Conclusions While pregnancy outcomes are similar between single blastocyst and multiple cleavage embryo transfer, the twin rate is reduced significantly among the single blastocyst transfers in patients with a low number of fertilized oocytes.