The Placenta is usually described as going through four grades, from 0 (most immature) to III (most mature). All placentas start at grade zero in early pregnancy. Changes can be seen from 12 weeks onwards. As pregnancy progresses, the placenta matures and calcifies. It is classified into the following grades at different stages in pregnancy, at approximately the following times: Grade 0. Before 18 weeks of pregnancy. Grade I. Around 18 to 29 weeks of pregnancy. Grade II. Around 30 to 38 weeks of pregnancy. Grade III. Around 39 weeks of pregnancy. It is usually not seen before 38 weeks. A grade III placenta is known as a severely calcified placenta. At this stage, a formation of indentations or ring-like structures can be seen within the placenta.