Objective: To clarify the effect of hyperemesis gravidarum (HG) on the 75g oral glucose tolerance test (OGTT) and gestational diabetes mellitus.Methods: This retrospective cohort study was conducted via an evaluation of the hospital database medical records of 700 pregnant women. Of these, 60 were included in the study group as a result of hospitalization due to HG, 41 were excluded, and the remaining 599 formed a control group. The body mass index (BMI), urine ketone levels, and ages of all participants were separately recorded, both in the initial examination and during the 75g OGTT.Results: At initial examination, no significant differences in maternal age and BMI were observed between the two groups. There was a significant decrease in BMI after 75g OGTT in the study group. No significant difference in fasting serum glucose levels was found between the two groups, but significant differences in first and second hour serum glucose levels were observed.Conclusions: HG may improve in many women in the late second trimester, and loss of fatty tissue may affect the 75g OGTT screening results. The appropriate cutoff value of 75g OGTT for HG should be reevaluated following future, larger, studies.