The role of ascending amniotic infection in the genesis of premature separation of the normally implanted placenta

Placental abruption (PA) is premature separation of the normally implanted placenta (placental abruption). It is potentially dangerous obstetric complication both for mother and fetus. It increases the risk of massive hemorrhage, preterm labor, and antenatal fetal death. Perinatal mortality in pregnancies with placental abruption is almost 15 times higher than in patients with timely separation of the placenta from the uterine wall [1]. In twin pregnancy the risk of PA increases by 2 times versus singleton pregnancy [2].

 Over the past two decades, there is stability in the incidence of placental abruption in Russia – from 9.5 per 1000 births in 2002 to 9.2 per 1000 births in 2022. [3]. The global data do not differ from the data in the Russian Federation – placental abruption occurs in 0.5–1.2% of all pregnancies [4].

The concept of “the great obstetrical syndromes” was introduced by G.C. Di Renzo. It includes preterm birth, premature rupture of membranes, preeclampsia, fetal growth restriction, excessive fetal growth, and stillbirth [5]. Romero R. complemented the list of syndromes with characteristics that fully comply with premature rupture of membranes: 1) the multiple etiologies; 2) long preclinical period; 3) maternal-fetal adaptive responses; 4) fetal involvement in the pathological process; 5) the result of genomic maternal-fetal and environmental interactions [6]. Ananth C.V., Vintzileos A.M. proposed the term "ischemic placental disease" to describe adverse pregnancy outcomes such as preeclampsia, small for gestational age and premature separation of the normally implanted placenta” [7].

According to Amsterdam Placental Workshop Group Consensus Statement, placental lesions are classified into 3 groups: vascular damage, inflammatory lesions and other [8]. Among vascular damage, there are maternal and fetal vascular malperfusions.

According to this classification, PA is a condition associated with impaired maternal blood flow caused by abnormal gestational remodeling of spiral arteries and fibrinoid necrosis in smooth muscle cell layer, that with rises and falls in blood pressure leads to arterial wall rupture. Hemorrhage develops in the microvascular basal lamina. The clinical picture is different depending on coagulation properties and location of the lesion. When the marginal zones of placenta are affected, the clinical picture predominantly includes complaints of genital tract bleeding. When the vascular injury is located closer to the central region of placental disc, the volume of arterial bleeding increases and is characterized by retroplacental accumulation of blood, destruction of the part of placental area, hypertonicity of the uterus, that lead to a life-threatening condition for the fetus. When the pathological process is stopped, a small retroplacental hematoma can develop [4, 9]. 

Clear diagnosis of placental abruption includes blood clots in the placental maternal surface with breakdown of red blood cells and reactive leukocyte infiltration; necrobiosis and subsequent necrosis of the decidual plate; changes in the placental villi with the similar consecution – necrobiosis, hemorrhagic necrosis, and ischemic necrosis. Changes in the villi occur only after a certain period of time (at least 3–7 days after the onset of the pathological process). This can be explained by the fact that there is no vascularization in the area of ​​the retroplacental hematoma. The blood does not circulate between the villi, fibrin deposits are transformed into dense fibrinoid and strangulate the villi.  This results in obstruction of feto-placental circulation. When the marginal zones of placenta are affected, placental abruption is in the crescent-shaped area, the changes in the villi may not occur. The blood from the adjacent lateral areas reaches the affected zone and bathe the villi

Another underlying mechanism of placental abruption is the development of acute inflammation, accompanied by increased activity of trophoblast metalloproteinases, impaired intercellular relationship with the basal membrane and rupture of the vessels of the decidual membrane [10].  The high virulence of the microbial pathogens destructively influences to the walls of arteries and veins during the spead of infection. Microvascular basal lamina damage generally occurs in the marginal zones of placenta, and is accompanied by bleeding without pain. With the increased coagulation potential, the signs of prolonged placental abruption occur, that is formation of marginal organized hematoma and infarctions at the placental implantation site due to ischemia. In 2014, Kobayashi A. et al. reported that membranitis was histologically confirmed in all women with marginal prolonged placental abruption and oligohydramnios [11].

The role of infectious factors is not clearly defined in publications on PA. The conclusions made by the authors are based on the clinical diagnosis of PA, in one-third of the reported cases, placental hematoma or hemorrhage is histologically confirmed [10].

It can be assumed that discrepancy between the clinical diagnosis (presence of genital tract bleeding) and pathomorphological diagnosis of placental abruption (absence of histological signs of PA) is explained by: 1) the severity of the process and performance of a rapid cesarean delivery and 2) rupture of the vessels of the decidual plate outside the placental bed.

Oltean I. et al. (2022), Redline R.W. et al. (2023) indicated that chorioamnionitis is associated with placental abruption [12, 13]. It can be suggested that inflammatory changes in the placenta combined with PA, contribute to increasing preterm birth rate and worsen neonatal outcomes. Infection is an independent risk factor for preterm birth. Ascending amniotic fluid infection in the presence of localized uterine vasculopathy increases the likelihood of PA. However, there are only few studies on the given topic, and no definitive evidence of cause-and-effect relationship between placental infection and PA was found.

The aim of the study was to explore the influence of premature separation of the normally implanted placenta combined with ascending amniotic infection on obstetric outcomes and conditions in newborns.

