Normal Placenta

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. It is composed of a placental disk, extraplacental membranes, and a three-vessel umbilical cord. The maternal surface is the basal plate , which is divided by clefts into portions—termed cotyledons. These clefts mark the site where internal septa originating from the decidua basalis push up into the intervillous space. 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 placental parenchyma. When examining the placenta, either following delivery or during fetoscopic surgery—for example, during laser therapy for twin-twin transfusion syndrome—the fetal arteries almost invariably cross over fetal veins. The chorionic plate and its vessels are covered by amnion.

Sonographically, the placenta in the first and early second trimesters appears homogeneous in echotexture and mildly hyperechoic compared with the underlying myometrium. It then becomes more isoechoic with advancing gestation ( Fig. 19-5 ). As examples, after midpregnancy, it is common to identify small placental sonolucencies, and in the third trimester, the placenta may appear more heterogeneous, with visible calcifications.

Second-trimester placenta. A, The placenta is isoechoic with its adjacent myometrium, its thickness is marked by the bracket, and an arrow points to the retroplacental space. B, With color Doppler, vessels are seen spanning the retroplacental space.

(Courtesy of Adriana Male, RDMS, Parkland Health and Hospital Systems, Dallas, Texas.)

The sonographic thickness is usually greater than that of the gross specimen owing to collapse of intervillous spaces as blood drains from the placenta. As a general rule, the placental thickness in millimeters roughly approximates the gestational age in weeks. It does not normally exceed 4 cm in the second trimester or 6 cm in the third trimester. The retroplacental “clear” space normally measures less than 1 to 2 cm and, as the name suggests, appears hypoechoic. The retroplacental space is a common location of hematoma development. Inability to visualize the retroplacental space in a woman at risk for placenta accreta raises concern.

Abnormal Placental Shape

In contrast to the normal architecture described previously, placentas may form as separate disks of nearly equal size. This bilobate or bilobed placenta is also known as placenta duplex. The cord inserts between the two placental lobes, either into a connecting chorionic bridge or into intervening membranes. Sonographic detection of a bilobate placenta that has a chorionic bridge—normal placental tissue connecting the two lobes—is not likely, particularly if not specifically sought. However, if placental lobes are separated by intervening membranes and the cord inserts directly into these membranes, such that the cord insertion is velamentous , then sonographic detection may help to avoid cord avulsion at delivery. A placenta containing three or more equal-sized lobes is rare and termed multilobate.

A succenturiate lobe is an accessory placental lobe that develops away from the main placental disk. The umbilical cord inserts into the main body of the placenta, and prominent vessels may be visible coursing along the intervening membranes ( Figs. 19-6 and 19-7 ). If an accessory placental lobe is suspected, color Doppler sonography may be particularly helpful in identifying the location and path of these vessels. Occasionally, the vessels that connect the main body of the placenta to a succenturiate lobe overlie the cervix, a form of vasa previa. Clinically, an accessory lobe may be retained in the uterus after delivery and cause postpartum uterine atony and hemorrhage. Succenturiate lobe is encountered more frequently with in vitro fertilization and twin gestations.

This succenturiate lobe has vessels that travel within intervening membranes and connect it to the main placental disk.

(Courtesy of Dr. Jaya George, University of Texas Southwestern Medical Center, Dallas, Texas.)

Succenturiate lobe. A, In this 20-week gestation, the umbilical cord inserts at the main body of the placenta, which is implanted anteriorly. B, Color Doppler sonography shows vessels connecting the main portion of the placenta to a posterior succenturiate lobe. C, Spectral Doppler interrogation of the arterial vessels connecting the succenturiate lobe to the placenta demonstrates a normal fetal heart rate (HR), 147 beats per minute (bpm).

There are other rare placental shapes in which varying portions of the fetal membranes are covered by functioning villi. With placenta membranacea, all or nearly all of the membranes are covered with villi. This form of abnormal placentation may lead to serious hemorrhage, preterm delivery, and hysterectomy because of associated placenta previa or accreta. With ring-shaped placenta, the placenta is annular, and a partial or complete ring of placental tissue is present. With placenta fenestrata , the central portion of a placenta disk is missing. These variants are not typically visible sonographically.

