Objective. The purpose of this study was to characterize the incidence of EM in Ross 308 strain eggs. Materials and methods. A prospective cohort study was performed, through candling and embryodiagnosis, in 4 groups of eggs coming from 4 different age breeders. Other data such as percentage of infertility, malformation, malposition, cracked and contaminated eggs were reported. Results. General EM reached 16.08% IC95% (14.69; 17.60) and was different among the different age-groups (p<0.001), being higher for eggs from 64 week-old breeders with 27.66% IC95%(23.92; 31.99) and lower for the 47 weeks old ones (8.84% IC95%) (6.97; 11.22). In the first week of incubation 57.53% of the embryos died and mortality was at its highiest during days 1 and 3; in the second week EM was 38.42% died, with a maximum of deaths between days 19 and 21. Conclusions. This study allowed confirmation on the biphasic behaviour of EM, although there are variations in the peaks of mortality possibly attributed to differences in production conditions for each enterprise.