There is currently no consensus on how success should be defined after medical management of first-trimester missed abortion. The aim of this study was to de termine the transvaginal ultrasound criterion associated with highest success ra te and, at the same time, lowest long-term complications. Prospective observati onal study of consecutively enrolled patients. A tertiary care university hospit al in northern Portugal.: Forty-four women submitted to medical management of f irst-trimester missed abortion using a regimen of vaginal misoprostol, with his tologically confirmed conception products passed vaginally. A transvaginal ultra sound scan was performed by an experienced sonographer in the morning after trea tment, to characterise uterine content. Patients were provided with a chart for daily registration of axillary temperature, vaginal bleeding and lower abdominal pain. Transvaginal ultrasound was repeated 2-3 weeks later, and again after th e following menses. Success rates of medical management when post-treat-ment t ransvaginal ultrasound criteria for subsequent expectant management were: absenc e of intra-uterine sac, largest anteroposterior diameter of hyperechogenic cont ent, and maximum area of hyperechogenic intra-uterine content in a sagittal vie w. Self-reported duration of vaginal bleeding and abdominal pain after medical treatment. Success rate was 86%(38/44) when absence of gestational sac on the 1 2 h transvaginal ultrasound was used as the main criterion for subsequent expect ant management and there was no need for further intervention. The success rate using the ultrasound criterion anteroposterior diameter ≤15 mm was 51%(22/43), and with maximum sagittal plane area under 7.5 cm2, 72%(31/43). Mean duration of vaginal haemorrhage was 9 days (minimum 2 days, maximum 14 days) and of lower abdominal pain 6 days (minimum 0 days, maximum 14 days). No patient recorded an axillary temperature exceeding 37°C. No apparent relationship between the size of ultrasound-estimated intra-uterine content and duration of symptoms was ob served. Absence of gestational sac on transvaginal ultrasound should be the crit erion used to document success after medical manage-ment of first-trimester mi ssed abortion, as it is associated with the highest short and long-term success rates, as well as mild and self-limited symptoms in the days following treatme nt. There is currently no consensus on how success should be defined after medical management of first-trimester missed abortion. The aim of this study was to de termine the transvaginal ultrasound criterion associated with highest success ra te and, at the same time, lowest long- term complications. Prospective observati onal study of consecutively enrolled patients. A tertiary care university hospit al in northern Portugal .: Forty-four women submitted to medical management of f irst-trimester missed abortion using a regimen of vaginal misoprostol, with his aggressiveness conception products passed vaginally. A transvaginal ultra sound scan was performed by an experienced sonographer in the morning after trea tment, to characterise uterine content. Patients were provided with a chart for daily registration of axillary temperature, vaginal bleeding and lower abdominal pain. Transvaginal ultrasound was repeated 2-3 weeks later, and again after th e following menses. Success rates of m edical management when post-treat-ment t ransvaginal ultrasound criteria for subsequent expectant management were: absenc e of intra-uterine sac, largest anteroposterior diameter of hyperechogenic cont ent, and maximum area of ​​hyperechogenic intra-uterine content in a sagittal vie w. -reported duration of vaginal bleeding and abdominal pain after medical treatment. Success rate was 86% (38/44) when absence of gestational sac on the 1 2 h transvaginal ultrasound was used as the main criterion for subsequently expect ant management and there was no need for further intervention. The success rate using the ultrasound criterion anteroposterior diameter of 15 mm was 51% (22/43), and with maximum sagittal plane area 7.5 cm2, 72% (31/43). Mean duration of vaginal haemorrhage was No patient relationship between the siz (minimum 2 days, maximum 14 days) and of lower abdominal pain 6 days (minimum 0 days, maximum 14 days). No patient recorded an axillary temperature exceeding 37 ° C. e of ultrasound-estimated intra-uterine content and duration of symptoms was ob served. Absence of gestational sac on transvaginal ultrasound should be the crit erion used to document success after medical manage-ment of first-trimester mi ssed abortion, as it is associated with the highest short and long-term success rates, as well as mild and self-limited symptoms in the days following treatme nt.