Placental findings in singleton stillbirths: a case-control study from a tertiary-care center in India

Introduction

The placenta plays a critical role in fetal development and in in utero health of fetus. Its detailed examination in any adverse perinatal event is of paramount importance and is justified by the facts that it lies at the maternal and fetal interface; serves as an essential organ to provide nutrition and blood supply to fetus; is affected by varieties of pathogenic or vascular insult affecting mother or fetus and any placental pathology can jeopardize fate of fetus in utero or of infant in later life. A systematic review of studies of placental pathology and stillbirth reported that placental pathologies can be responsible for fetal death in up to 65% of cases [1].

In previous studies, gross and microscopic pathologies of placenta and umbilical cord have been linked to stillbirths [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Smaller/larger placenta, long/short cord, thick/thin cord, circumvallate or circummarginate insertion of membranes, fetal vascular malperfusions (FVM), maternal vascular malperfusions (MVM) as well as various fetal and maternal inflammatory responses in placental disc have been linked to stillbirths. These findings have also been linked to individual clinical entity/disease in mothers (diabetes, hypertensive disorder of pregnancy, thrombophilia, premature rupture of membranes) or in fetus (fetal anomalies and fetal growth restriction) separately or in combination. However, majority of these studies provide insight into single/few placental findings, lack study of clinical factors (in each category of placental abnormality group) in same cohort of patients, have used non-standardized protocol for placental examination or lack control group to compare findings. Moreover, only few studies have compared findings in term and preterm stillbirth/livebirth placentas [2, 12, 18]. Furthermore, placentas from apparently healthy infants also have reports of abnormal histopathological lesions (particularly chorioamnionitis) [19]. Hence, we conceptualized this case-control study about placental findings in singleton stillbirths with the following objectives: (1) to compare placental and umbilical cord pathologies in singleton stillbirth placentas and singleton livebirth placentas; (2) gestation-specific analysis of pathologies in preterm stillbirth placentas vs. preterm livebirth placentas; and in term stillbirth placentas vs. term livebirth placentas; (3) prevalence of various maternal and fetal clinical factors in mothers with findings in their placentas. We believe that since there is dearth of literature on this topic from the country which shares the maximum burden of stillbirth in the world [20], therefore this study will help in identifying high risk mothers for stillbirths in current or subsequent pregnancies and will foster data in the field of stillbirth.

Materials and methods

Results

Various placental findings and maternal/fetal clinical factors

Placental gross findings

The absolute number of women in both the groups (cases and controls) with concerned clinical factor was compared in each category of placental finding (Figure 1A–F). The most prevalent maternal factor among cases in gross placental category was hypertensive disorders (n=15) followed by abruption (n=12) and growth restricted fetus were more in number (n=45) than anomalous fetus (n=34) (Figure 1A). Out of total 96 stillbirth hypoplastic placentas, 40 were associated with SGA fetus while out of 15 livebirth hypoplastic placentas, 10 had SGA baby.

Figure 1:

Prevalence of various maternal and clinical factors in different category of placental findings in stillbirth group and livebirth group.

(A) Placental gross category. (B) Umbilical cord gross category. (C) Membranes gross category. (D) Maternal vascular malperfusion category. (E) Fetal vascular malperfusion category. (F) Maternal/fetal inflammatory response category.

Discussion

The placenta plays a very crucial role in the development of fetus. Hence, placental evaluation holds paramount importance, particularly in adverse perinatal outcomes. We evaluated and compared placental and umbilical cord pathologies in singleton stillbirth placentas with singleton livebirth placentas, and did a gestation-specific analysis of placental pathologies in stillbirths and livebirths. We found that placental findings in all the six categories were more common in stillbirths than livebirths. Also, the placental findings associated with stillborn placentas with highest odds included placental hypoplasia, necrotizing chorioamnionitis and avascular villi.

