Grade and looseness of the inner cell mass may lead to the development of monochorionic diamniotic twins.

Objective To examine the relationship between the inner cell mass (ICM) grade and its morphological configuration on the occurrence of monochorionic diamniotic (M-D) twinning. Design Retrospective embryo cohort study. Setting Private IVF clinic. Patient(s) Evaluation of frozen-thawed single blastocyst transfers with hormone replacement treatment in 8,435. This cohort included 71 blastocysts and their ICMs observed by time-lapse photography. Intervention(s) Any changes in configuration of the ICMs observed by time-lapse photography were analyzed retrospectively. Main Outcome Measure(s) The amount of loosening of blastomeres within the ICM was evaluated by time-lapse observations. The number of cells that were involved in the loosening process was also assessed. Both of these parameters were correlated with the type of monozygotic twinning that eventuated. Result(s) The M-D twinning incidence resulting from blastocysts with a high grade ICM (grade A) were transferred was 0.38% (3/796), whereas it was significantly higher, 1.38% (34/2,463), when blastocysts with a poorer (B and C) grade ICM were transferred. Among 71 transferred frozen-thawed blastocysts that were studied with time-lapse photography, there were two dichorionic diamniotic and one M-D twins. Careful observations of the embryo that resulted in the one M-D case, revealed that the ICM acquired a looser appearance due to decompaction of at least eight cells. This type of decompaction was not observed in the ICMs of other transferred blastocysts. Conclusion(s) The occurrence of M-D twinning may be avoided by excluding blastocysts that contain decompacting ICMs.