Patricia M. McShane MD Medical Director The World Egg Bank
A recent large series reviewing oocyte warming has given us more reason to be confident using banked donor eggs for our patients. One of the pioneers of egg banking, Ana Cobo reported an oocyte-to-baby rate of 6.5% with the warming of 3610 eggs (Cobo A et al. Six years’ experience in ovum donation using vitrified oocytes: report of cumulative outcomes, impact of storage time, and development of a predictive model for oocyte survival rate. Fertil Steril 2015;104:1426-34). Although 6.5% seems like a low number, it compares favorably to the rate with fresh donor and autologous oocytes.
The oocyte-to-baby rate is a new metric that is particularly important in the oocyte donation and vitrification arena, but is not familiar to many practitioners. Let’s review some of the history of the various metrics used in ART.
The history of IVF includes many efforts to get the data right, yielding the many iterations of the annual CDC and SART reports. For clinicians and patients, one relevant question is the likelihood of a birth given a commitment to a certain stage of therapy, such as live birth per stimulation or per retrieval.
Over time with increasing success rates, the chance that an individual embryo would implant became the most clinically relevant question. For many years, implantation rate for the best prognosis groups has been the benchmark indicator of the quality of an IVF practice. Knowing program-specific data based on a given patients’s age, quality of embryos and other factors helps to determine the number of embryos to transfer. Of course, given the increasing implantation rates and the improved success with embryo cryopreservation, elective single embryo transfer has become the norm in several countries and many practices in the US.
Although the first successful IVF was the result of a natural cycle, controlled ovarian hyperstimulation (COH) has been used for the vast majority of cycles over the last thirty plus years. Even with high egg numbers, the efficiency of IVF remains fairly low. Currently human reproduction is fairly inefficient on a per egg basis, especially working with large number of eggs resulting from COH. Scientists and clinicians alike recognize that the chance of any egg yielding a pregnancy potential is very limited. This can be expressed as the oocyte-to-baby rate.
In the late 1990’s and early 2000’s, several groups recommended using the oocyteto-baby rate as a way to advance our understanding of the physiology and practice of ART. It is challenging to calculate this rate since not all eggs and resulting embryos are utilized in a single cycle, and use of frozen embryos may occur much later. Understanding the uncertainty, an oocyte-to-baby rate of 4.5 to 7% for IVF with autologous eggs has been deemed a good estimate. Another way of expressing this information is that approximately 15 to 22 eggs are required to achieve a pregnancy, utilizing current stimulation protocols. Rates on the high end of this range (7.3%) have been reported for eggs from some young donors (Martin J et al, Live babies born per oocyte retrieved in a subpopulation of oocyte donors with repetitive reproductive success. Fertil Steril 2010;94:2064-8).
In the current era of donor egg banking, the relevant question immediately became the number of warmed vitrified eggs likely to yield a viable embryo. Cobo’s observation of 6.5% oocyte-to-baby appears to be in line with the reported rates from fresh oocyte donation, further reassuring clinicians and recipients considering vitrified banked eggs.
The oocyte-to-baby rate also informs the number of eggs needed in the case of elective egg vitrification for medical or social indications. Current recommendations in many programs for women under age 38 are to bank 15 to 20 eggs.