A 36-year-old Japanese woman (gravida 1, para 1) visited our clinic because of secondary infertility. Her first pregnancy at 31 years of age was natural and she delivered a 3320 g infant by vaginal delivery at 39 weeks and 5 days of gestation without perinatal complications. The patient had regular menstrual cycles for 30 days. Her body mass index (BMI) was 18.6 (height 159.5 cm, weight 47.2 kg). Medical and family history were unremarkable. An internal examination revealed no abnormalities in the uterus or ovaries. Ultrasound examination revealed a polycystic pattern in the right ovary; however, the patient was not diagnosed with polycystic ovarian syndrome. Hormone levels of anti-Müllerian hormone (AMH), cycle day 3 (CD 3) of estradiol (E2), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) were 7.44 ng/mL, 23.0 pg/mL, 7.3 mU/mL, and 7.4 mU/mL, respectively. Hysterosalpingography showed bilateral fallopian tube passage. The husband’s semen analysis revealed no abnormalities in semen volume, sperm count, or sperm motility according to the WHO 2010 criteria. After five cycles of timed intercourse followed by five cycles of intrauterine insemination with the husband’s semen, no pregnancy was established. Therefore, the patient underwent in vitro fertilization (IVF). Controlled ovarian stimulation was performed using a gonadotropin-releasing hormone (GnRH) antagonist protocol. Purified urinary FSH (150 IU, Gonapure, ASKA Pharmaceutical, Tokyo, Japan) was administered every alternate day, and clomiphene citrate (50 mg/day, Clomid, Shionogi Co. Ltd., Osaka, Japan) for 5 days was started on CD 3. On CD 10, human menopausal gonadotropin (225 IU, Ferring Pharma, Tokyo, Japan) with GnRH antagonist (0.25 mg, Ganirest subcutaneous syringes, MSD, Tokyo, Japan) were administrated. On CD 12, transvaginal ultrasound showed 14 follicles larger than 16 mm, and the serum E2 level was 1332 pg/mL. Final oocyte maturation was triggered by subcutaneous injection of recombinant human chorionic gonadotropin (rhCG) (0.25 μg, Ovidrel, Serono Inc., Tokyo, Japan) and nasal spray of a GnRH agonist (600 μg, Buserecure, Fuji Pharma, Tokyo, Japan). Oocyte retrieval was performed 35 h after the trigger under general anesthesia. A total of 15 cumulus-oocyte complexes were retrieved, and 12 oocytes reached the MII stage. Conventional IVF and intracytoplasmic sperm injection (ICSI) were performed according to semen parameters. All embryos were cultured in ONESTEP Medium (NAKA Medical Inc., Tokyo, Japan) under 6% O2, 5% CO2, and 90% N2 gas. Cleavage-stage embryo quality was evaluated on day 3 according to Veeck’s criteria [] and blastocysts were evaluated according to Gardner’s classification []. One cleavage embryo (10 cells, G2) and seven blastocysts (three blastocysts for 4AA, one blastocyst for 4AB, and two blastocysts for 4BB) were cryopreserved by vitrification using the Cryotop carrier system (Kitazato Biopharma Co., Tokyo, Japan) according to the manufacturer’s instructions. FET was scheduled, and endometrial preparation was performed using a hormone replacement cycle (HRC) or modified natural cycle. In the HRC, transdermal estradiol (0.72 mg, Estrana TAPE, Hisamitsu Pharmaceutical, Tokyo, Japan) was initiated on CD 3. Progesterone treatment with vaginal progestin tablets (300 mg/day, LUTINUS Vaginal Tablet, Ferring Pharmaceuticals Co., Ltd., Tokyo, Japan) and oral dydrogesterone tablets (30 mg/day, Duphaston, Mylan EPD, Tokyo, Japan) was initiated at an endometrial thickness of 8 mm. FET was scheduled 3–5 days after the start of progesterone treatment. In the modified natural cycle, rhCG (0.25 μg, Ovidrel, Serono Inc., Tokyo, Japan) was injected when the endometrial thickness and follicle diameter reached more than 8 and 16 mm, respectively. Vitrified-warmed ET was scheduled 4–6 days after rhCG administration, according to the embryo stage. In the first cycle of FET, one blastocyst-graded 4BB was transferred under the HRC. In the second cycle of FET, one early cleaved embryo, graded G2, was transferred under a modified natural cycle. In the third cycle of FET, one blastocyst-graded 4AB was transferred under a modified natural cycle. All embryos used in these three embryo transfers were of conventional IVF origin. However, the three FET cycles did not result in pregnancy. After three cycles of FET failure, the patient decided to undergo an ERA test (IGENOMIX, Valencia, Spain). Endometrial preparation for the ERA test was performed using the standard HRC described above. The initial day of progesterone administration was set as 0 h of progesterone exposure. Endometrial sampling was performed after 125 h of progesterone exposure using a Pipelle endometrial sampler (Laboratoire CCD, Paris, France). Specimens were processed and shipped according to the manufacturer’s instructions. The ERA results were tabulated as reported by IGENOMIX and classified as receptive or non-receptive. Non-receptive results were considered either pre-or post-receptive, and details of endometrial adjustment recommendations or rebiopsy were documented. In addition, the presence or absence of CE in the biopsied endometrium was examined. The diagnostic criteria for CE were based on a previous study []. Accordingly, CE was defined as the presence of one or more plasma cells/high-power field (hpf) (× 40) in CD138 immunostaining and classified as score 1 for 1–5 cells/hpf, score 2 for 6–20 cells/hpf, and score 3 for more than 20 cells/hpf. The first ERA test was performed 125 h after progesterone exposure. The laboratory reported that the endometrium was in a non-receptive (post-receptive) phase, and recommended retesting 101 h after progesterone exposure. A simultaneous CE test showed a score of 3. She was administered antibiotics for 2 weeks with levofloxacin 0.5 g/day and metronidazole (1 g/day) to treat CE. Subsequently, the second ERA test was performed after 101 h of progesterone exposure. The laboratory reported that the endometrium had not reached the WOI and estimated the WOI to be 113 ± 3 h after progesterone exposure. We questioned the results of the estimated WOI and completed the third ERA test after obtaining informed consent from the patient. The third ERA test was performed 113 h after progesterone exposure. The laboratory reported that the endometrium was in a non-receptive (pre-receptive) phase and estimated the WOI to be 137 ± 3 h after progesterone exposure. A CE test performed at the same time as the second and third ERA tests showed a score of 1 for the collected endometrium. According to the third ERA test results, the vitrified-warmed blastocyst graded 4AA, which was derived from ICSI, was transferred at 137 h of progesterone exposure, which corresponded to ET 12 h later than the normal HRC. Pregnancy was achieved in this ET cycle, and the patient had an uncomplicated vaginal delivery of a male neonate weighing 2970 g at 39 weeks and 5 days. One year later, another FET of one 4AA blastocyst was performed in the HRC, and pregnancy was achieved after 137 h of progesterone exposure. She delivered a viable 2168 g female neonate at 33 weeks and 2 days after an unexpected rupture of the membrane occurred at 33 weeks and 1 day. The timing of endometrial sampling and the recommended WOI by the three ERA tests are shown in Fig..