There are two main types of IVF cycles. These are fresh embryo transfer and frozen-thawed embryo transfer.
Fresh embryo transfer is the most common, and is associated with national average live birth rates of about 45-50% per transfer and implantation rates in the 30-40% range in patients less than 35 years of age. At The Fertility Center of Las Vegas, our implantation rates with fresh embryos have been higher, in the 40-50% range for many years. The implantation rate is the proportion of transferred embryos that implant and then progress to have fetal heartbeats. The implantation rate is a very valuable measure of effectiveness because it cannot be artificially inflated by transferring a large number of embryos into the uterus.
However, our implantation rates with FET have been even greater, particularly in dedicated cycles following the freeze of an entire cohort of embryos. In two randomized trials in patients up to 40 years old, we observed implantation rates of 67% in high responders and 71% in normal responders who had an entire embryo cohort frozen, and the clinical pregnancy rates (proportion of transfers resulting in at least one fetal heart) were 80% and 84%, respectively. These rates were achieved in pre-specified trials that were continually reviewed by an independent monitor, and without any genetic testing nor the associated added costs.
Our success rates with this method greatly exceed national averages for fresh transfer or FET in any age group of patients using their own eggs for IVF. National averages are clearly not representative of these cycles. This is true for two reasons. First, FET avoids the compromised uterine environment that is often present and unpredictable in fresh non-donor cycles. Second, our FET protocol involves freezing the entire cohort at an early stage (the bipronuclear stage) and culturing to blastocyst after thaw, so that the blastocyst is confirmed to have resumed development. Most frozen embryo transfers in the national averages are “second best” embryos left over after the patients’ best embryos were transferred in fresh cycles, and they are also recently thawed embryos with no confirmation of resumed development.
The transfer of a vigorously developing blastocyst (not merely “survived”) into an unimpaired uterine environment is characteristic of fresh oocyte donor cycles, and indeed our FET method achieves implantation and pregnancy rates as high as the best fresh egg donor cycles, when performed in young patients.
Many clinical and basic science studies have found the uterine environment is disturbed by ovarian stimulation, and multiple studies have found reduced implantation rates in fresh embryo transfer when compared to frozen-thawed embryo transfer. A summary of all randomized trials published to date comparing fresh cycles and FET found that the overall chance of pregnancy was improved by embryo freezing, as has been reported in the mainstream media. Our own randomized trials found this to be especially true in normal responders. A compromised endometrium is not limited to patients with very high estrogen levels during stimulation, and, in fact, appears to be worse in average patients than in high responders.
The greatly improved success rates are not the only benefit of embryo freezing.
FET is also associated with reduced rates of ectopic pregnancy and almost no risk of ovarian hyperstimulation syndrome (OHSS), two risks that are increased in fresh transfer when compared to natural pregnancies or FET. An ectopic pregnancy occurs when an embryo implants outside the uterus. OHSS is a potentially dangerous condition associated with abdominal swelling through uncontrolled fluid shift following ovarian stimulation and exposure to hCG.
Another risk associated with fresh IVF is low birthweight. It is well established that infants resulting from fresh non-donor transfer are smaller than those from natural pregnancies or from FET, and it has also been shown that they are smaller than those from fresh egg donor cycles. One theory is that this is due to poor attachment (placentation) of the embryo to the uterine wall in fresh non-donor cycles, because the uterus was exposed to ovarian stimulation in that cycle. This suggests the poor uterine environment following ovarian stimulation contributes to the reduced birthweight following fresh non-donor transfers.
In summary, our dedicated FET protocol is associated with implantation rates far above national averages for fresh transfer or FET in each age group, FET reduces risks to the mother, and FET may even improve some aspects of infant health.