Background: The aim was to analyse multinucleation in relation to its incidence in time and in the population, and its correlation with clinical variables, with other morphological characteristics and with the implantation rate of cleavage stage embryos.
Methods: Retrospective analysis of 10 388 cleaved embryos from 1395 consecutive IVF/ICSI cycles in 700 patients between January 1, 1999 and June 30, 2002.
Results: Multinucleation was observed in 3491 (33.6%) embryos in 1107 cycles (79.4%) of 609 (87%) patients, more frequently on day 2 than on day 3: 2848 (27.4%) versus 1567 (15.1%) [relative risk (RR) = 1.82; 95% confidence interval (CI) = 1.72-1.92]. Its incidence increased with fragmentation: 31.0, 34.4 and 36.5% for fragmentation </=10%, 10-20% or 20-30%. It was increased in stimulation cycles that were shorter (34.9 versus 32.0%, RR = 1.09; 95% CI = 1.03-1.15), required a higher FSH dose (34.6 versus 32.0%, RR = 1.08; 95% CI = 1.02-1.14) and yielded more oocytes (34.5 versus 29.7%, RR = 1.16; 95% CI = 1.08-1.25). Four-cell embryos on day 2 showed minimal multinucleation (16.8%) as well as 8-cell embryos on day 3 (15.5%). Embryos counting both 4 blastomeres on day 2 and 8 on day 3 showed minimal multi nucleation (11.6%). Multinucleated embryos had a decreased implantation rate: 4.3% in single and 5.7% in double embryo transfers.
Conclusions: Multinucleation is a frequently observed phenomenon. It is associated with impaired cleavage and increased fragmentation and is compromising the ongoing implantation rate. Multinucleation should be part of embryo assessment.