The placenta is an important, complex and special organ between the mother and the fetus. The placenta is composed of both fetal and maternal tissues and is an important organ for material exchange, nutrient metabolism, hormone secretion and barrier to foreign microbial invasion to ensure normal fetal development. It is unique to the mammalian embryonic development process. In fetal animals, fetal growth is linked to the mother through the placenta. The placenta is an indispensable organ for the maintenance of the whole process of pregnancy and for the growth and development of the fetus. What are the functions of the placenta? In fact, the placenta has complex physiological functions, which can be divided into the following points: 1. metabolic function: including gas exchange, nutrient supply and discharge of metabolic products from the fetus. 2, defense function: the fetal blood and the mother’s body are separated by the placental barrier, which has a protective function for the fetus. But this function is not perfect, various viruses (such as rubella virus, influenza virus, cytomegalovirus, etc.) can directly enter the fetus through the placenta, which can cause fetal malformation, miscarriage and stillbirth. 3, immune function: the production of various protein hormones, such as alpha-fetoprotein has immunosuppressive effect. 4.Endocrine function: mainly secrete various hormones and enzymes. Such as estrogen and progesterone, human chorionic gonadotropin, contractile hormone enzyme, etc. What is the structure of placenta? The placenta is composed of three parts: amniotic membrane, lobulated chorionic membrane and basal meconium. Amnion: It forms the fetal part of the placenta and is the innermost layer of the placenta. The amnion is smooth, free of blood vessels, nerves and lymph, and has a certain degree of elasticity. Lobular chorionic membrane: constitutes the fetal part of the placenta and accounts for the major part of the placenta at full term of pregnancy. The main structure of the placenta, the chorion, is formed gradually when the embryo develops from 13d to 21d. The chorionic villi in contact with the basal meconium are called lobulated chorionic villi because they are rich in nutrients and well developed. Basal metaplasm: It forms the maternal part of the placenta. The surface of the bottom metaplasm is covered with a layer of trophoblast cells from the fixed villi, which together with the bottom metaplasm form the bottom of the interstitial space of the villi, called the metaplastic plate, from which some metaplastic intervals extend in the direction of the chorionic villi, dividing the maternal surface of the placenta into about 20 maternal lobes. What are the positions of the placenta? Placenta praevia is divided into three types: complete placenta praevia, partial placenta praevia and marginal placenta praevia: complete placenta praevia: the endocervix is completely covered by placental tissue; partial placenta praevia: the endocervix is partially covered by placental tissue; marginal placenta praevia: the lower edge of the placenta is attached to the lower part of the uterus, but does not go beyond the endocervix. Low placenta state: after the formation of placenta, the edge of placenta reaches or is close to the endocervix, this is all called low placenta state, with the increase of gestational weeks, the placenta will slowly rise with the increase of uterus, after 32 weeks, if the placenta does not rise to more than 2.5 cm from the endocervix, we call it placenta praevia. What are the special types of placenta? Rotated placenta: the incidence is less than 1/6000, it refers to the concave center of the fetal side of the placenta, surrounded by thickened gray-white ring, Rotated placenta can be divided into complete (forming a complete ring of placental tissue) and partial (forming an incomplete ring of placental tissue), partial Rotated placenta will not cause any fetal abnormalities, while complete Rotated placenta is associated with placental abruption, preterm birth, IUGR, fetal malformation, perinatal The complete placenta is associated with placental abruption, preterm birth, IUGR, fetal malformation, and increased perinatal mortality. Membranous placenta: A membranous placenta is a large, thin placenta surrounded by a layer of functional villi. This abnormal form of placenta is very rare, with an incidence of about 1/3300, and is caused by excessive blood supply to the meconium, making it difficult for the smooth chorionic membrane connected to the meconium to atrophy. Paraganglionic placenta: one or several placental lobules develop within a distance from the periphery of the main placenta, and there are blood vessels connected between the main placenta and the paraganglionic placenta. It usually does not cause adverse effects to the mother and fetus, but it may cause placenta residual leading to hemorrhage.