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The placenta is a vital organ with multiple functions, such as endocrine, immune, and physiological. The placenta is formed gradually during the first three months of pregnancy, while, after the fourth month, it grows parallel to the development of the uterus. Once completed, it resembles a spongy disc 20 cm in diameter and 3 cm thick. It is a temporary organ, whose genetic characteristics are identical to those of the developing child. The placenta interacts with the environment in which it is present and vice versa. Proper development of the placenta is essential for a successful pregnancy. There are several layers of tissue that make up this delicate organ that need to develop normally for proper function during gestation. Without proper function, there can be devastating consequences to the pregnancy.

Normally, there is no mixing of fetal and maternal blood. Although, this is possible with specific infectious agents or small breaks in the placental membrane that can happen during parturition. If there are breaks in the membrane, maternal red blood cells are able to cross into the fetal circulation and vice versa. Treponema pallidum (syphilis infection) can cross the barrier without breaks in the membrane. Toxoplasma gondii (toxoplasma infection) can create its own breaks in the membrane to get across into fetal circulation and infect the fetus. Erythroblastosis fetalis, also referred to as hemolytic disease of the newborn, happens when the mother makes antibodies to the fetus’s Rh factor after an Rh-negative mother becomes exposed to the fetus’s Rh-positive blood. The mom makes IgG anti-Rh antibodies, which can cross the placenta. The first pregnancy is not an issue because it takes time for mom to elicit this response, but can cause problems in future pregnancies with an Rh-positive fetus. The antibodies can cross the placenta and attack the fetal red blood cells, causing fetal hydrops (anemia and edema).[18] Placenta accreta is when the placenta grows too far into the myometrium due to a lack of decidua, which allows the villi to anchor to the myometrium. In placenta increta, the villi bury even deeper into the myometrium. Placenta percreta is when the placenta grows through the full thickness of the myometrium and reaches the serosa of the uterus. This invasion of myometrium can cause bleeding during pregnancy and bleeding complications postpartum.[19][20]

Placenta accreta is when the placenta grows too far into the myometrium due to a lack of decidua, which allows the villi to anchor to the myometrium. In placenta increta, the villi bury even deeper into the myometrium. Placenta percreta is when the placenta grows through the full thickness of the myometrium and reaches the serosa of the uterus. This invasion of myometrium can cause bleeding during pregnancy and bleeding complications postpartum.[19][20] A healthy umbilical cord should contain two umbilical arteries and one umbilical vein. If there is only one artery, there may be other defects present in the fetus, such as growth restriction or genetic abnormalities.[21] There can also be multiple arteries, which may also be associated with genetic abnormalities or congenital defects.[22] There can also be cysts in the umbilical cord filled with fluid. They are visible on ultrasound throughout the pregnancy.[23] Cysts may resolve on their own, or if there are multiple cysts, there may also be chromosomal abnormalities.[24] A rare complication from a cyst could be torsion or hematoma that could potentially cause fetal death.[25] True and false knots are also an abnormality of the umbilical cord. A false knot is when the vessels inside the cord become tortuous, but there is no knot in the cord itself. There is no adverse outcome associated with this type of knot. A true knot, however, can be dangerous to the fetus. A true knot is where the cord actually twists into a knot, which could potentially cause fetal demise if it is too tight.[26][27] The umbilical cord normally attaches to the center of the placenta. The umbilical cord can implant abnormally both in the fetus and the placenta. In the placenta, there can be a velamentous insertion or a marginal insertion. A velamentous insertion is when the vessels separate as they develop between the amnion and chorion as they grow toward the placenta. As they get closer to the placenta, they are exposed and not protected by Wharton’s jelly; this can be dangerous, especially in vasa previa where the vessels are especially prone to rupture.[28] Marginal insertion is where the cord inserts on the edge of the placenta. There is still an increased risk of placenta previa and abruption but not quite as high as a velamentous insertion.[29][30]

The umbilical cord normally attaches to the center of the placenta. The umbilical cord can implant abnormally both in the fetus and the placenta. In the placenta, there can be a velamentous insertion or a marginal insertion. A velamentous insertion is when the vessels separate as they develop between the amnion and chorion as they grow toward the placenta. As they get closer to the placenta, they are exposed and not protected by Wharton’s jelly; this can be dangerous, especially in vasa previa where the vessels are especially prone to rupture.[28] Marginal insertion is where the cord inserts on the edge of the placenta. There is still an increased risk of placenta previa and abruption but not quite as high as a velamentous insertion.[29][30] Many pathologies of the placenta are visible on histological examination, but some pathologies are visible grossly.[31] Gross pathologies include meconium myonecrosis green discoloration, vasculitis yellow-green discoloration, abscesses, placental infarction orange discoloration, masses, cysts, thrombi, and incomplete tissue collection indicating retained placenta. The placenta may be stored fresh in a refrigerator or placed in fixative if needed for further examination after parturition.[11][32] The detachment of the placenta is a very dangerous circumstance: if it happens, in fact, the placenta detaches from the uterus causing a strong haemorrhage, putting not only the life of the fetus at risk, which does not receive more oxygen and nutrients needed to survive but also of the expectant mother. Placental detachment may be due to: hypertension, overdistention of uterine walls due to excess of amniotic fluid (polyhydramnios), multi-twin pregnancy, diabetes, drug use.