Embryo transfer: fresh or frozen cycle?

Currently available data suggests that freezing all embryos and transferring them in subsequent cycles increases the chances of getting pregnant and reduces the likelihood of complications during pregnancy and childbirth.

Aristotle University of Thessaloniki, Scientific Partner, FIVI Medically Assisted Reproduction Unit, Inter-Balkan Medical Centre, Thessaloniki

According to the pregnancy rates published every year by the European Society of Human Reproduction and Embryology, the average success rate for embryo transfer right after egg retrieval, in a fresh cycle, is around 29%, both for traditional in-vitro fertilization and for microinsemination.

If a couple has excess embryos following a fresh embryo transfer, the surplus embryos are frozen to be thawed and transferred in a subsequent cycle. This increases their odds of conceiving by 8%.

Consequently, on the basis of the above, a quick response to the dilemma ‘fresh or frozen embryo transfer’ would be that it is preferable to transfer the embryos in a fresh cycle, as pregnancy rates are higher.

However, this approach does not take into account the fact that embryos transferred in the frozen cycle are those that were not selected for transfer in the fresh cycle and, thus, of a lower quality, which means they are less likely to lead to pregnancy.

More importantly, we need to know whether the endometrium that will receive the valuable embryos is affected by ovarian stimulation. If this is the case, it would be better to consider the all-freeze strategy, i.e. freeze all embryos and transfer them in a later cycle.

In this subsequent cycle, the endometrium will not have suffered the consequences of our effort to allow more follicles to develop, in order to create several eggs and embryos and be able to select the best in quality for the transfer.

Does ovarian stimulation have an impact on endometrial quality?

It is now clear that ovarian stimulation is linked to an abnormal endometrial development at both the histological and genetic level, regardless of the stimulation method.

Do the chances of pregnancy increase after embryo transfer in a frozen cycle as compared to the transfer of embryos in a fresh cycle?

Although there is only a small number of studies answering this question, the data published so far suggest that the likelihood of pregnancy is higher after frozen cycle transfer. It should be noted that the women who took part in these studies were randomized to either have their embryos immediately transferred in a fresh cycle or have them all frozen and then thawed and transferred in a later cycle.

Are the pregnancies resulting from fresh cycles different from those resulting from frozen cycles?

Pregnancy following frozen-thawed embryo transfer – as compared to pregnancy after fresh cycle transfer – is associated with:

• a smaller chance of intrauterine growth retardation
• a smaller chance of birth weight < 2500 g
• a smaller chance of premature birth < 37 weeks
• a lower rate of perinatal mortality
• a lower rate of postpartum bleeding for the mother

On the basis of the above data, it is obvious that freezing all embryos for later thawing and transfer at a subsequent cycle has significant advantages and should be discussed with the couple, especially if their previous IVF attempts were unsuccessful.