OBJECTIVE: To assess ultrasound and hematological changes during the early luteal phase following triggering of final oocyte maturation with human chorionic gonadotropin (hCG) in women at high risk for developing ovarian hyperstimulation syndrome (OHSS).METHODS: This was a retrospective cohort study of 319 women undergoing in-vitro fertilization who were at high risk for OHSS following administration of hCG for the triggering of final oocyte maturation. Patients were treated with a gonadotropin-releasing hormone agonist or antagonist protocol and were monitored for 5 days post-oocyte retrieval (early luteal phase). Severe OHSS was diagnosed in the presence of at least moderate ascites and two or more of the following: maximum ovarian diameter (MOD) > 100 mm, hematocrit (Ht) > 45%, white blood cell count (WBC) > 15 000/mm , hydrothorax, dyspnea and oliguria. Outcome measures included change in Ht, ascites grade, WBC and MOD, as well as the association between these changes during the early luteal phase.RESULTS: Ascites grade, Ht and WBC increased significantly (P ≤ 0.001) during the early luteal phase, both in patients who developed and in those who did not develop severe early OHSS. MOD increased significantly (P = 0.001) only in patients who developed severe early OHSS. On multivariable analysis, both time following oocyte retrieval and whether severe early OHSS developed were significantly associated with ascites grade, Ht, WBC and MOD; furthermore, there was also a significant interaction between time and development of severe early OHSS for all four variables (P ≤ 0.001).CONCLUSIONS: In women at high risk of OHSS, ascites grade, Ht and WBC significantly increased with time over the 5-day observation period, in line with the pathophysiology of the syndrome. Our data support the use of MOD in the diagnosis of severe early OHSS, and provide novel evidence for the role of change in Ht as a patient-specific hemoconcentration marker during development of OHSS. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.