Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of blastocyst-stage versus cleavage-stage embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis

Data from observational studies show that ET at the blastocyst stage is associated with a higher risk of very preterm delivery. However, we were not able to adjust for confounders. Perinatal outcome data from existing randomized trials are needed to determine the safety of ET at the blastocyst stage compared with the cleavage stage.