Question special

It is interesting that when put to the test of randomization, there is no difference in the preterm delivery in both groups.
Previous large datasets except Kato et al. ( 2012) have suggested increase in preterm and very preterm delivery for even singletons born as a result of fresh embryo transfer as compared to those born after frozen embryo transfer. Mechanisms thought to be hyper-estrogenized endometrium and abnormal placentation leading to increased preterm delivery in babies born after fresh embryo transfer.
Any suggestions why results could be different in this study?