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Definitions
Basal plate: maternal surface of placenta.
Chorioangioma: benign tumors arising from the fetal surface of the placenta.
Chorionic plate: fetal surface of placenta.
Complete placenta previa: occurs when the placenta completely covers the internal os.
Extrachorial placenta (circummarginate and circumvallate): attachment of placental membranes to the fetal surface of the placenta rather than the villous placental margin.
Low-lying placenta/low implantation: occurs when the inferior placental edge is within 2 cm of the internal cervical os.
Marginal placenta previa: occurs when the placenta extends over the cervix and abuts the internal cervical os.
Partial placenta previa: occurs when the placenta partially covers the internal os.
Placental abruption: separation of a normally implanted placenta.
Succenturiate placenta: the presence of one or more accessory lobes separate from the main placental body.
Trophotrophism: a dynamic process of placental remodeling where areas with poor vascularity atrophy and areas of good perfusion grow.
Uterine synechia (amniotic sheets): broad-based tissue that extends into the amniotic cavity but is external to the amnion and chorion.
Vasa previa: occurs when fetal vessels that run in the fetal membranes cross the internal cervical os.
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The placenta, cord, and membranes are the building blocks of human pregnancy. Vast changes take place in the first trimester and throughout pregnancy that can be followed sonographically. The placenta provides the essential connection between the mother and developing fetus. Many clinical problems are attributed to the placenta, despite the fact that they cannot always be explained after pathologic examination.
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A thorough understanding of the anatomy of the normal placenta and its variations, as well as the pathologic conditions that are known to occur, is necessary to correctly interpret the sonographic appearance. Placental location with respect to the internal cervical os and the maternal urinary bladder will be reviewed, especially in light of the increasing cesarean delivery rate and abnormal placentation. The umbilical cord and membranes share similar developmental origins, which will be considered. Intrauterine membranes can be delineated with sonography in patients who have bleeding episodes and in uncomplicated pregnancies. Their assessment is clinically important in multifetal pregnancies.
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This chapter examines the embryology of the placenta, cord, and membranes. Normal and abnormal processes of placentation and umbilical cord development will be reviewed. Pertinent clinical aspects are highlighted.
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DEVELOPMENT OF THE PLACENTA: EMBRYOLOGY
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The endometrium undergoes changes in preparation for embryo implantation known as the decidual reaction or decidualization, where secretory endometrium is transformed into deciduas.1,2 This change is prompted by estrogen, progesterone, and other factors secreted by the invading blastocyst.2 There are 3 parts of the decidua: (1) decidua basilis, the modified portion of decidua directly beneath the implantation site; (2) decidua capsularis, the portion overlying the blastocyst; and (3) decidua parietalis...