Placental maturity level one at 21 weeks of pregnancy generally has no effect if the fetus grows and develops normally. The maturity of the placenta is usually determined by ultrasound, and currently the commonly used method is the Grannum placenta grading method, which divides the maturity of the placenta into four grades: Grade 0 placenta: the chorionic plate is a bright straight line, the substance is uniform and consistent fine particles, can not see the base. Most of them appear before 28 weeks of pregnancy. Grade 1: the chorionic plate is a mildly undulating bright line, the substance of the placenta is slightly thicker particles, slightly stronger echoes, the basal layer still does not appear, most often 29-36 weeks of gestation. Grade 2 placenta: the chorionic plate is incised and extends to the placental parenchyma but does not reach the basal layer; the placental parenchyma is coarsened and has strong short columnar echoes; the basal layer may have irregular strong stripes of echoes that run parallel to the muscular wall. The gestational week is mostly 36-40 weeks. Grade 3 placenta: the chorionic villus tracing has reached the basal layer, the placenta parenchyma appears to have multiple strong echogenic rings, there may be small pools of anechoic echoes, and sometimes reflective foci of enhanced calcification can be seen. These grades are only a performance of ultrasound, can not fully represent the function of the placenta, and there is a certain error, 21 weeks gestation placenta I, such as fetal growth and development is normal, generally do not need to worry about, if the fetal growth and development is delayed, consider the existence of placental dysfunction may be, need to consult the doctor in a timely manner.