Our patient was a 29-year-old, 161-cm-tall Japanese woman weighing 62 kg, with a body mass index of 23.9 kg/m2. She had previously undergone an induced abortion at age 20 years and had received conservative therapy with methotrexate for left tubal pregnancy at age 27 years. She had been a housewife since she had married at age 25 years, had no other medical history, and had taken no medications. She did not like smoking or alcohol. Infertility-related testing at her previous infertility clinic revealed that her antimüllerian hormone level was 2.65 ng/ml, and her basal levels of estradiol (E2), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin were 16.2 pg/ml, 2.1 mU/ml, 5.1 mU/ml, and 11.7 ng/ml, respectively. Her thyroid-stimulating hormone (TSH) level was 0.86 μIU/ml, and her menstrual cycle was 28 days. She did not demonstrate any ultrasonographic findings characteristic of polycystic ovary syndrome. Hysterosalpingography revealed bilateral tubal obstruction with right-sided hydrosalpinx. The patient was scheduled to undergo assisted reproduction for 1.5 years of secondary infertility. Salpingectomy was discussed and planned in case of repeated implantation failure. In blood taken on day 1 of the patient’s last menstrual cycle, her levels of E2, LH, and P4 were 26 pg/ml, 4.4 mIU/ml, and 0.23 ng/ml, respectively. After a negative result was confirmed in an hCG urine test, she was started on oral dydrogesterone 20 mg/day and began daily self-injection of urinary FSH 300 IU on the same day. In blood taken on day 9 of ovarian stimulation, her levels of E2, LH, and P4 were 4569 pg/ml, 1.35 mIU/ml, and 3.5 ng/ml, respectively. Therefore, the urinary FSH was changed to human menopausal gonadotropin 300 IU, which contains high levels of LH. On day 11 of ovarian stimulation, her levels of E2, LH, and P4 were 8679 pg/ml, 0.1 mIU/m, and 16.3 ng/ml, respectively, prompting suspension of ovarian stimulation. The patient had no symptoms during the controlled ovarian stimulation (COS), and no abnormal ultrasound finding was detected during COS. Eleven days after ovarian stimulation was suspended, the patient had abdominal distension and demonstrated ascites extending to the upper abdomen under ultrasonography. She was diagnosed with OHSS and started on cabergoline 0.5 mg/day and aspirin 100 mg/day, but these failed to improve her condition. Therefore, 13 days after ovarian stimulation was suspended, she was transported to our hospital for intensive care for severe OHSS. Upon arrival, the patient perceived abdominal distension as well as a soft abdomen, lower abdominal pain, and low back pain during palpation. Her blood pressure, heart rate, body temperature, and oxygen saturation were 113/88 mmHg, 100 beats/minute, 37.0 °C, and 99%, respectively. Her blood test results were as follows: white blood cell count 19,800/μl, hemoglobin 14.2 g/dl, hematocrit 41.3%, C-reactive protein 1.2 mg/dl, total protein 5.1 g/dl, albumin 2.7 g/dl, aspartate transaminase 31 U/L, alanine transaminase 23 U/L, lactate dehydrogenase 210 U/L, sodium 130 mEq/L, potassium 4.4 mEq/L, creatinine 0.48 mg/dl, and uric acid (UA) 5.8 mg/dl. The results of serologic testing for hepatitis B, hepatitis C, and syphilis were negative. Ultrasonography revealed bilateral ovarian enlargement (right ovary length, 8.6 cm; left ovary length, 5.5 cm) as well as ascites extending to the upper abdomen. A chest x-ray showed that both costophrenic angles were sharp, and no pleural effusion was observed. On the basis of these findings, the patient was diagnosed with severe OHSS and hospitalized for further care. Suspicion of late-onset OHSS in a pregnancy cycle prompted measurement of her serum hCG level, which was 27,778 mIU/ml. When the patient was asked again about her medical history, she stated that her menstrual cycle occurred 28 days prior to the beginning of ovarian stimulation and that she had last had sexual intercourse 16 days prior to the beginning of ovarian stimulation. Thus, at the start of ovarian stimulation, at 4 weeks, 2 days (after her true last menstrual period), bleeding that was originally assumed to be menstruation was deemed to be abnormal uterine bleeding in early pregnancy. When she was transported to our hospital, she was 7 weeks, 3 days pregnant. A second ultrasound revealed a hollow structure on the lateral aspect of the ovary but did not show an embryo inside the ovary. In addition, the corpus luteum was indistinct due to enlargement of the ovary. On the basis of these findings, emergency laparoscopic surgery was performed for suspected right tubal pregnancy. We performed bilateral laparoscopic salpingectomy, and chorionic villi were macroscopically observed in the right fallopian tube. We also observed approximately 5000 ml of light-yellow ascites. We prevented postoperative thrombus with oral administration of aspirin 100 mg/day and intermittent pneumatic leg compression. We also administered additional oral administration of cabergoline 0.5 mg/day until OHSS improved. On day 4 postoperation, the patient was discharged after demonstrating improvement of ascites, improvement of hemoconcentration, and a favorable reduction in serum hCG (751 mIU/ml). Oral administration of aspirin and cabergoline and aspirin was continued until day 11 postoperation. Her serum hCG level returned to negative on day 24 postoperation, and she resumed her infertility treatment at her previous infertility clinic 3 months after the surgery. Her pathology result was determined to be right ampullary tubal pregnancy. We performed a repeated serum hCG test on a specimen of blood preserved by the patient’s previous physician. According to this test, the patient’s serum hCG level at the start of ovarian stimulation was 12 mIU/ml. The patient’s clinical course is shown in Fig..