Under certain conditions, embryo freezing may significantly improve IVF treatment outcomes, according to a study published in the Fertility and Sterility journal in early August of this year.
Recent technological improvements (such as vitrification) made egg and embryo cryopreservation increasingly common. One advantage of the technique is that it allows fertility specialists to test for genetic defects or chromosomal diseases. However, until recently, the impact of embryo freezing on treatment success rates remained unclear.
In the largest ever study comparing fresh and frozen embryo transfers, a research team led by scientists from Stanford University’s Medical Center analyzed data from close to 3000 IVF cycles in the US. Patients in the two groups had similar causes of infertility, medical histories and hormone levels, as well as similar numbers of retrieved oocytes and created embryos.
It should be noted that to reduce the effect of varying embryo quality, fresh/frozen cycles (where the best-quality embryos were transferred first and the rest frozen) were discarded from the sample. Additionally, experts did not assess live birth rates – the criteria for a positive outcome was the patient being transferred back to her treating specialist(s) for follow-up pregnancy care.
Overall, 52% of frozen and 45% of fresh transfers resulted in pregnancy. However, these were not uniform throughout the cohorts. The biggest difference in success rates was seen amongst women aged over 35 who had high progesterone levels. In that group, frozen cycles were 73 per cent more likely to lead to a pregnancy.
This is consistent with previous research data that showed a link between lower progesterone levels and higher embryo implantation success rates. For example, a 2013 study found that women with progesterone levels below 8 ng/mL were 70% more likely to become pregnant following embryo transfer than those whose progesterone levels exceeded 12 ng/mL.
This difference is related to elevated progesterone levels negatively impacting embryos’ ability to attach themselves to uterine walls. Thus, delaying the transfer (by means of embryo freezing) allows for more optimal conditions with better odds of a positive outcome.