Abnormal Placental Thickness

Placentomegaly , an abnormally thickened placenta, is diagnosed if the placental thickness exceeds 4 cm in the second trimester or 6 cm in the third trimester ( Fig. 19-8 ). There are numerous causes of placentomegaly, including maternal diabetes, severe maternal anemia, severe fetal growth restriction, aneuploidy, and congenital infections. The latter include syphilis, parvovirus infection, cytomegalovirus infection, toxoplasmosis, herpesvirus infection (rarely), and if at risk, rubella or schistosomiasis. Placentomegaly is a component of hydrops fetalis, and any cause that can result in immune or nonimmune hydrops may present with placentomegaly as a component. In some cases, placentomegaly may result from collections of blood or fibrin within the placenta. Examples include massive perivillous fibrin deposition, intervillous or subchorionic thromboses, and large retroplacental hematomas, which are described later. Neoplasia is a rare cause. Benign vascular lesions include chorioangioma and chorangiosis, also described in subsequent sections.

Placentomegaly. A, This anterior placenta measured 7.1 cm in thickness at 28 weeks’ gestation. The fetus was hydropic in the setting of trisomy 21. B, The placental thickness measures 4.6 cm at 20 weeks’ gestation. Oligohydramnios was also present, and the maternal serum alpha-fetoprotein level exceeded 18 multiples of the median. Pathologic examination following delivery confirmed a large intervillous thrombus.

Gestational trophoblastic disease creates a thick cystic-appearing placenta. Cystic vesicles are also seen with placental mesenchymal dysplasia. Vesicles in this rare condition correspond to dilated stem villi that result from stromal expansion.

A placenta is not generally considered too thin, although focal attenuation may occur if a hematoma develops and subsequently resolves. A pregnancy with severe polyhydramnios may appear to have a thin placenta as a function of compression by fluid. A small placenta associated with a growth-restricted fetus may also appear thinner.

Placenta Previa

Derived from the Latin praevia, for going before , placenta previa indicates that the placenta is the presenting part rather than the fetus. Sonographic identification of placenta previa is essential because affected pregnancies are at risk for vaginal bleeding, which is often initially painless—a herald bleed —but requires urgent evaluation as hemorrhage may ensue. Women with prior cesarean delivery and placenta previa are at risk for placental invasion, and the risk increases with the number of prior cesarean deliveries.

Terminology

As sonography has replaced physical examination of the cervix in cases of suspected previa, terminology has changed. Largely based on findings from the clinical “double set-up” examination in preparation for delivery of suspected placenta previa, three general categories were formerly used:

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  Complete placenta previa referred to a placenta that completely covered the cervix, or more specifically, the internal cervical os.

  
  
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  Partial placenta previa indicated that the placenta partially covered the cervical os. Clinically, this could occur and be detected only when cervical dilatation was present, because one had to visualize the placental edge crossing over the dilated cervix.

  
  
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  Marginal placenta previa indicated that the placental edge just reached the margin of the internal cervical os, generally noted via gentle palpation during double set-up examination.

Sonographically, it is not usually possible to differentiate between the categories of partial and marginal placenta previa, because if the cervix is closed the internal cervical os appears as a “point” ( Fig. 19-10 ). All women in whom the placenta covers the cervix or reaches the cervical os will require cesarean delivery, such that a distinction between complete, partial, and marginal previa as defined earlier does not change pregnancy management.

Placenta previa. A, In this transvaginal image at 32 weeks’ gestation, the anterior placenta previa completely covers the internal cervical os. B, In this image from a different patient, also at 32 weeks’ gestation, the inferior edge of the posterior placenta just reaches the level of the internal cervical os. Whether the placenta partially covers the closed os or just reaches its margin is not a distinction that is technically feasible or clinically helpful. Short arrows depict the endocervical canal in both images.

Recently, the classification of placenta previa was revised by a workshop that included the National Institute of Child Health and Human Development (NICHD), Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound. Using the new classification, the diagnosis of placenta previa is made when the placenta covers or just reaches the internal cervical os, eliminating the terms complete, partial, and marginal. If the inferior placental edge encroaches upon the cervix, such that it is within 2 cm of the internal os but does not overlie it, the diagnosis is low-lying placenta . If the inferior placental edge is 2 cm or more from the internal cervical os, the placental location is considered normal.

