P-125 Day 5 endometrium, Is the pregnancy rate following day 5 or day 6 vitrified/thawed blastocysts transfer affected?
Authors: Gomez-Sanchez E , Castelló D , Diaz R , Almunia N , Valcárcel M , Jiménez R , Villaquirán A M , Diez C , Quintero L , Castilla J A 
Published: June 2023

Study question

Are ongoing pregnancy rates of embryos vitrified on day 5 and day 6 when transferred into a day 5 endometrium comparable?

Summary answer

Day 5 blastocysts implant better in day 5.5 endometrium than day 6, whether they were vitrified on day 6 or 5 and cultured 24 hours.

What is known already

Preparation of the endometrium for transfer of cryopreserved embryos is usually done so that transfer takes place on endometrial day 5, although embryos may be at day 5 or 6 of development, so there may be asynchrony between the endometrium and the embryo if a day 6 embryo is transferred. Fresh transfer of day 5 blastocysts has been reported to have better results than day 6 blastocysts, predictably due to this endometrial asynchrony. However, data from frozen blastocyst transfers are less clear, possibly due to heterogeneity of the patient population and/or embryo quality.

Study design, size, duration

In this study we aim to analyse whether ongoing pregnancy after vitrified/thawed embryo transfers is affected if day 5 or day 6 embryos are transferred into day 5 endometrium. For this purpose, we examined 2266 cryotransfers performed in three Next Fertility clinics in Spain between 2016 and 2022. Day 5 (n = 1583) and 6 (n = 519) blastocyst transfers and day 5 embryos that devitrified one day before transfer (n = 164) were retrospectively compared.

Participants/materials, setting, methods

In this work blastocyst transfers were performed from donor or autologous oocytes, with or without PGT-A. Embryos were vitrified on day 5 or 6. Endometria were prepared with hormone replacement therapy or in natural cycle. All FET were performed after 5.5 days of progesterone exposure, regardless of embryo stage (day 5 or 6). The results were analysed considering all the above groups.

Main results and the role of chance

Ongoing pregnancy are superior with day 5 blastocysts versus day 6 and day 5 cultured for 24 hours prior to transfer. However, day 6 blastocysts were similar to cultured ones.

Own oocytes: Day 5 44.98% (n = 538) vs Day 6 35.75% (n = 207) (p &lt; 0.03); Day 5 44.98% vs Day 5 cultured 32.58% (n = 89) (p &lt; 0.03); Day 6 35.75% vs Day 5 cultured 32.58% (p &gt; 0.05).

Donated Oocytes: Day 5 45.86% (n = 726) vs Day 6 28.51% (n = 221) (p &lt; 0.00001); Day 5 44.98% vs Day 5 cultured 39.34% (n = 61) (p &gt; 0.05); Day 6 28.51% vs Day 5 cultured 39.34% (p &gt; 0.05).

PGT-A Own oocytes: Day 5 51.72% (n = 203) vs Day 6 25.39% (n = 63) (p &lt; 0.003); Day 5 51.72% vs Day 5 cultured 40.00% (n = 10) (p &gt; 0.05); Day 6 25.39% vs Day 5 cultured 40.00% (p &gt; 0.05).

PGT-A Donated Oocytes: Day 5 42.24% (n = 116) vs Day 6 35.71% (n = 28) (p &gt; 0,05); Day 5 42.24% vs Day 5 cultured 40.00% (n = 4) (p &gt; 0.05); Day 6 35.71% vs Day 5 cultured 50.00% (p &gt; 0.05).

Miscarriage rates were similar across all groups.

Limitations, reasons for caution

Not all cohorts in the study were equally large, so some results have less statistical power, especially in abortion rates.

Wider implications of the findings

Day 5 blastocysts should always be transferred into a 5.5-day endometrium on the day they are devitrified, as this is when they are most synchronous with the endometrium. Further studies are needed to assess whether day 6 embryos implant better in a 6.5-day endometrium than in a 5.5-day endometrium.

Trial registration number

not applicable