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The placenta, as a rule, presents more or less rounded outlines, but now and again when inserted in the neighbourhood of the internal os it may take on a horseshoe-like appearance, its two branches running partially around the orifice.
—J. Whitridge Williams (1903)
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The placenta is a fantastic organ in its own right. As discussed in Chapter 5 (Blastocyst), it provides the indispensable interface between mother and fetus. Indeed, placental anatomy, physiology, and molecular structure remain some of the most intriguing and understudied topics in obstetrics.
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Although a placental examination by the obstetrician is recommended, by consensus, routine pathological examination is not mandatory. Indeed, specific conditions that merit submission for detailed inspection are still debated. By way of example, the College of American Pathologists has recommended placental examination for an extensive list of indications (Langston, 1997). Data, however, are insufficient to support all of these. At minimum, the placenta and cord should be inspected in the delivery room. The decision to request pathological examination should be based on clinical and placental findings (Redline, 2008; Roberts, 2008). Listed in Table 6-1 are some of the indications at Parkland Hospital for placental anatomical and histopathological examination.
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At term, the typical placenta weighs 470 g, is round to oval with a 22-cm diameter, and has a central thickness of 2.5 cm (Benirschke, 2012). It is composed of a placental disc, extraplacental membranes, and three-vessel umbilical cord. The disc surface that lies against the uterine wall is the basal plate, which is divided by clefts into portions—termed cotyledons. The fetal surface is the chorionic plate, into which the umbilical cord inserts, typically in the center. Large fetal vessels that originate from the cord vessels then spread and branch across the chorionic plate before entering stem villi of the placenta parenchyma. In tracing these, fetal arteries almost invariably cross over veins. The chorionic plate and its vessels are ...