The inherent limitation in evaluating a dynamic process by a few snapshots at discrete time points is reflected in the recent observation that the result of embryo scoring can change markedly within few hours ( Montag et al., 2011). The dynamic nature of cell cleavage and embryo development is, however, well known as demonstrated with respect to fragmentation, evenness of blastomeres, appearance and disappearance of pro-nuclei (PN) and nuclei ( Payne et al., 1997 Hardarson et al., 2002 Lemmen et al., 2008) along with the change in blastomere numbers over time due to cell divisions.
The restricted use of alternative methods may largely be explained by the simplicity and cost-effectiveness of static morphological grading, when compared with most of the alternative methods, and by the lack of documentation for superiority of alternatives. There is a well-documented close correlation between morphological appearance and developmental stage of the embryo at given time points and developmental competence. Although many of these methods are promising, grading systems based on morphology remain the preferred way of assessing embryonic competence. aneuploidy screening, O 2 respiration, metabolic profiling and gene expression analysis ( Mastenbroek et al., 2007 Ottosen et al., 2007a, b Jones et al., 2008 Scott et al., 2008 Seli et al., 2010). Therefore, several areas have been investigated in search of additional markers of viability to supplement current criteria for selection, e.g. As eSET becomes increasingly applied in clinical practice, the challenge of identifying the single embryo with highest developmental competence in a cohort becomes crucial. Elective transfer of a single embryo (eSET) is an efficient method of reducing the risk of multiple gestations. To maximize the probability of pregnancy, multiple embryos are often transferred simultaneously, which increases the risk of multiple pregnancies and the associated neonatal complications and maternal pregnancy-related health problems ( Stromberg et al., 2002 Pinborg et al., 2003 Pinborg et al., 2004 Walker et al., 2004). Time-lapse monitoring, development kinetics, morphology, embryo selection IntroductionÄespite efforts to optimize current procedures, implantation rates of IVF embryos remain relatively low with a clinical pregnancy rate of ∼30% per transfer ( Andersen et al., 2008). This mini-review summarizes the current knowledge about dynamic markers of viability and discusses the potential clinical role of time-lapse analysis in embryo assessment and selection. Before such randomized controlled studies are organized, the most promising parameters to evaluate must be identified. Due to lack of larger, randomized clinical studies it remains to be elucidated whether embryo selection using dynamic parameters improves clinical outcome and which parameters are of significance.
The introduction of time-lapse equipment approved for use on human embryos offers novel clinical opportunities for continuous monitoring of embryos, enabling flexible evaluation of known morphological parameters and potentially introducing new dynamic markers of viability. Lately, more attention has been given to the assessment of dynamic embryo development as a tool for evaluating embryonic potential. Embryo morphology and developmental stage at given time points are closely correlated with developmental competence and assessment of morphological parameters at discrete inspection points thus remains the preferred way of evaluating embryonic potential. Selecting the most competent embryo therefore constitutes a major challenge in assisted reproductive technology. As elective transfer of a single embryo (eSET) becomes increasingly accepted, the need to improve implantation rates becomes crucial.