Blastocyst Developmental Rate in Two Different Single Step Culture Media: a Sibling Oocyte Prospective Non-interventional Study

SUMMARY:

In ART procedures, blastocyst transfer can be considered the most physiological situation, as the embryo is placed in the uterine cavity at a stage most similar to that which occurs in nature. These potential benefits have induced more and more centres to move from cleavage stage to blastocyst stage embryo transfer all over the world (Maheshwari et al., 2016). Despite this, there are also some potential disadvantages. This approach decreases the total number of usable embryos (defined as those available for transfer or cryopreservation) (Glujovsky et al., 2016). Some concerns regarding the safety of this approach have also been raised. In particular, the extended duration of the in vitro culture may have long-term effect in the embryo development. In a recent meta-analysis, blastocyst stage transfer was in fact associated with increased risks of preterm birth (<37 weeks), very preterm birth (<32 weeks), large for gestational age and perinatal mortality, although the latter was only identified from one study. Conversely, blastocyst transfer was associated with a decrease in the risk of small for gestational age and vanishing twins, although this latter was reported by only one study (Martins et al., 2017).

Culture media plays a crucial role in this context. Two distinct approaches aimed at supporting embryo development to the blastocyst stage have been proposed: the "back to nature" sequential approach and the "let the embryo choose" single medium approach (Summers et al., 2003; Machtinger et al., 2012, Quinn 2012).

Sequential culture media system is designed to meet identified changing metabolic and nutritional requirements of the developing embryo from day 1 to day 5 (Gardner et al. 1996, 1997, 1998, 1999). With this approach, embryos are grown up to day 3 in a first medium and then, at the cleavage stage, are moved to a second one. The goal is to expose the embryos to stage-specific media, designed to reflect observed changes in concentrations of pyruvate, lactate, and glucose in the Fallopian tube versus the uterus (Gardner et al., 1996).

On the other hand, single culture media aim at allowing developing embryos to choose the nutrients they require, while at the same time minimizing stress from exposure to an abrupt change in their culture environment on day 3. Single media can be used with a medium change on day 3, or in a single-step uninterrupted culture, in which there is no medium refreshing on day 3 [Biggers et al., 2008]. Available data comparing the performance of single vs. sequential culture media suggest that both types of media seem to provide similar support to the developing embryo (Sfontouris et al., 2017).