(See the article by lwasenko et al, on pages 1526–33.)
Among the most devastating obstetric complications for parents is stillbirth, defined as fetal death in the uterus after 20 weeks of gestation. These tragic deaths, of which half are due to maternal or fetal infections, occur in 1 in 160 infants . Approximately 50% of stillbirths are associated with fetal growth restriction . Human cytomegalovirus (CMV), the most frequent viral cause of congenital infection and birth defects, has now been implicated as a cause of stillbirth. In this issue of the Journal, Iwasenko and colleagues analyzed autopsy specimens from 130 stillbirths for bacterial and viral infections. Evidence of CMV infection—viral DNA and foci of replication—was detected in fetal tissues and placentas from 15% of stillborn infants, greatly outnumbering other pathogens. Histologic analysis revealed that fetal thrombotic vasculopathy in the placenta was significantly associated with congenital CMV infection, compelling evidence that infection and resulting vascular fibrosis contribute to stillbirth.
About half of the women in the United States are seronegative for CMV and in danger of a first-time infection during pregnancy . Mothers are seldom screened for antiviral antibodies, and most are unaware of the risk of infection from toddlers in daycare . Pregnant women with new infections have a 32% risk of transmitting virus, and 15% of infected fetuses will have lifelong disabilities. Every year about 8000 infants are born with congenital CMV disease , exceeding the rates of better-known disabilities such as Down syndrome, fetal alcohol syndrome, or neural tube defects . Birth defects from symptomatic CMV infection include mental retardation, cerebral palsy, and progressive hearing loss [7, 8, 9]. An additional 40,000 infants, asymptomatic at birth, will have varying degrees of hearing deficits. Nonneurologic symptoms of congenital infection, resolved after delivery , include intrauterine growth restriction (IUGR), hepatosplenomegaly, thrombocytopenia, and extramedullary hematopoiesis. These temporary symptoms could result from reduced oxygen and nutrient transport from persistent injury and fibrosis in infected placentas [10, 11, 12].
Despite the magnitude of the problem, little attention has been focused on the etiology of stillbirth . Approximately 27,000 stillbirths were reported to the National Center of Health Statistics in the United States in 2001, and about the same number of infant deaths was reported. Stillbirth rates declined only 17% from 1985 to 2001, whereas infant mortality declined twice as much in the same period. A significant racial disparity exists: There are 12.1 stillbirths for every 1000 live births among black mothers with singleton births, but half as many among white mothers . One national health objective for 2010 was to reduce stillbirths to 4.1 for every 1000 live births and to reduce the number of fetal deaths for all racial and ethnic groups. Without identifying, treating, and preventing infections associated with stillbirth, these ambitious goals cannot be accomplished.
Broad screening for etiologic agents that may have infected the mother and fetus is seldom conducted in cases of stillbirth. In low-income countries, ascending bacterial infections are the most frequent cause of stillbirth . In high-income countries, viral infections that cause congenital disease are a frequent cause [1, 13, 16]. Using molecular analytic techniques, Iwasenko and colleagues detected CMV DNA by polymerase chain reaction (PCR) and foci of viral infection by immunostaining formalin-fixed, paraffin-embedded fetal kidney, liver, and placentas from stillbirths. When all tissues were CMV DNA–positive, the placentas had histologic evidence of stromal hemorrhage of villi, extensive vascular sclerosed villi, and vascular proliferation. Stillborn infants had extramedullary hematopoiesis and petechial hemorrhages. Fetal hydrops and IUGR were also present, but these symptoms were not exclusively related to CMV infection. In contrast, fetal thrombotic vasculopathy was significantly more likely to occur in stillbirths associated with CMV infection than in those associated with other causes. It was recently reported that 16% of stillbirths had evidence of congenital CMV infection, based on detection of viral DNA in frozen placental biopsy specimens . Histological analysis of formalin-fixed, paraffin-embedded placentas showed that chronic villitis was more common in fetuses with CMV infection than in uninfected fetuses, even in the absence of fetal symptoms.
Whether CMV infection is found in autopsy tissues after stillbirth could depend on the techniques employed—PCR to detect viral DNA, immunostaining of viral proteins, or histologic analysis of biopsy specimens. The use of formalin-fixed tissues to detect viral DNA is less sensitive than the use of frozen specimens and requires high copy numbers for positive results. In the Iwasenko et al study, fewer placentas than fetal tissues were found to contain CMV DNA. Inasmuch as CMV infects the placenta before the fetus, it is likely that focal sites of viral replication were missed or had been cleared by macrophages and dendritic cells . Taking multiple biopsy specimens from the center of the placenta improves detection of CMV DNA and localization of sites of viral replication . Molecular analysis of frozen biopsy specimens from infected placentas in early gestation  and at term [18, 20] revealed CMV DNA in placentas from mothers with low- to moderate-avidity immunoglobulin G, suggesting recent infection. Immunostaining for CMV proteins confirmed replication in cytotrophoblasts expressing virion receptors at the uterine–placental interface [17, 21].
CMV infection triggers a constellation of molecular mechanisms that alter differentiation of the placenta  and could explain reduced functions associated with IUGR. Infected cytotrophoblasts and fetal endothelial cells impair invasion and migration [17, 23], downregulate matrix metalloproteinases, release viral factors with paracrine effects that impair functions of neighboring uninfected cells , and alter expression of cell–cell and cell–matrix adhesion molecules . Infected fetal endothelial cells induce expression of an epithelial integrin, activate transforming growth factor β and increase collagen deposition, a possible explanation for fibrosis in the villous core . Chronic villitis from congenital CMV infection and thrombosis in main stem and surface vessels reduce fetal blood flow, leading to villous infarction, fibrosis and avascular villi [12, 27]. Interestingly, early studies reported a 14% incidence of extensive avascular villi in placentas of stillborn fetuses, significantly higher than the overall occurrence of 4.5% . Avascular villi and terminal villi with fibrotic stroma indicate fetal deprivation and a stressful intrauterine environment . Like newborns with congenital CMV infection, stillborn infants in the Iwasenko et al study had extramedullary hematopoiesis, a symptom of hypoxia. Recent studies showed that congenitally infected placentas have significantly increased avascular villi and fibrosis in the villous core, reduced blood flow, and more fetal capillaries than uninfected placentas . Vascular endothelial growth factor and its receptor are upregulated, suggesting an underlying molecular mechanism that enables responsiveness to a hypoxic environment and compensatory remodeling during development.
Current estimates of congenital CMV infection are based on the diagnosis of primary maternal infection using serology, presence of fetal anomalies by ultrasound, and quantification of viral DNA in amniotic fluid by PCR . Such diagnostic procedures are not generally applied in cases of stillbirth. Studies from Australia (Iwasenko et al, this issue) and Greece  strongly implicate CMV infection and fetal thrombotic vasculopathy as contributing factors in stillbirth, but a lack of serologic analysis failed to link the problem to primary maternal infection. A comparable analysis of viral infection in autopsy specimens remains to be performed in the United States. The Stillbirth Collaborative Research Network was established by the National Institute of Child Health and Human Development to standardize postmortem and placental examination and thereby improve diagnosis of fetal or placental conditions that contribute to stillbirth . Continued investigation of viral infections in stillbirths should better define the extent to which CMV is a contributing factor. A recent report suggests that treatment of the mother with CMV hyperimmune globulin after primary infection could reduce vertical transmission and symptomatic disease . Application of novel antibody treatments and the introduction of a vaccine to prevent maternal CMV infection  could finally help us reach the goal of reducing congenital disease and the incidence of stillbirths.