Optimal Planning of a Day 3 Cryopreserved(Frozen)-Thawed Embryo Transfer in a Natural Cycle With hCG Administration or After Spontaneous LH Peak?

For surplus embryos after fresh IVF-cycles, cryopreservation has become common medical practice. These frozen embryos are mostly replaced in an artificial cycle with exogenous estrogen and progesterone or in a natural cycle. Often, hCG is administered as an ovulation induction agent for scheduling purposes.

Successful implantation requires a co-ordinated series of events allowing a timely dialogue between a receptive endometrium and the intrusive blastocyst . The period of receptivity is thought to be 3 days in human. It is suggested that blastocyst apposition begins about day LH+6 and is completed by day LH+10

In general, the aim is to transfer the embryo during the 'window of implantation', what is defined as the period during which the uterus is receptive for implantation of the free-lying blastocyst. This has been a subject of debate since many years.

A prospective study by Fatemi et al. (2010) revealed a significantly higher ongoing pregnancy rate after transferring frozen-thawed embryos in natural cycles with a spontaneous LH peak compared with natural cycles controlled by hCG for final oocyte maturation and ovulation (31.1% vs. 14.3%, respectively). In this trial, FrET (frozen embryo transfer) was planned 5 days after the LH surge or 5 days after the administration of 5000IU of hCG.

In order to optimize the synchronization in the hCG group, and therefore enhance the pregnancy rates, the aim is to plan a FrET 6 days after hCG administration instead of 5 days. The rationale behind is that day 3 frozen embryos are thawed the day before embryo transfer, which means they are already at day 4 of the embryonic development.