Embryo density may affect embryo quality during in vitro culture in a microwell group culture dish

Culturing embryos in groups is a common practice in mammalian embryology. Since the introduction of different microwell dishes, it is possible to identify oocytes or embryos individually. As embryo density (embryo-to-volume ratio) may affect the development and viability of the embryos, the purpose of this study was to assess the effect of different embryo densities on embryo quality. Data of 1337 embryos from 228 in vitro fertilization treatment cycles were retrospectively analyzed. Embryos were cultured in a 25 mu l microdrop in a microwell group culture dish containing 9 microwells. Three density groups were defined: Group 1 with 2-4 (6.3-12.5 mu l/embryo), Group 2 with 5-6 (4.2-5.0 mu l/embryo), and Group 3 with 7-9 (2.8-3.6 mu l/embryo) embryos. Proportion of good quality embryos was higher in Group 2 on both days (D2: 18.9 vs. 31.5 vs. 24.7%; p < 0.001; D3: 19.7 vs. 27.1 vs. 21.2%; p = 0.029; Group 1. vs. Group 2. vs. Group 3). Cell number on Day 3 differed between Groups 1 and 2 (6.8 +/- 2.2; 7.3 +/- 2.1; p = 0.004) and Groups 2 and 3 (7.3 +/- 2.1 vs. 7.0 +/- 2.0; p = 0.014). Culturing 5-6 embryos together in a culture volume of 25 mu l may benefit embryo quality. As low egg number, position, and distance of the embryos may influence embryo quality, results should be interpreted with caution.