When a couple undergoes an assisted fertilization treatment, it may be that they are offered by the center they have contacted to freeze the embryos obtained or to transfer them fresh, that is, in the same stimulation cycle. Are there any differences in the outcome of the treatment? This is the question that the authors of the study published in August 2020 on TheBMJ asked themselves.
This choice is not always possible. There are situations that make freezing mandatory, for example in the case of a pre-implantation diagnosis of the blastocysts (which leads to subsequent vitrification of the embryos analyzed for subsequent transfer), or in the case of hyperstimulation.
When we talk about the total freezing of the embryos obtained, after an ovarian stimulation treatment we talk about the “freeze-all” strategy.
This elective freezing of embryos is becoming very common as pregnancy rates after frozen transfer approach those of fresh transfers.
A freeze-all strategy can also minimize the risk of ovarian hyperstimulation syndrome but, prior to this study, there was a lack of adequate publications comparing the two strategies.
To help fill this gap, a team of researchers set out to test whether the total freeze strategy resulted in a higher pregnancy rate than embryo transfer during the stimulation cycle (and therefore “fresh”).
The study comparing fresh transfer and frozen embryo transfer
The study published in TheBMJ took the considerations 460 women between the ages of 18 and 39 with regular menstrual cycles who have started their first, second or third course of in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) treatment at fertility clinics in eight public hospitals in Denmark, Sweden and Spain.
This was a randomized controlled trial, and the participating women were randomly divided into one of two transfer groups.
Women with endometriosis, uterine abnormalities or thyroid disease were excluded from the study. Also not included were women who had a comorbid condition that could lead to adverse pregnancy outcomes, such as diabetes, gastrointestinal, cardiovascular, pulmonary, liver or kidney disease.
There ovarian stimulation was started on the second or third day of the cycle for both groups. In the frozen embryo transfer group, a gonadotropin-releasing hormone antagonist was administered on day 5 or 6. In the fresh embryo transfer group, participants received the gonadotropin-releasing hormone agonist if they had more than 18 follicles with a diameter greater than 11 mm, to avoid ovarian hyperstimulation syndrome.
Both groups received a single blastocyst transfer.
Women in the fresh transfer group who had an excessive number of mature follicles on the day of induction had to resort to elective freezing of all embryos and the transfer was delayed as a safety measure.
THE results show that the pregnancy rate (defined by the detection of the fetal heartbeat detectable after eight weeks of gestation) it did not differ significantly between the freeze-all and transfer-to-fresh groups (62 out of 223 or 27.8% vs 68 out of 230 or 29.6%).
Furthermore, no significant difference was found in the live birth rate, with 61 out of 223 or 27.4% for the group with frozen embryos and 66 out of 230 or 28.7% for the group that had fresh blastocyst transfer.
No significant differences were observed between the two groups in miscarriage rates, and none of the women had severe ovarian hyperstimulation syndrome.
The risks of complications they did not differ between the two groups except for a higher mean birth weight after frozen blastocyst transfer and an increased risk of prematurity after fresh blastocyst transfer.
The researchers pointed to some limitations of the study that may have influenced their results, but at the same time concluded that “In women with regular menstrual cycles, a freezing strategy did not lead to an increase in ongoing pregnancy and live birth rates compared to fresh transfer “. They also added that “Results call for caution in the indiscriminate application of a freezing strategy when there is no apparent risk of ovarian hyperstimulation syndrome“.
In practice, these results underline that the increasingly widespread use of frozen embryo transfer may not be necessary and reassure the safety of fresh embryo transfer in women with moderate to normal numbers of follicles.
“If all goes well and the woman is not at high risk of ovarian hyperstimulation syndrome, then it is perfectly fine to have a fresh embryo transfer.“.
Freeze-all versus fresh blastocyst transfer strategy during in vitro fertilization in women with regular menstrual cycles: multicentre randomized controlled trial ,