The placenta attaches to the uterine wall and allows metabolic exchange between the fetus and the mother. The placenta has both embryonic and maternal components. The embryonic portion comes from the outermost embryonic membrane. The maternal portion develops from the decidua basalis of the uterus. The placental membrane separates the embryonic blood from maternal blood but is thin enough to allow diffusion and transport of nutrients and waste. A normal placenta is round or oval shaped and about 22 cm in diameter. It is 2 cm to 2.5 cm thick and weighs about a pound.
Placenta accreta is the abnormal adherence of the placenta to the myometrium, associated with a partial or complete absence of the decidua basalis and an abnormally or incompletely developed fibrinoid Nitabuch layer. When normally developed, these layers represent the cleavage line allowing a normal third stage of labor. The prevalence of this condition has been increasing and now occurs in 1 of 2500 pregnancies. The incidence of placenta accreta increases in women with previous cesarean delivery, other uterine surgery, advanced maternal age, high gravidity, multiparity, previous curettage, and placenta previa. The highest risk for a placenta accreta is in pregnancies with a history of a cesarean section and a current placenta previa.
Ultrasound features suggestive of placenta accreta include deficiency of retroplacental sonolucent zone, vascular lacunae, myometrial thinning, and interruption of the bladder line. Grayscale sonography has a sensitivity of 77% to 87% and a specificity of 96% to 98% for placenta accreta. MRI does not appreciably improve the diagnostic accuracy when compared to ultrasonography. An attempt to deliver an adherent placenta can result in hemorrhage, shock, and uterine inversion. Hysterectomy traditionally treated placenta accreta, but uterus-conserving treatments are now commonly used.
Placenta increta is a form of placenta accreta in which the placental villi penetrate into the uterine muscle (myometrium), but do not penetrate the uterine serosa. Placenta increta accounts for approximately 15% to 17% of all placenta accreta cases.
Placenta percreta is a form of placenta accreta in which the placental villi penetrate through the myometrium to the uterine serosa. Placenta percreta accounts for approximately 5% to 7% of all placenta accreta cases.
Placenta previa occurs when the placenta implants totally or partially in the lower segment of the uterus rather than in the fundus. In complete previa, the internal os is completely covered by the placenta. In partial previa, a portion of the internal os is covered by the placenta. In marginal previa, the edge of the placenta extends to the edge of the cervical os. A low-lying placenta will commonly be defined as within 2 cm of the cervical os without covering any portion of it. These conditions occur in approximately 1 in 200 to 250 pregnancies, and risk factors include prior cesarean delivery, previous abortion, prior intrauterine surgery, smoking, multifetal gestation, increasing parity, and maternal age. A woman with a history of placenta previa is 12 times more likely to have placenta previa in a subsequent pregnancy. Ultrasound screening programs during first and early second trimester pregnancies now include placental localization. Transvaginal ultrasound can also make a diagnosis. Delivery should be by Cesarean section as dilation of the cervix causes separation of the placenta leading to bleeding from the open vessels. Vaginal delivery is an option for those with low-lying placentas as the bleeding morbidity has proven to be limited.
Bilobed placenta (placenta bilobate, bipartite placenta, placenta duplex) is a placenta with two roughly equal-sized lobes separated by a membrane. It occurs in 2% to 8% of placentas. The umbilical cord may insert in either lobe, in velamentous fashion, or in between the lobes. While there is no increased risk of fetal anomalies with this abnormality, bilobed placentas can be associated with first-trimester bleeding, polyhydramnios, abruption and retained placenta. A placenta with more than two lobes is rare and is termed a multilobate placenta.
The succenturiate placenta is a condition in which one or more accessory lobes develop in the membranes apart from the main placental body to which vessels of fetal origin usually connect them. It is a smaller variant of a bilobed placenta. The vessels are supported only by communicating membranes. If the communicating membranes do not have vessels, it is called placenta supuria. This condition occurs in 5% of placentas. Advanced maternal age and in vitro fertilization are risk factors for the succenturiate placenta. Other factors leading to succenturiate placentas include implantation over leiomyomas, in areas of previous surgery, in the cornu or over the cervical os. Ultrasound, particularly color Doppler, can be used to identify this condition. The risks of vasa previa and retained placenta are increased with this condition like with bilobed and multilobate placentas.
Circumvallate placenta is an extrachorial placenta that is an annularly-shaped placenta with raised edges composed of a double fold of chorion, amnion, degenerated decidua, and fibrin deposits. In this condition, the chorionic plate is smaller than the basal plate, resulting in hematoma retention in the placental margin. Within the ring, the fetal surface has the usual appearance except that the large vessels terminate abruptly at the margin of the ring. Circumvallate placenta is associated with poor pregnancy outcomes due to increased risk of vaginal bleeding beginning in the first trimester, premature rupture of the membranes (PROM), preterm delivery, placental insufficiency, and placental abruption. The diagnosis is difficult to make during pregnancy and is made most often on visual inspection of the placenta after delivery.
Circummarginate placenta is an extrachorial placenta that is similar to a circumvallate placenta except that the transition from membranous to villous chorion is flat. This form is clinically insignificant.
Placenta membranacea is a rare placental abnormality where chorionic villi cover fetal membranes either completely (diffuse placenta membranacea) or partially (partial placenta membranacea), and the placenta develops as a thin structure occupying the entire periphery of the chorion. Clinically the abnormality can present with vaginal bleeding in the second or third trimester which is often painless or during labor. Other placenta abnormalities, such as placenta previa and placenta accreta, can be associated with this condition. Ultrasound has been reported as a diagnostic tool for this condition, but due to the condition’s rarity, there is no data on its sensitivity and specificity.
The ring-shaped placenta is an annularly-shaped placenta that is a variant of placenta membranacea. It can sometimes be a complete ring of placental tissue, but more often tissue atrophy in a portion of the ring results in a horseshoe shape. Incidence is less than 1 in 6000. The ring-shaped placenta can cause antepartum and postpartum bleeding as well as fetal growth restriction.
Placenta fenestrata is a rare condition in which the central portion of the discoid placenta is missing. Rarely, there may be an actual hole in the placenta, but more often the defect involves the villous tissue, and the chorionic plate remains intact. At delivery, this finding may cause concern for retained placenta.
Battledore placenta (Marginal cord insertion) is a condition in which the umbilical cord is inserted at or near the placental margin rather than in the center. The cord can be inserted as close to 2 cm from the edge of the placenta (velamentous cord insertion). The incidence is 7% to 9% of singleton pregnancies and 24% to 33% in twin pregnancies. Complications associated with battledore placenta are preterm labor, fetal distress, and intrauterine growth restriction.
The placenta is a maternal-fetal organ which begins development at implantation of the blastocyst, and it is delivered at birth with the fetus. The fetus relies on the placenta for nutrition and many developmentally essential functions.
Abnormalities range from those anatomical associated with degree or site of implantation, those of structure and placental function, to placenta-maternal effects such as pre-eclampsia and fetal erythroblastosis, and finally to mechanical abnormalities associated with the umbilical cord.
The evaluation of the placenta and cord plays an important role in evaluating the viability of the fetus, and ultimately the infant delivered.
While routine pregnancy is managed by the obstetrician, complex cases that involve placental abnormalities are usually managed by an interprofessional team that include the intensivist, hematologists, labor and delivery nurses, and ansethesiologists. Most placental abnormalities come to light just before or during delivery. With the advent of ultrasound, most serious placental abnormalities are identified before the delivery takes place. One of the most serious morbidities with placental abnormalities is the potential for hemorrhage, hence, a team approach is necessary to ensure safety of the mother and the infant.
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