While comparing placental weight, we found that smaller placentas had significant association with stillbirths especially with term stillbirths. Similar to our finding Hutcheon et al. found association between lower placental weight and risk of stillbirth [4]. Furthermore, SGA and hypertensive disorders were the most prevalent finding in this category, implying that hypertensive disorders may affect placental development and its weight which in turn can affect fetal growth [5]. Various reasons have been hypothesized for this association. McDonald et al. hypothesized that low level of pro-angiogenic factor leads to smaller placenta, which in turn is responsible for inadequate nutrient support to fetus and adverse perinatal outcome [6]. The role of high level of anti-angiogenic proteins and low proangiogenic proteins have been also seen in SGA fetus and pre-eclampsia in literature [28]. Placentas with succenturiate lobe were significantly associated with stillbirth after 37 weeks of gestation in our study as has been previously reported also [7]. Machin et al. and Tantbirojn et al. concluded that abnormal coiling index is associated with risk of stillbirth [8, 9]. We also found that umbilical cord coiling abnormalities significantly were more prevalent in the both the preterm and term stillbirth group. We also found that the higher numbers of babies were SGA in cord gross category. Sharma et al. conducted a cross-sectional study in 408 antenatal women and found that abnormal coiling index calculated at antenatal scan at 18–20 weeks of gestation was associated with preterm birth and low birth weight [29]. The clinical implication of above findings is that placental and cord (coiling index) assessment during antenatal scans is very important and can detect mothers at high risk of SGA babies, hypertensive disorders and fetal demise.

Meconium staining of placenta may indicate in utero fetal hypoxia. Fetal anomalies and prolonged labor both have been established as a risk factor for fetal hypoxia and hence may lead to passage of meconium that in turn may lead to fetal death [10]. Therefore, anytime during pregnancy or labor, presence of meconium in liquor or prolonged labor should alarm the obstetrician and such mothers should be kept under intense monitoring to save fetus. We could not find any significance of circumvallate and circummarginate insertion with studied outcome. However, contrary to our findings, Suzuki S found that incidence of intrauterine fetal death and other complications (premature delivery, oligohydramnios, etc.) was significantly higher in patients with circumvallate placentas than that in controls [11].

The normal and adequate blood flow to maternal vessels is essential to provide oxygen and nutrients to fetus and therefore, any maternal vascular pathology can affect growth of fetus and further outcome of pregnancy. Uterine vasculature aberrations from the normal remodeling process are central to the pathologic process in development of MVM. This leads to either abnormal high velocity and turbulent flow through un-remodeled spiral arterioles or reduced blood flow secondary to reduced vascular capacity. Among the livebirths, MVMs were the most frequent microscopic pathologies observed in our study (23%). Similarly, Pathak et al. found that chronic placental underperfusion was found in a substantial number of normal singleton pregnancies (7%) [19]. Man et al. found that MVMs were the most frequent placental abnormalities associated with stillbirth [18]. Similarly, MVMs were found in 56% of stillbirths in our study. Pre-eclampsia has strong association with MVM histopathological findings. Autoimmune disorders such as antiphospholipid antibody syndrome and systemic lupus erythematosus have also been found associated with MVM [30]. Similar to above findings; in our study, 55.55% of total hypertensive mothers and 63.63 % of total mothers with autoimmune disorders in stillbirth group had MVM in their placenta. The recurrent rate of such lesions can be upto 25% in next pregnancy [5]. Similar to our findings, MVM have been associated with fetal death in previous studies also [12], [13], [14, 30]. In literature, the association between fetal anomalies (especially congenital heart defect) and risk of pre-eclampsia in mothers have been described and placental vascular malperfusions have been found as a common finding in both type of placentas implying the role of common etiologic factors in fetal anomalies and hypertensive disorders that in turn may lead to stillbirth [31]. If MVM are diagnosed in a case of stillborn placentas, further complete maternal check-up to look for hidden/latent hypertensive disorders or autoimmune disorders are warranted and more intensive monitoring in next pregnancy is of paramount importance. Various maternal conditions (thrombophilic disorders [protein C deficiency, protein S deficiency, factor V leiden mutation], preeclampsia and anti-phospholipid antibody syndrome) and various fetal conditions (fetal growth restriction, fetal anomalies, fetal infection and fetal death) have been associated with changes of FVM [15]. We also found that FVM were significantly more common in stillbirth placentas and 37.5% women with thrombophilia had FVM changes in placenta. More number of stillbirths had congenital anomalies in this category implying that screening for hypertensive disorders and thrombophilia is needed for such mothers who had placenta with FVM. However, we agree that it is difficult to differentiate that whether these FVM changes are before or after demise of fetus in utero and this can be a major limitation in interpretating data in this specific category.

