Most of disproportionate infants born small for gestational age (SGA) have an history of placental dysfunction with no explained cause. We report a case of an unexplained SGA infant with placental infarctions and thrombosis. Maternal thrombophilic disorder tests revealed that the patient was heterozygous for the A20210 prothrombin gene variant a newly identified thrombotic risk factor. It may be suggest that prothrombin gene variant, as factor V Leiden, could be a genetic risk factor for placental insufficiency.