A recent randomised controlled trial and previous large cohort studies have shown that the uterine immune environment is a crucial element in improving the performance of Assisted Reproductive therapy (ART). As previous studies mixed Day-3, Day-5, single or doble embryo transfer, the clear influence of the endometrial environment on the embryo itself and its type of transfer (fresh or freeze thawed) need further investigation.

To complement previous studies, the present matched- pair study aims to select the population who exclusively received a single Day-5 embryo transfer (SET) and benefitted of a uterine immune profiling between 1 January 2020 and 30 June 2023 before the SET Day-5.

This population will be matched to a population. who did not have uterine immune profiling in the nine months prior to the single Day 5 embryo transfer between 2018 and 2023.

The matching criteria are the maternal age (+/-1 year), the past history of ART (same number of previous oocytes pick-up, same number of previous embryo transfer) and the same type of transfer (IVF, ICSI, frozen transfer) and the same category of expansion of the blastocoel (B1-B2/ B3-B4/ B5-B6).

The primary end-point is the live birth rate (LBR) following the Day 5 SET in the population who benefited from pre-SET immune profiling versus the LBR following the Day 5 SET in the pair- matched population who did not benefit from pre-SET uterine immune profiling Secondary end point are the clinical pregnancy rate and miscarriage rate per SET in the two paired populations

The PRECONCEPTIO software will be used to extract all the uterine immune profiles carried out between January 2020 and June 2023 for patients attending the two reproductive centres, Bluets and Diaconnesse.

In parallel, data will be extracted from the MEDIFIRST software selecting all patients who benefited from a single day 5 embryo transfer between January 2018 and March 2024.

The case group represents the selection of patients who had both uterine immune profiling prior to ET and Day 5 single embryo transfer within the nine months following uterine immune profiling.

Each patient in the treated group is matched to patients who had a single Day 5 ET without immune profiling in the nine months prior to ET.

Matching criteria will be maternal age (+/-1year) and, number of previous egg retrievals (including the ones without any transfer) and, number of previous ETs and technique used (IVF, ICSI, frozen embryo transfer) and degree of expansion of blastocoel (B1-B2/B3-B4/B5-B6) to reflect the quality of the embryo transferred The primary end-point is the live birth rate (LBR) following the Day 5 SET in the population who benefited from pre-SET immune profiling versus the LBR following the Day 5 SET in the pair- matched population who did not benefit from pre-SET uterine immune profiling Secondary end point are the clinical pregnancy rate and miscarriage rate per SET in the two paired populations

Impact of the uterine immune profiling on the live birth rate

• In function of the type of embryo transfer (fresh or freezed thawed)
• In function of the quality of the blastocyst transferred
• In function of the range of embryo transfer

For case: Uterine immune profiling followed by a Day-5 fresh SET or day-5 or Day-6 freeze-thawed SET within the nine months following the uterine immune profiling between January 2020 and June 2023.

Will be excluded patients with no SET day-5, with uterine immune profiling more than nine months before, or with no matching pair For control: Blastocyst SET without uterine immune profiling within the year prior the embryo transfer matched to the case group between January 2018 on June 2023

• maternal age (+/-1 year) and
• the past history of ART (same number of previous oocytes pick-up, same number of previous embryo transfer) and
• the same type of transfer (IVF, ICSI, frozen transfer) and
• the same category of expansion of the blastocoel (B1-B2/ B3-B4/ B5-B6).

Female involved in IVF

Patients with a single embryo transfer at Day-5 case group: With an immune profile within the nine month before ET Control group: without immune profiling performed