The growing placenta starts at 5 weeks of pregnancy and forms at 12 weeks of pregnancy. The placenta begins to grow at 5 weeks of pregnancy. At this time there is already a relatively obvious fetal placental circulation established, just because it is too small and too early ultrasound is not visible, 8 weeks of pregnancy when the placental tissue gradually formed, 12 weeks of pregnancy the placenta is fully formed, ultrasound can be detected. The formation of the placenta is a gradual process. In the very early stages of pregnancy, ultrasound can often see where the gestational sac is attached to the uterus through the period of time, and at 12 weeks of pregnancy the placenta is fully formed, and ultrasound can detect the placenta more clearly. As the placenta grows, it is important to monitor its position by ultrasound. Being too close to the cervical opening, or covering the cervical opening, is considered placenta praevia, which is a dangerous condition that requires close attention during pregnancy.