Low-Lying Placenta

Placentation is termed low-lying if the inferior placental edge encroaches upon the cervix, such that it does not overlie but is within 2 cm of the internal os ( Fig. 19-11 ). The prevalence of low-lying placenta is approximately 3/1000 pregnancies at approximately 36 weeks’ gestation, thus similar to placenta previa. As with placenta previa, a low-lying placenta identified in the early second trimester often resolves prior to delivery. In one series of 1240 pregnancies with low-lying placenta diagnosed between 16 and 24 weeks’ gestation, 90% had resolved by 32 weeks and 98% had resolved by delivery.

Low-lying placenta. In this transvaginal image at 34 weeks’ gestation, the measurement from the inferior edge of the posterior placenta to the internal os is approximately 8 mm.

Velamentous Cord Insertion

With this abnormality, the umbilical cord does not insert directly into the placenta but rather into the membranes usually a short distance away from the placental edge ( Fig. 19-13 ). Sonographically, the umbilical arteries and vein may be visible traversing the membranes along the wall of the uterus, unprotected by Wharton jelly, before inserting at the margin of the placenta ( Fig. 19-14 ). Color Doppler sonography assists with visualization. Theories regarding the development of velamentous cord insertion include trophotropism, described earlier, or an error in initial blastocyst attachment that orients the fetus a distance away from the decidua basalis rather than directly opposite it.

Velamentous cord insertion. In this gross placental specimen, several large vessels are supported only by intervening membranes as they extend from the umbilical cord to the placental disk.

(Courtesy of Dr. David Nelson, University of Texas Southwestern Medical Center, Dallas, Texas.)

Velamentous cord insertion. A, In this transvaginal color Doppler ultrasound image at 22 weeks’ gestation, the umbilical cord is seen inserting into the membranes along the anterior wall of the lower uterine segment, a short distance away from the anterior placenta. B, The umbilical vessels run along the anterior uterine wall. C, The umbilical vessels insert into the inferior margin of the anterior placenta.

In population-based series, velamentous insertion occurs in approximately 0.5% to 1.5% of singleton births and 6% of twins. It is more prevalent in monochorionic twins, particularly in the smaller twin of a monochorionic pair, and it is associated with birth weight discordance. In a population-based review of more than 600,000 singleton births, velamentous insertion was associated with an approximately twofold increased risk for placental abruption, SGA neonate, and perinatal death. It has also been reported in 7% of fetal deaths prior to 24 weeks’ gestation. The risk for placenta previa is increased, and this is the most common precursor of vasa previa, discussed next. Care should be taken at delivery to avoid avulsing the umbilical cord when delivering the placenta, as velamentous insertion is associated with a greater than 10-fold increase in need for manual placental extraction.

Vasa Previa

In approximately 1/2500 deliveries, fetal vessels course within the membranes that overlie the cervix, a potentially devastating complication termed vasa previa ( Fig. 19-15 ). Vasa previa can develop in two settings: either with a velamentous cord insertion—connecting the umbilical cord to the placenta, or with a succenturiate or bilobed placenta—connecting the two placental lobes. In one review, each of these causes accounted for approximately 50% of prenatally diagnosed vasa previa cases. Multiple gestations are a recognized risk factor, and twins comprise 12% to 25% of those with vasa previa in published series. Pregnancies resulting from assisted reproductive technologies appear to be at particular risk.

Vasa previa. The placenta is posterior and low-lying, and the umbilical cord insertion is velamentous, directly into the membranes just anterior to the cervix, such that unprotected fetal vessels course directly over the internal os.

There are no standardized criteria for how close the fetal vessels must be to the internal cervical os to constitute vasa previa, and a threshold of “within 2 cm of the os” has been proposed. In one series, all emergent deliveries with vasa previa had a fetal vessel within 2 cm of the internal cervical os.

Vasa previa is potentially catastrophic if unrecognized, because the fetal vessels are prone to compression during labor and because the fetus can quickly exsanguinate when the membranes rupture. With prenatal detection, reported survival rate exceeds 95%. In the absence of prenatal diagnosis, however, the perinatal mortality rate is as high as 50%.

If the diagnosis is suggested sonographically in the second trimester, up to 25% of cases may resolve prior to delivery. If vasa previa persists, scheduled cesarean delivery is recommended prior to the onset of labor. Some have advocated that this occur prior to 36 weeks’ gestation.

