By Edward Ditkoff MD
In order to understand the pros and cons of an earlier versus later embryo transfer, we need to review basic biology. An embryo begins its growth as a single cell, and divides daily. By day 3, it typically grows into a 6-8 cell multicell embryo. Over the next 2-4 days (5-7 days of life), the embryos should continue to divide, and become a blastocyst, (ideally by day 6). The cells in the blastocyst consist of either inner cell mass cells (become the fetus) or trophoblast/trophectoderm cells (become the placenta). A blastocyst is considered more special than an earlier staged embryo since it has developed further and is starting to differentiate. Further developed embryos have a higher probability of being genetically normal than the earlier embryos, and not all early embryos will continue to develop. Thus, by delaying embryo transfer, the embryologist is better able to select the embryos with a higher probability for continued development. If embryos were routinely transferred earlier, we would have less time to observe their development, have less information and confidence in their potential to develop into a fetus, and possibly transfer too many embryos at an earlier stage.
If a patient produces a small number of eggs or has a small number of embryos (fertilized eggs), she may be concerned whether this small number (1-2) of embryos may not continue to divide and/or progress to the more developed blastocyst stage (not have a transfer). However, a delayed transfer might improve the uterine environment and contribute to a higher success rate. In addition, she might also desire to delay her transfer in order to have more information about the embryos at the time of transfer (for prognostic information for possible future management). At CNY, we are flexible and support our patient’s decision with which they are most comfortable. In these cases, regardless of the transfer day, embryos (1-2) could be transferred, and the risk for a multiple pregnancy could be less.
If a patient produces a larger number of embryos (fertilized eggs), and many appear similar in quality after three days, it is difficult to determine the best to transfer. Typically it is beneficial to allow these embryos to sort themselves out, and see which progress to blastocysts, the strongest appearing embryos may then be selected for transfer (typically 1-2).
We realize women’s reproductive success rates significantly decrease starting at the age of 35 due to aneuploidy (abnormal amount of genetic material in the embryo). Aneuploidy rates increase with age, and could reach above 50% at the age of 36. Thus, we refer to ASRM guidelines when advising how many embryos to transfer. At CNY, again we are flexible and support the patient’s decision with which they are most comfortable. We will support their decision since they are informed of the increased risk for a higher risk pregnancy if they desire a higher number of embryos transferred than advised.
In summary, the ability of growing an embryo to the blastocyst stage in the laboratory has been an important breakthrough for IVF that maximizes pregnancy rates while minimizing the risk of a multiple pregnancy. By growing embryos beyond the traditional three days, the embryology team may determine, with greater certainty, which embryos have the most potential for implantation.