and Video S1: A 32-year-old primigravid woman underwent obstetric ultrasound at 32 weeks of gestation because of a small-for-date uterine size. Her medical history as well as familial history was unremarkable; no underlying medical disease and no history of any pelvic surgery. The antenatal course of the current pregnancy was uneventful. Ultrasound screening for fetal anomaly at 20 weeks of pregnancy revealed normal structures and fetal biometry, with no records of pelvic pathology on sonographic examination. Ultrasound examination at this visit (32 weeks of pregnancy) showed a slightly delayed growth of the fetus (estimated fetal weight of 15th percentile). Interestingly, a cystic mass at the right adnexa, close to the lower uterine segment was noted. The mass was measured as 3.0 × 4.5 × 3.0 cm in size, well-circumscribed, unilocular, and had homogeneous low-level echoes, with swirling flow in the mass, which can be clearly visualized on the simple 2D ultrasound as seen in Figure 1 and video S1 (a supplementary file). A uterine artery aneurysm was highly suspected upon 2D ultrasound. Aneurysms of the uterine artery are rare and with an unknown true prevalence. The entity can be categorized into two groups, pseudoaneurysms and true aneurysms. Pseudoaneurysms are abnormal outpouchings or the dilatation of arteries which are bounded only by the tunica adventitia, the outermost layer of the arterial wall, whereas true aneurysms are bounded by all three layers of the arterial wall. Pseudoaneurysms typically occur when a blood vessel wall is injured and the leaking blood collects in the surrounding tissue. They can occur in patients of all ages, typically following penetrating or blunt trauma, infection, dissection, excessive effort, or as a complication of a cesarean section [1]. True aneurysms occur when the artery or vessel weakens and bulges, sometimes forming a blood-filled sac. True aneurysms of uterine arteries are extremely rare. To the best of our knowledge, a very limited number of isolated case reports have been published in the literature [2,3]. Moreover, a true aneurysm of the uterine artery has never been described during pregnancy. Therefore, its natural history remains unexplored. Nearly all cases reported in the literature are pseudoaneurysms. Additionally, most published pelvic true aneurysms involved ruptured ovarian artery aneurysms during peripartum periods [4,5,6,7,8]. The case presented here is unique in terms of being an unruptured true aneurysm of the uterine artery at the time of diagnosis and with the spontaneous development of thrombosis in the aneurysm in late pregnancy.