There is a certain “art” in Assisted Reproductive Technologies. Part of which is the successful culturing of embryos. Providing the best environment for embryos to thrive is one part of the infertility puzzle that can be very complex. Based on many years of research, embryologists study the biology of embryos to determine their specific needs at specific developmental stages. These needs are met by culture media, culture conditions, incubator conditions, pH and temperature stability.
There are different protocols in which to grow embryos, one method is to culture embryos in individual drops of culture media under oil. The benefits of individual culture allow the embryologist to track each embryo’s development to choose the best one or two for transfer. This method is especially useful in PGD cases where it is necessary to keep embryos separated. Individual culture is not as essential when culturing embryos to the blastocyst stage, in fact, there is literature supporting group culture (growing embryos on feeder layer of cells) as beneficial to the growth and development of embryos. Some research shows an increase in compaction and blastocyst developmental rates which lead to a higher efficiency of an IVF cycle (with many blastocysts to choose from, any remaining blastocysts after transfer will be vitrified, so if the fresh ET does not lead to a pregnancy, many subsequent FETs [frozen embryo transfer] are available).
Reproductive experts have established time points linked to development to score embryos and which scores are definitive markers for pregnancy and live birth rates; all work in the laboratory is dictated by the time of hCG injection before the egg retrieval. Although scoring embryos is subjective, there are some checkpoints an embryo needs to meet to be deemed superior. Many checkpoints have been studied: oocyte assessments, fertilization checks, early cleavage, day 2, day 3, day 4, day 5 and day 6 checks. Oocytes are not exposed to ambient air nor handled during natural conception. Therefore, one goal in IVF is to limit the exposures to these un-natural events to the embryo. This means determining which assessment times are more crucial, it is our job to know when the embryos need to be assessed.
Maternal genes drive embryo development until the 6-8 cell stage, after this stage the embryonic genome turns on. Embryos that develop to the blastocyst stage have an increased prognosis versus transferring these embryos on day 2 or 3, especially for certain subgroups of patients (ex. male factor patients). A good day 2 score is an indicator of a good day 3 embryo; therefore it is not completely necessary to score the embryos both days. Also, scoring on day 2 allows the embryologist to remove any poor quality embryos from culture with the better embryos. With many blastocysts to choose from on day 5, a day 2 score will help with the selection process. It is well known that a good trophectoderm score is linked to good implantation rates. If a pregnancy does not result from blastocysts with good morphometrical scores, the physician will better understand the etiology of the infertility.