Descriptive Analysis of Serum Immunological Markers During an Euploid Frozen Embryo Transfer in a Natural Cycle.

Inclusion Criteria: * Women aged 18 years to 40 years * Having at least one good quality (grade A or B for inner mass cell and trophectoderm) day 5 or 6 chromosomally normal cryopreserved blastocyst available for transfer * Single embryo transfer in a natural cycle (NC) or an Hormone Replacement Therapy (HRT) protocol * Fresh ejaculate used for fertilization Exclusion Criteria: * Body mass index lower than 18.5 or equal or higher than 29 kg/m2 * Endometriosis or adenomyosis suspected by medical history (dysmenorrhea, dyspareunia, heavy or prolonged menstrual bleeding (\> 8 days), chronic pelvic pain, catamenial rectal or bladder symptoms) or diagnosed by imaging (magnetic resonance imaging or ultrasonography) * Uterine abnormalities * Hydrosalpinx * Insulin resistance or diabetes mellitus diagnosed by HbA1c 5.7 % * Antiphospholipid syndrome * Polycystic ovarian syndrome according to Rotterdam criteria: presence of at least two of the following: irregular cycles (\< 21 or \> 35 days or \< 8 cycles a year), biochemical or clinical hyperandrogenism, ovarian ultrasound morphology (\> 20 follicles per ovary on transvaginal scan) or Anti-mullerian hormone \> 5,98 ng/ml * History of recurrent miscarriage, defined as the loss of 2 or more pregnancies according to ESHRE guidelines * History of implantation failure, considered as the lack of pregnancy after the transfer of 2 good-quality (grade A or B for inner mass cell and trophectoderm (29)) euploid embryos * History or suspicion of Asherman syndrome * Autoimmune disease * Couple first degree consanguineous