|Sonographic Evaluation of the Normal and Abnormal Placenta
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Course: Sonographic Evaluation of the Normal and Abnormal Placenta
Target Audience: Physicians, sonographers and others who perform and/or interpret obstetrical ultrasound.
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Estimated Time for Completion of tutorial: One hour
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Dr. Lyndon Hill discloses no relevant financial relationships with commercial interests.
IAME discloses no relevant financial relationships with commercial interests.
Sonographic Evaluation of the Normal and Abnormal Placenta
The eventual position and shape of the normal placenta is due to the degeneration of villi from all areas, except those with the best blood supply. As a result, villi in the lower uterine segment tend to atrophy, while villi develop within the uterine fundus. This process is referred to as trophotropisim (tropho [Greek], to feed; tropic [Greek], turn toward) 4 .
The placenta should be considered a fetal organ. Intrauterine growth restriction (IUGR) is preceded by reduced placental growth in the first half of pregnancy 5 . In early onset IUGR there is a failure to develop a normal placental villous tree, as well as a failure by the placenta to respond to hypoxia with trophoblastic proliferation and angiogenesis 6 .
Enlarged "jelly-like" Placenta
Enlarged "jelly-like" placentas have a prevalence of 0.6 to 7.6% 7 (Fig. 1). The range in the estimated frequency is due to variation in terminology between authors. In general, these placentas contain numerous sonolucent spaces with turbulent flow; the placental weight is < 10th percentile. They are associated with IUGR, pre-eclampsia and elevated 2nd trimester maternal serum alphafetoprotein 8 . Defective transformation of the spiral arteries results in a globular "jelly-like" placenta. Because of the poor vascularity, the majority of jelly-like placentas are located laterally.
The intervillous space varies in size (0.3 to 3.0 cm) 9 . With uterine contractions, spiral artery flow is altered, resulting in a change in the shape of the intervillous space. The slow turbulent flow within the intervillous space, as well as the spiral artery's entrance into the intervillous space can be appreciated with color Doppler. One or more intervillous spaces are visualized in 2.2% of placentas between 15 and 34 weeks' gestation 10 .
An intervillous thrombus (Fig. 2), therefore, consists of coagulated maternal and fetal blood within an intervillous space. These lesions occur in up to 40% of placentas; they are not associated with an increase in fetal morbidity. Sonographically, they are initially isoechoic. As fibrin accumulates around its periphery, an intervillous thrombus becomes increasingly echogenic 11,12 .
These cysts are frequently located close to the placental cord insertion (Fig. 3). The cysts may be simple or complex. If the cysts are larger than 4.5 cm or > 3 in number, the frequency of intrauterine growth restriction may be increased 13 .
In mid-gestation the placenta occupies 50% of the uterine surface. By 40 weeks' gestation, the placenta occupies 17 - 25% of the uterine volume 14 . This, in part, explains the decreasing prevalence of placenta previa as gestation advances.
The incidence of placenta previa is approximately 6% in the 1 st trimester 15 and 0.5% at term 15 .
Predisposing factors for placenta previa are outlined on Table I. The recurrence risk for placenta previa increases with each cesarean section (Table II) 16 . A prior cesarean section does not affect the site of placental implantation. The association between a prior cesarean section and placenta previa is due to a lack of resolution of a previa, suggesting that trophotropism is affected by lower uterine segment scarring 17 .
With transvaginal sonography, Farine et al 18 reported a 100% sensitivity and negative predictive value for the sonographic diagnosis of placenta previa (Fig. 4). If the placenta extends over the internal cervical os by >= 2.5 cm at 20 - 23 weeks' gestation, Becker et al 19 had a 100% cesarean delivery rate (12 cases) (Fig. 5).
Transvaginal sonography has been used to define a low-lying placenta as < 2.0 cm from the internal cervical os 20 (Fig. 6). Openheimer et al 20 did not have to perform any cesarean sections for vaginal bleeding when the placenta was > 2 cm from the internal cervical os (14 cases). However, 7 of 8 patients with a placental edge < 2 cm from the internal cervical os required a cesarean section for bleeding. The only patient in this group who did not have a cesarean section had a scan to delivery interval of 11 weeks. If the lower edge of a low-lying placenta is thick (> 1 cm) there is a higher risk of hemorrhage, emergency cesarean section and placenta accreta 21 .
Placental abruption has been defined as the premature separation of a normally implanted placenta after 20 to 24 weeks' gestation. External vaginal bleeding is not always present; approximately 40% of cases have concealed retroplacental hemmorhage (Fig. 7). The incidence of placental abruption is determined by the prevalence of high-risk patients at a given institution and is between 1/50 and 1/270 deliveries. The recurrence risk after 1 or 2 prior placental abruptions is 5.6% and 17%, respectively 22,23,24 . Factors that have been associated with placental abruption are outlined in Table III 25 .
