Intrauterine thrombosis of umbilical cord vessels is a rare event (2.5-4.5/10,000) and usually followed by poor fetal outcome. We present the rare case of spontaneous intrauterine thrombosis of an umbilical artery leading to severe intrauterine growth restriction (IUGR) and provide clinical and pathological findings. A 28-year-old nulliparous third gravida was referred to our institution because of IUGR at 32+4 weeks of gestation. Fetal growth had been appropriate until the 31st week of gestation and had stopped thereafter. There were no signs of abruption of the placenta and no structural abnormalities except an absent paravesical colour Doppler flow in the region of the right umbilical artery. Other Doppler measurements, karyotype and TORCH serology were normal. Intermittent non-reassuring fetal heart rate led to cesarean section at 34+3 weeks of gestation. A healthy girl with measurements on the 3rd centile was born (weight of 1,590 g, length of 41 cm and head circumference of 29 cm). Gross examination displayed an elongated, highly twisted umbilical cord with a length of 70 cm, central insertion and three umbilical vessels. Microscopic examination confirmed the diagnosis of umbilical artery thrombosis along the entire length of the umbilical cord. Calcification within the thrombus and microcalcification in occluded chorionic vessels were observed as well as hemorrhagic endovasculitis and endangiopathia obliterans in the stem villi arteries. This fetal thrombotic vasculopathy (FTV) comprised about 40% of the parenchyma. The coagulation parameters and blood counts of the mother and the infant were normal apart from transient neonatal thrombocytopenia. The reason for thrombosis remained unclear but could be attributed to the elongated and highly twisted umbilical cord. Intrauterine arterial thrombosis may cause severe IUGR. This condition might be detectable by ultrasound in the course of an IUGR workup, especially when no other reasons can be found.