to identify all women who were pregnant at their initial TTP presentation. Among 592 adulthood-onset TTP patients with a severe ADAMTS13 deficiency, 42 patients with a pregnancy-onset TTP were included. Surprisingly, the proportion of USS patients (n ‫؍‬ 10 of 42 patients [24%]; confidence interval, 13%-39%) with pregnancy-onset TTP was much higher than that in adulthood-onset TTP in general (less than 5%) and was mostly related to a cluster of ADAMTS13 variants. In the present study, subsequent pregnancies in USS patients not given prophylaxis were associated with very high TTP relapse and abortion rates, whereas prophylactic plasmatherapy was beneficial for both the mother and the baby. Pregnancyonset TTP defines a specific subgroup of patients with a strong genetic background. This study was registered at www.clinicaltrials.gov as number