Effects of embryo density on cell number of day 3 embryos cultured in a 30-µl drop: a retrospective cohort study.

ABSTRACT

For individual cultures, findings on regulating embryo density by changing the microdrop volume are contradictory. The aim of this study was to investigate the relationship between embryo density and the developmental outcome of day 3 embryos after adjusting covariates. In total, 1196 embryos from 206 couples who had undergone in vitro fertilization treatment were analyzed retrospectively. Three embryo densities were used routinely, i.e. one embryo in a drop (30 µl/embryo), two embryos in a drop (15 µl/embryo) and three embryos in a drop (10 µl/embryo). Embryo quality on day 3 was evaluated, both the cell number of day 3 embryos and the proportion of successful implantations served as endpoints. Maternal age, paternal age, antral follicles and level of anti-Müllerian hormone, type of infertility, controlled ovarian stimulation protocol, length of stimulation, number of retrieved oocytes, number of zygotes (two pronuclei) and insemination type were covariates and adjusted. After adjusting fully for all covariates, the cell number of day 3 embryos was significantly increased by 0.40 (95% CI 0.00, 0.79; P = 0.048) and 0.78 (95% CI 0.02, 1.54; P = 0.044) in the 15 µl/embryo and 10 µl/embryo group separately, compared with the 30 µl/embryo group. The proportions of implanted embryos were 42.1%, 48.7% and 0.0% in the 30 µl/embryo, 15 µl/embryo and 10 µl/embryo groups respectively. There was no statistical significance (P = 0.22) between the 30 µl/embryo group and the 15 µl/embryo group. After adjusting for confounders that were significant in univariate analysis, embryo density was still not associated with day 3 embryo implantation potential (P > 0.05). In a 30-µl microdrop, culturing embryos with an embryo density of both 15 and 10 µl/embryo increased the cell number of day 3 embryos, which did not benefit embryo implanting potential, compared with individual culture of 30 µl/embryo.