On the day of egg collection, some patients often receive >20 eggs and have their transplant cancelled. This can be seen in the medical record as “Cancellation due to OHSS”. While patients may be happy about the high number of eggs harvested, they may wonder why they cannot have a fresh embryo transfer. So let’s find out what is Ovarian Hyperstimulation Syndrome (OHSS) and the reasons why transfer cannot be performed due to OHSS! OHSS often occurs in women with ovulation induction by ovulatory drugs or controlled superovulation with artificial insemination or in vitro fertilization, during the luteal phase (from ovulation to the day before the onset of menstruation) or early pregnancy (before the end of the 13th week of pregnancy). Due to the extensive use of ovulation-promoting drugs, the incidence of OHSS is no longer rare, with an overall incidence of 20%, and is one of the major complications of assisted reproduction techniques. The etiology and pathophysiological changes of OHSS are closely related to hCG and serum E2. The production of large amounts of vasoactive substances is the pathophysiological basis for the occurrence of OHSS, resulting in damage to the capillary wall, its increased permeability, and leakage of intravascular fluid, causing ascites, pleural fluid, diffuse edema, etc. III. Clinical manifestations Abdominal distension, nausea, vomiting, diarrhea, which further develops into drowsiness and fear of food. Pleural fluid may lead to respiratory distress. Physical signs: rapid weight gain, oliguria or anuria. Auxiliary examination: hemoconcentration, increased white blood cells, hypovolemia, electrolyte disorders, peritoneal/pleural/pericardial effusion, hypercoagulable state of blood. IV. Diagnosis: 1. History of ovulation-promoting drugs and hCG use; 2. Occurs during the luteal phase or early pregnancy; 3. Basic features: enlarged ovaries; intravascular fluid transfer to the third cavity, ascites, pleural fluid, generalized edema; hematoconcentration. Early onset OHSS: occurs <10 days after hCG injection Late onset OHSS: occurs ≥10 days after hCG injection V. Risk factors 1. young (<35 years old) and thin women; 2. ovaries are highly sensitive to ovulatory drugs, such as PCOS patients; 3. high AMH levels; 4. previous history of OHSS and allergic individuals; 5. multiple follicular development; 6. serum E2 levels high; 7, endogenous hCG secretion during early pregnancy; 8, use of hCG to promote ovulation or maintain the corpus luteum of pregnancy, etc. Patients with >20 fertile eggs usually have high serum E2 levels and are prone to OHSS. If still transplanted and pregnant, hCG stimulation during early pregnancy will lead to the occurrence, persistence, or even aggravation of OHSS. Clinically, patients usually have gastrointestinal discomfort and other symptoms that are detrimental to both the physical and psychological aspects of the patient. We hope that you will be in the best state of preparation for the arrival of your baby, so don’t be discouraged if your transplant is cancelled due to OHSS, and please actively cooperate with your doctor’s treatment plan. VI. Treatment principles OHSS is a self-limiting disease, and most patients can heal on their own. Treatment is mainly symptomatic. Mild: outpatient observation and follow-up, generally no special treatment; moderate: hospitalization for observation, bed rest and fluid replacement; severe: hospitalization, active treatment, absolute bed rest, close monitoring of blood pressure, pulse, respiration, ascites, pleural fluid, correction of electrolyte imbalance, supplemental albumin and volume expansion.