Vasa previa should be considered whenever one encounters a low-lying placenta or a resolved placenta previa, as well as in the setting of velamentous cord insertion, succenturiate lobe, or bilobate (bilobed) placenta. Careful sonographic assessment should include color and spectral duplex Doppler evaluation to identify fetal vessels overlying the cervix, which confirms the diagnosis ( Fig. 19-16 ). Routine evaluation of the placental cord insertion site may help to avoid missing this rare but devastating entity. However, it is recognized that even with transvaginal imaging and color Doppler sonography, not all cases of vasa previa will be detected antepartum.

Vasa previa. A, A tubular structure ( arrow ) is shown overlying the cervix on this transabdominal sagittal sonographic image. B, With color and spectral duplex Doppler interrogation, this structure is shown to be a fetal vessel, confirming vasa previa. In many instances, the unprotected fetal vessel crossing the cervix is not appreciated on gray-scale images and is only detected with careful Doppler evaluation.

Epidemiology and Risk Factors

The prevalence of placental invasion has increased in recent years in parallel with the rise in cesarean delivery rates. Series from the past decade suggest that the prevalence has increased to 1/500 to 1/900 pregnancies.

An overwhelming majority of cases occur in the presence of two factors: placenta previa and prior cesarean delivery. The risk increases sharply with each subsequent cesarean delivery. In the setting of placenta previa, the risk for placental invasion is approximately 3% at the first cesarean delivery, 11% by the second, 40% by the third, and 60% to 70% for subsequent cesarean deliveries. However, in the absence of previa, the risk for placenta accreta is only 0.03% at the initial cesarean and remains less than 1% up to the fifth cesarean. Regardless of whether the placenta previa is anterior or posterior, placental invasion also may develop if the placenta overlies uterine scarring, resulting from prior uterine leiomyoma resection, endometrial ablation, or Asherman syndrome.

Sonographic Findings: Second and Third Trimesters

Sonographic criteria for detection of morbidly adherent placenta or placental invasion include the following:

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  Multiple placental lacunae . These irregularly shaped hypoechoic vascular spaces may give the placenta a “Swiss cheese” appearance. They contain turbulent flow shown with color Doppler imaging ( Fig. 19-18 ).

  
  

  
  

  
  

  

  
  

  
  

  Placental lacunae. A and B, Transvaginal images in the sagittal plane depict placenta previa with multiple placental lacunae. The images are from two different pregnancies, each at 28 weeks of gestation. C and D, Color Doppler ultrasound imaging (from the patients shown in A and B, respectively) demonstrates turbulent flow within these irregularly shaped vascular spaces.

  
  
  
  
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  Thinning of the retroplacental myometrium, with the smallest myometrial thickness measuring less than 1 mm in the sagittal plane ( Fig. 19-19 ).

  
  

  
  

  
  

  

  
  

  
  

  Thinning of retroplacental myometrium. This transvaginal image at 32 weeks of gestation shows the placenta (previa) filling the lower uterine segment, and there is marked attenuation of the retroplacental myometrium ( arrows ). The smallest myometrial thickness is below 1 mm.

  
  
  
  
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  Irregularity or disruption of the echogenic interface between the maternal bladder and uterine serosa— the bladder-serosal interface —on gray-scale imaging.

  
  
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  Increased vascularity of the bladder-serosal interface, with bridging vessels that course from the placenta to the region of the bladder-serosal interface (and sometimes farther into the bladder) shown with color Doppler imaging ( Fig. 19-20 ).

  
  

  
  

  
  

  

  
  

  
  

  Bridging vessels. This transvaginal sonographic image at 32 weeks’ gestation was obtained in the transverse plane and depicts prominent bridging vessels, shown with color Doppler, that course from the placenta to the bladder-serosal interface. They correspond to the irregularities along the bladder-serosal interface visible with gray-scale imaging.

  
  
  
  
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  Loss of the normal linear hypoechoic area between the placenta and the wall of the uterus, called the retroplacental clear space (see Fig. 19-19 ).

