Biomarkers of Endometrial Receptivity

Inclusion Criteria - Arm A - stimulated cycle: * female aged less than 37 years (maximally 36y + 364d) * no smoker * normal menstrual cycles lasting between 25 to 35 days * had been infertile for less than five years * normal responder * fewer than three failed cycles of assisted reproduction treatment, including fresh IVF/ intracytoplasmic sperm injection (ICSI) embryo transfer cycles and/or frozen-thawed embryo transfer cycles * sperm obtained through ejaculation * spermiogram more than 5 million sperm/mL * BMI 19-29 kg/m2 * follicle stimulating hormone (FSH) \< 10 IU/L on the third day * basal antral follicle count of 5-15 * undergoing the same routine gonadotrophin-releasing hormone agonist (GnRHa) long depot or gonadotrophin-releasing hormone antagonist (GnRH-ant.) protocol * informed consent Exclusion Criteria - Arm A - stimulated cycle: * genetic disease * metabolic and/or endocrine disorders * polycystic ovary syndrome (defined by the Rotterdam criteria) * women with prior diagnosis of endometriosis or adenomyosis * previous gynecological/pelvic surgery except for salpingectomy * repeated spontaneous abortions (two or more) * previously less than 5 oocytes and/or serum anti-Mullerian hormone value \< 1.0 mIU/ml or more than 20 oocytes, milli-International unit (mIU) * previous ovarian hyperstimulation syndrome (OHSS) * presence of any structural abnormality of the reproductive system * donor oocyte cycles * severe male factor infertility \< 5 million sperm/mL * low response to stimulation * endometrium \< 8 mm at the day of human chorionic gonadotropin (hCG) or ET * number of retrieved oocytes 5 - 20 * low fertilization capacity (rate of fertilization \< 20% and late ICSI following IVF fertilization failure) * OHSS * IVF cycle cancelled before ET * other than easy one high-quality blastocyst transfer (at least grade 3BB) Inclusion criteria - Arm B - substituted cycle: * female aged less than 37 years (maximally 36y + 364d) * no smoker * normal menstrual cycles lasting between 25 to 35 days * had been infertile for less than five years * normal responder at stimulation * fewer than three failed cycles of assisted reproduction treatment, including fresh IVF/ intracytoplasmic sperm injection (ICSI) embryo transfer cycles and/or frozen-thawed embryo transfer cycles * sperm obtained through ejaculation * spermiogram more than 5 million sperm/mL * BMI 19-29 kg/m2 * FSH \< 10 IU/L on the third day * undergoing the same routine estrogen/progesterone substituted cycle * informed consent Exclusion criteria - Arm B - substituted cycle: * genetic disease * metabolic and/or endocrine disorders such as diabetes, metabolic syndrome, and thyroid disorders * polycystic ovary syndrome (defined by the Rotterdam criteria), hyperprolactinaemia * women with prior diagnosis of endometriosis or adenomyosis * previous gynecological/pelvic surgery except for salpingectomy * repeated spontaneous abortions (two or more) * previously less than 5 oocytes and/or serum anti-Mullerian hormone value \< 0.5 mIU/ml in the stimulated cycle * previous OHSS * presence of any structural abnormality of the reproductive system * severe male factor infertility \< 5 million sperm/mL in the stimulated cycle * number of retrieved oocytes 5 - 20 in the stimulated cycle * low fertilization capacity (rate of fertilization \< 20% and late ICSI following IVF fertilization failure) * endometrium less than 8 mm at the day of thawing and transfer indication * thawed blastocyst cycle cancelled before ET * other than easy one best quality frozen/thawed blastocyst transfer (at least grade 3BB)