The sensitivity of an ultrasound examination for detecting placental abruption is between 25% and 50% 24,26,27 . However, if a retroplacental clot is visualized, the positive predictive value is quite high. In one study, if the delivery was within 2 weeks of positive ultrasound finding, the diagnosis was confirmed in every case. Because of its low sensitivity, the primary role of an ultrasound examination is to exclude a diagnosis of placenta previa.
Some of the numerous sonographic manifestations of placental abruption are outlined in Table IV 24,28,29 . Retroplacental abruptions generally have a worse prognosis than abruptions without a contained hemorrhage. Chronic or recurrent abruption with subsequent placental insufficiency can result in fetal growth restriction 30 .
Succenturiate Placental Lobe
There may be one to several succenturiate or accessory placental lobes (Fig. 8). They each would have a vascular connection to the main body of the placenta. The incidence of succenturiate placental lobes is approximately 5% 31 . Villous atrophy results in the isolation of a placental lobe from the main body of the placenta. The primary concern with a succenturiate placental lobe is that the vessel connecting to the main placental body traverses the cervix (vasa previa). The second complication associated with a succenturiate placental lobe is the potential retention of the accessory lobe within the uterus after delivery 32 .
With placenta accreta the decidua basalis does not separate the myometrium and trophoblast. As a result the placenta adheres directly to the myometrium (accreta), grows into the myometrium (increta); or grows completely through the myometrium (percreta).
The incidence of placenta accreta has increased markedly over the past 70 years. In the 1930's the reported incidence was approximately 1 case/30,000 deliveries 33,34 . Wu 35 has recently reported an incidence in 1/533 deliveries. This marked increase is due to the increase in cesarean sections. With the increase in cesarean sections, there has also been an increased incidence of placenta previa - an additional risk factor for placenta accreta 34 . Usta et al 36 have reported a 6.3% rate of placenta accreta with placenta previa.
The loss of the hypoechoic interface between the placenta and myometrium was one of the first sonographic signs associated with placenta accreta. However, the isolated loss of this clear space as the only sign of placenta accreta has subsequently been found to have a low positive predictive value for placenta accreta between 15 weeks' gestation and term 37 .
When placenta accreta is present, the placenta has irregularly shaped vascular sinuses (lacunae) that have detectable low velocity blood flow (Fig. 9). Placental lacunae have been detected in cases of placenta accreta as early as 15 weeks' gestation. Placental lacunae have been reported to have a 93% sensitivity and positive predictive value for placenta accreta 38 .
The interface between the bladder and uterus is usually fairly thick. In patients with placenta accreta, varicies give the appearance that the bladder wall is interrupted (Fig. 10). This sonographic sign has a 93% sensitivity for detecting placenta accreta 38 .
The presence of a gestational sac in the lower uterine segment at 10 weeks or earlier is a 1 st trimester sonographic sign associated with palcenta accreta 37 .
Chorioangiomas are detected in 1% of placentas evaluated pathologically after delivery. However, those that are large enough to be clinically significant occur only once in every 3,500 to 9,000 pregnancies 39,40 . In the past, only chorioangiomas > 5 cm were considered large enough to produce fetal complications 41 . More recently, it has been determined that the vascularity of a tumor, rather than its size, determines the likelihood of fetal complications 42 . Chorioangiomas are most commonly detected around the placental cord insertion. As they enlarge, chorioangiomas protrude into the amniotic cavity (Fig. 11).
Color Doppler should be used to evaluate a placental mass for blood flow. Chorioangiomas that produce fetal effects have either significant internal flow or a large feeding vessel 41 (Fig. 11). Their echo pattern may vary from isoechoic with the placenta to having a mixed echodense/echospared appearance due to the type and degree of degenerative changes that may have occurred. Fibrotic degeneration of a chorioangioma reduces the amount of blood flow through the lesion and is, therefore, considered a good prognostic sign.
The fetal effects from a vascularized chorioangioma are outlined in Table V. Non-immune hydrops results from the shunting of blood to the tumor and secondary high-output failure. Hemolytic anemia may also contribute to fetal hydrops 43 . Polyhydramnios is associated with the amount of vascular flow to the chorioangioma rather than to its size 42 . It has been hypothesized that the associated polyhydramnios is due to a transudate through the walls of the abnormal vessels within the chorioangioma. The fluid then passes through the placenta to the amniotic cavity.
The placenta should be evaluated, not only as a necessary organ for fetal growth the development, but also as a potential source of fetal disease and/or compromise.
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