Morula and Cavitating Morula, is it Really Almost a Blastocyst?

Introduction: It is generally accepted that a morula on day 4 is the normal stage of a healthy developing embryo, but data is limited regarding etiology and prognosis when the embryo is still at the morula/ cavitating morula (CAVM) stage on day 5.

Aim: The aim of this study was to describe and compare the ongoing pregnancy rate between morula and CAVM transferred on day 5 and to describe and compare the blastulation rate between day 5 morula and CAVM.

Materials and methods: A single center, retrospective cohort study between January 2012 and November 2016. Three thousand three hundred and twenty-one cycles included 10,304 delayed development embryos on day 5 that were cultured until day 6.

Results: 186 patients had fresh embryo transfers of 82 embryos at the morula stage and 104 embryos at the CAVM stage. The pregnancy rate (15.8% vs. 21.1% p=NS) and the ongoing pregnancy rate (15.8% vs. 17.3% p=NS) were comparable between the two groups. The study group included 10304 day 5 delayed embryos: 5395 morulas and 4909 CAVM on day 5. The blastulation rate was significantly higher in the CAVM group compared to the morula group (39.2% vs. 20.4% p<0.001) and the progression to top quality embryo was significantly higher in the CAVM group (35.9% vs. 17.7% p<0.001).

Conclusions: Although the blastulation rate is improved in the CAVM group the pregnancy rate of fresh slow developing embryos is comparable between morula and CAVM.