The main goal of this study was to determine the embryo with high adherence potential to the endometrium that would increase the clinical pregnancy rate in ICSI cycles by morphological parameter evaluation. The result clearly showed that MBD higher than 49.73 μm could predict embryos more ideal for transfer, thus increasing the clinical pregnancy rates in ICSI cycles.
Embryo morphology evaluation on day 3 is useful in the determination of the best embryos [10]. In the past, more than two embryos were transferred to increase pregnancy rates. However, it has been found that this approach also increases the rate of multiple pregnancy and poses additional medical risks for the mother and baby [11]. Changing embryo selection criteria over the years has brought innovations in the selection of the best embryo to implant, and the process of recovery in clinical pregnancies and a decrease in multiple pregnancies has started [11, 12]. The risks that develop with the transfer of excess embryos have been included in the relevant scope of international authorities on ART, and many associations have published guidelines on the subject [13]. All these developments have put pressure on IVF centers to transfer a small number, preferably a single embryo. This has led to the intensification of research on the best embryo selection criteria.
Although many morphological evaluation methods related to blastomeres have been defined, the effect of MBD on ART success is still unknown.
In 2003, Johansson et al. studied the DNA content of blastomeres/fragments using donated excess embryos. They included grade 3 embryos, which were defined as >20% but <50% fragments and/or blastomeres/cells of all sizes and/or heavily granulated cytoplasm or vacuolation. According to this study, the blastomere size should be over 45 μm in the second day embryo and over 40 μm in the third day embryo in order to be called a blastomere. They concluded that cells smaller than this were always anucleated and should be considered fragments [14]. This suggests a relationship between the increase in blastomere diameter and impaired development of the embryo. It is possible that this situation will affect pregnancy rates in IVF.
According to the results of the current study published by Gardner et al., low, medium, or high viability classification determined by morphometric evaluation of the embryo is important to the implantation of the embryo and clinical pregnancy rates. Ideal day 3 embryos with high viability are described as the ones with mononucleated blastomeres, equal cell size, <20% fragmentation, and at least 4 blastomeres. Thus, ideal embryos are related to higher implantation, pregnancy, and birth rates [15]. Our study is the first to measure MBD and relate the cutoff value to the clinical pregnancy rate. The researchers concluded that bio- and/or morphokinetic markers are important in the selection of the embryo for transfer, among which morphological data play a key role [15].
Alpha Scientists in Reproductive Medicine and the ESHRE Special Interest Group of Embryology organized a meeting in 2010 in Istanbul. At this meeting, oocyte, zygote, and embryo grading criteria were evaluated in the IVF laboratory, and a consensus was reached [6]. According to this consensus, the embryo is expected to have 4 cells on the second day and 8 cells on the third day, depending on the time of ICSI or IVF. Embryo fragmentation was evaluated as mild when below 10%, moderate when it was 10–25%, and severe when it was above 25%. If the appearance of multinucleation is detected, the implantation potential of the embryo will be low, the rate of chromosomal anomalies will be high, and the rate of spontaneous abortion will be high [6, 15]. Criteria such as blastomere size evaluation, cytoplasmic granularity scoring, membrane appearance scoring, and cytoplasmic vacuole presence grading in the second- or third-day embryo are parameters that can be used in terms of morphometric evaluation. These results were obtained from large series and data from many centers. However, MBD was not defined as a morphometric feature, as in other studies.
According to the results of the study on cases with another single embryo transfer, the effects of parameters such as blastomere volume index, blastomere symmetry index, and mean ovality on the ongoing pregnancy rate were investigated. The third day embryo was evaluated in this context [16]. It was concluded that these parameters, which were established with the help of a computer and determined by special formulas, were not useful in terms of embryo selection criteria [16].
Another question is how important morphometric evaluation before embryo transfer might be or whether multiple evaluations will be required. In a systematic review, Kaser and Racowsky investigated the selection of embryos with a time-lapse monitoring system [17]. In this review, the effects of many morphometric parameters (pronuclear dynamics and morphology, first cytokinesis time, nucleus disappearance time after cleavage, time to reach cleavage phases, cleavage cycle time, time to reach morula, time to reach blastocyst phase, etc.) were investigated. Although many studies have reported that embryos that pass the cleavage stage faster have higher implantation potential, it has been concluded that a single morphokinetic parameter cannot be used to evaluate the embryo with the best implantation potential [17].
There are no studies reporting the relationship between blastomere size and pregnancy rates in IVF. However, of course, we estimate that the pregnancy rate will be high in cases with a high blastomere size, up to a certain limit. As the blastomere size increases, the embryo surface area will also increase. The increase in surface area will also increase the area of interaction between the endometrium and the blastomere surface to be implanted. In this way, pregnancy rates may increase, as there will be a large number of endometrial and embryo receptor interactions.
The most important strength of this study is the selection of the embryo with the best implantation potential using the embryo morphometric measurement to be performed on the day of transfer in cases with single embryo transfer. In this way, an embryo with a high pregnancy rate will be transferred. To our best knowledge, MBD has not previously been proposed as a predictor for ICSI outcome. When an embryo with an MBD measurement greater than 49.73 is selected for transfer, in addition to obtaining a high clinical pregnancy rate, multiple pregnancies and their maternal fetal complications may be prevented.
There are some limitations to present study. In this study, the embryo morphological assessment was individual, dependent, and subjective, and there were no intermittent measurement values [18]. Another limitation is the small number of cases. In addition, although all MBD measurements were made by a single embryologist, diameter markings were made manually. The whole study was built on measurements of blastomeres from a single photo, which means that we may have at least 2 blastomeres with no clear outline. This could have some subjective consequences. For the most objective evaluations, a computer-based system would be helpful.