Several observations and investigations regarding the sonographic findings of accreta have been reported. Loss of the retroplacental clear space is a common cause of false positive reports and is therefore best used in conjunction with other findings. As single factors, lacunae and interruption of the bladder-serosal interface, particularly when color Doppler sonography confirms hypervascularity, appear to have higher positive-predictive values for invasion than other characteristics. Placental lacunae can vary in degree. Finberg and Williams (1992) developed a grading scale for lacunae: grade 1+ for one to three lacunae, usually small; grade 2+ for four to six lacunae, typically larger; and grade 3+ for diffuse lacunae throughout the placenta. More extensive lacunae throughout the placenta have been associated not just with placental invasion but with placenta percreta. Another finding that may be associated with placenta percreta is hypervascularity of the entire bladder-serosal interface shown with three-dimensional (3D) power Doppler sonography.

In a recent meta-analysis of 23 series that included 3700 pregnancies at risk for placenta accreta, the pooled sensitivity of sonography to identify invasion was 91% and the specificity was 97%. However, the reported sensitivity for the individual studies ranged from 60% to 100%, and the specificity ranged from 50% to 100%. Thus, although the overall utility of sonography for suspected placental invasion is good, there is considerable variability in published reports. More recently, investigators have tried to combine the various sonographic findings with clinical parameters, such as number of prior cesarean deliveries, to more accurately predict placental invasion.

Magnetic Resonance Imaging for Placenta Accreta

Magnetic resonance imaging is not recommended for routine evaluation of suspected placental invasion, as its sensitivity and specificity are similar to sonography, and it has not been shown to affect mode of delivery or need for cesarean hysterectomy. It may be useful, however, when ultrasound findings are inconclusive or additional information is needed ( Fig. 19-21 ). For example, in a patient with prior uterine surgery, instrumentation, or injury involving the posterior myometrium, sonographic visualization may be limited by fetal position or other factors. In such cases in which concern for possible placental invasion is raised, targeted magnetic resonance imaging might be of benefit.

Placenta percreta. Placental invasion with involvement of the maternal bladder wall, indicating placenta percreta, is shown on these sagittal ( A ) and coronal ( B ) single shot fast spin echo magnetic resonance images, obtained for purposes of surgical planning. The heterogeneity of the placenta is due to lacunae. Irregularity of the placental interface is noted with extension into the bladder wall ( arrow ). Intraluminal material within the bladder represents blood clot.

(Courtesy of Dr. Liina Poder, University of Texas Southwestern Medical Center, Dallas, Texas.)

First-Trimester Placenta Accreta and Cesarean Scar Pregnancy

The rising prevalence of placental invasion, coupled with increasing use of first-trimester sonography, has led to recognition of placenta accreta in the first trimester. As previously stated, most pregnancies with placental invasion have placenta previa overlying a prior hysterotomy scar. Similarly, it is possible for implantation to occur in the region of the scar tissue at the anterior lower uterine segment—termed cesarean scar pregnancy —which may result in early uterine rupture. Multiple investigators have found that cesarean scar pregnancy and placenta accreta share the same pathogenesis and that cesarean scar pregnancy, rather than being a separate entity, may be a precursor to second- or third-trimester placenta accreta.

First-trimester sonographic findings of cesarean scar pregnancy include the following:

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  Gestational sac implantation that is low and anterior in the uterus, in the region of the hysterotomy scar. Before 8 weeks, the sac may fill the “niche” of the scar in a triangular configuration, and by 8 weeks, the gestational sac shape may appear more rounded.

  
  
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  Attenuation of the myometrial layer between the gestational sac and bladder, such that the measured distance from the anterior trophoblastic border to the uterine serosa is 1 to 3 mm ( Fig. 19-22 ). This measurement is comparable to the smallest myometrial thickness reported on sonographic imaging done later in pregnancy.

  
  

  
  

  
  

  

  
  

  
  

  Cesarean scar pregnancy with attenuation of retroplacental myometrium. A, In this transvaginal image of a 6-week gestation, the gestational sac is implanted low and anteriorly in the uterus (in the region of the hysterotomy scar) and appears to bulge outwardly; there is no visible myometrial layer ( asterisks ) between the gestational sac and maternal bladder. B, This transvaginal sonogram at 11 weeks shows thinning of the myometrial layer ( asterisks ) between the gestational sac and the bladder.

  
  
  
  
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  Visible irregularities along the placental-myometrial interface.

  
  
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  Echolucent areas within the placenta, a precursor to the lacunae visible later in gestation ( Fig. 19-23 ).

  
  

  
  

  
  

  

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