Placenta is formed by three major units: placental disc, chorioamniotic membranes and umbilical cord. Acute inflammatory lesions can involve either one or two or all of these three units. Acute chorioamnionitis is defined as histologic evidence of neutrophilic infiltration in these membranes but this response may be due to infective or to non-infective etiology. In our study, all cases of fetal inflammatory responses were associated with acute chorioamnionitis. Inflammatory disorders especially chorioamnionitis have been associated with stillbirths [15, 32]. Lahra et al. concluded that absence of fetal inflammatory response is associated with antepartum unexplained stillbirths [16]. We also found that more livebirth placentas had fetal inflammatory response than stillbirth placentas; however, this difference was not significantly associated in our study. Recent literature suggests that non-infective inflammation of chorioamniotic membranes can also be caused by endogenous mediators known as “damage-associated molecular patterns (DAMPs)” or alarmins which should be explored more in future [17]. Similar to our finding, Williams et al. studied total 2,579 infants and concluded that histologic chorioamnionitis was associated with markers of fetal growth restriction in both preterm and term infants [33].

The strength of our study was that it was conducted in a tertiary-care referral institute with patients coming from various parts of the country and therefore has a heterogenous patient population. Secondly, studying the placental lesions associated with stillbirths has more significance, when it is done in those parts of the world which contribute the largest proportion of the caseload, as was done in this study. Also, we included livebirths as controls further increasing the strength of the study. This study is more informative as patients’ clinical findings were also studied with their placental findings. However, this study had some limitations that should be considered while interpreting the results. First, the pathologists were not blinded to the outcome – stillbirth or livebirth. Secondly, on gestation-specific analysis, the sample size in the subgroups was relatively small. Therefore, more such kind of studies at larger scale are needed in evaluating the role and association of placental findings in stillbirths. However, as very few of this kind of studies are available from the developing world, our results will foster future research in this direction and will provide baseline data to the Obstetricians and the policy-makers alike. Also, there may be significant overlap between macroscopic findings and microscopic findings of placenta in our study, however, our categorization of the placental findings in the two broad groups (macroscopic and microscopic) was not mutually exclusive. The six categories of placental lesions were chosen to comprehensively characterize the placental findings including both macroscopic and microscopic findings. Moreover, some selection bias might be present in the study as majority of participants who had a stillbirth were of middle-class and were literate, contrary to the findings in literature. However, those contradictory findings might also reflect the population in the study area.

Our study identified that both gross (gross lesions of placenta, umbilical cord and membranes) and placental microscopic lesions (maternal vascular malperfusions, fetal vascular malperfusions and inflammatory response) were associated with stillbirths. Hypertensive disorders, fetal anomalies and growth restriction were the most prevalent clinical factors in stillbirth fetus with placental pathology. Many of such placental lesions (weight, succenturiate lobe, coiling index) can be diagnosed antenatally and these pregnancies should be monitored carefully to prevent intrauterine fetal death. Many placental lesions when identified postnatally (MVM, FVM) warrant evaluation of mothers to rule out hypertensive disorders and/or autoimmune disorders. Furthermore, identifying placental pathology in stillbirth placenta may allay anxiety of mother/parents to some extent where no direct fetal/maternal cause can be identified. To conclude, placental examination in a case of stillbirth can detect/diagnose many maternal/fetal conditions and thereby can help in preventing future stillbirths in high-risk mothers.