Deciding to undergo in vitro fertilization (IVF) is a big step in your fertility treatment. This complicated process requires several steps to ensure a successful IVF cycle. The embryo culture is a critical stage during in vitro fertilization.
Embryo culture begins immediately following egg retrieval and lasts for the next several days. Since this phase includes fertilization, it can often be a stressful time for you and your partner. You will learn how many eggs are successfully fertilized and will be eligible for transfer.
Your eggs and sperm are washed after they’re collected to remove any impurities or toxins and maximize the likelihood of successful fertilization. Once the eggs are optimally mature, usually after two to six hours, a sperm sample is introduced to each egg.
An embryologist will closely monitor the developing fertilized egg, or zygote. At 18 hours, the cell should contain two pronuclei, which means they contain genetic material from both partners. In another 24 hours, the embryologist will look for cell division. Depending on the lab, the embryo may be monitored further or transferred.
Embryos will be cultured for anywhere from two to five days before being transferred. The environment is critical to successful embryo cultures. During the monitoring process, your lab and fertility specialist will work closely to determine the optimal length of time before transfer.
After two days, the embryo is typically at the two to four cell stage. Your lab and fertility specialist may recommend transferring the embryos at this stage if they are developing poorly or only a small number of embryos were successful.
At three days, the embryo should be six to eight cells. Most laboratories will then opt for extended culture for 2 more days, to allow the embryo to develop into a blastocyst. Embryo development up until this point has been able to progress without the embryo processing its DNA and making new proteins. In order for the embryo to develop beyond the 8 cell stage, it must be able to activate its own genes and produce its own proteins. By thus proving its functionality, the later stage embryos now have a greater likelihood of implanting and developing into a viable fetus.
25-60% of embryos will survive to the blastocyst stage, which generally occurs after five days. With > 64 cells, the blastocyst is nearly ready for implantation. This is the stage at which a small number of cells from the area of the embryo which is destined to become the placenta can be removed to allow for genetic testing, or PGS (pre-implantation genetic screening).
It is typical for there to be what appears to be significant loss along the way in the process of early embryo development. 80-90% of retrieved eggs are mature. About 2/3 mature eggs will typically fertilize. About half of fertilized eggs develop normally to the 8 cell stage, and 80% of these develop into healthy appearing blastocysts.
Therefore, starting from retrieval only about 20-25% of initially recovered eggs will typically become healthy blastocysts, available for genetic testing or embryo transfer. And if PGS is performed, usually 50% of embryos are genetically normal and usable to transfer.
Understanding the different stages of in vitro fertilization can help you and your partner navigate the stress associated with this fertility treatment. Work with your fertility specialist to develop a plan and answer any questions you may have.
