Uterine septum is one kind of Müllerian duct anomaly with a prevalence ranging from 2.3% [1, 2]. Some cases are asymptomatic which are incidentally detected on the ultrasound and obstetric surveillance is recommended; others are associated with infertility, recurrent pregnancy loss or preterm labor [3- 5]. Expectant management and serial sonographic scanning are required. Salpingography and three-dimensional ultrasound helps to detect uterine anomalies [6, 7]. Hysteroscopic examination is valuable on the condition of either diagnosis or treatment before pregnancy [8]. However, ultrasonographic diagnosis turn to be difficult due to the septa shielded by the fetus and placenta. Here we present a case of delayed diagnosis of uterine septum developed from 14-33 gestational weeks (GW). The initial impression of the sonographic image interfered our judgment.