Material and methods

This retrospective study was conducted at the Clinical Hospital MD GROUP (Moscow).

Medical documentation of 109 patients, who gave birth from January, 2006 to April 2025, was analyzed. Medical documentation included birth histories (2006–2020), medical records of pregnant, parturient and puerperant women, who received medical care in inpatient healthcare facilities (2021–2025), and histories of development of 115 newborns.  

Inclusion criteria in the study were gestational age ≥ 22 weeks, histological diagnosis of placental abruption – morphological detection of retroplacental or retromembranous blood clots with significant hemorrhage within the decidual tissue. Non-inclusion criteria were absence of pathological findings of retroplacental hemorrhage, placenta previa (low-lying placenta), and placenta increta into the uterine scar after cesarean section.

According to Amsterdam Placental Workshop Group classification placental changes associated with ascending amniotic infection are classified into the stages of maternal and fetal inflammatory response. Stage I of maternal inflammatory response is characterized by acute subchorionitis, stage 2 by acute chorioamnionitis, and stage 3 by necrotizing chorioamnionitis. Stage 1 of fetal inflammatory response includes chorionic vasculitis or umbilical phlebitis. Stage 2 refers to umbilical phlebitis and inflammation of a single artery or panvasculitis of the umbilical cord, and stage 3 is characterized by necrotizing funisitis.

Based on histologically confirmed placental infection, the patients were divided into 2 groups. Group 1 (the main group) (n=40) included women (n=40) with placental abruption combined with ascending amniotic fluid infection. Group II (the control group included women (n=69) with placental abruption without the signs of ascending amniotic fluid infection. Stages I and II of maternal and fetal inflammatory response were in patients in group I.

In group I, the patients gave birth to 44 babies (including 4 twin babies). In group II, the women gave birth to 71 babies (including 2 twin babies). Based on gestational age at birth, the newborns in the groups were divided into subgroups.  Subgroup Ia (PA and ascending amniotic fluid infection) consisted of 29/44 (62.5%) preterm infants born before 258 days of pregnancy. Subgroup Ib (PA and ascending infection of the amniotic fluid) included 15/44 (37,5%) term babies. Subgroup IIa (PA) included 29/71 (40,8%) preterm babies. Subgroup II b (PA) included 42/71 (59.2%) term babies.

Based on the presence of ascending amniotic fluid infection, the frequency of preterm birth and birth before 33 weeks of pregnancy, extremely early preterm birth, and birth asphyxia in PA were compared between the groups.

The patients were examined in accordance with the Orders of the Ministry of Health of Russia: Order No 572 of November 01, 2012 “On approval of the Procedure for the provision of medical care in the field of Obstetrics and Gynecology (except for the use of assisted reproductive technologies)” [14]; Order No. 1130 of October 20, 2020 “On approval of the Procedure for providing medical care in the field of obstetrics and gynecology” [15]. 

Based on medical documentation, the following parameters were assessed: somatic, gynecological and obstetric history, timing and methods of delivery, conditions in newborns, their birth weight, the need for hospitalization in the Intensive Care Unit (ICU), duration of inpatient treatment of newborns, and the laboratory parameters.

Microbiological tests of vaginal discharge were performed upon hospital admission for delivery. Neonatal asphyxia was diagnosed based on the Apgar scores and the acid-base balance in umbilical cord blood.

Placental histological examination was performed using light microscopy of hematoxylin-eosin stained histological sections, fragments of fetal membranes, umbilical cords, and placental tissue obtained from various sites (at equal distances from each other) in enough volumes to obtain the mean score in assessment of the functional state along the entire length of the placenta), as well as the fragments of   pathologically altered regions. The diagnosis of ascending amniotic fluid infection was made based on the presence of pathological changes in the placental structure –  inflammatory infiltrate in fetal membranes, the subchorionic placental region, and umbilical cord vessels.

Echography was performed using ultrasound devices GE Voluson S8, Voluson E6, Voluson E10 with convex sensor. The retrospective data analysis was approved by the Ethical Committee of the N.I. Pirogov Russian National Research Medical University, Ministry of Health of Russia (Pirogov University).

Statistical analysis

Statistical data processing was performed using IBM SPSS Statistics V22.0 (IBM Microsoft, USA). The null hypothesis was tested using the Shapiro–Wilk normality test. The quantitative data with normal distribution are presented as arithmetic mean (M) and standard deviation (SD). The parameters with distribution different from normal are presented as Ме (Q1; Q3), where Ме – is median, Q1 and Q3 – the upper and lower quartiles. Nominal data are described as absolute and relative frequencies. The quantitative parameters between the groups were compared using Student’s t-test for normal distribution, and the Mann–Whitney U-test for non-normal distribution. Pearson’s chi-square test and Fisher’s exact test were used to compare the nominal attributes. The effect size (strength of associations in the data) was estimated using Cramer's V. Effect size was measured using the odds ratio (OR) with 95% confidence interval (95% CI).  The Kruskal–Wallis rank-based test was used to compare distribution of quantitative variables between the groups. Pearson’s chi-square test was used to compare the binary variables between more than two groups. When significant differences were identified, post hoc analysis was performed with the Bonferroni correction for multiple comparisons. The differences were considered statistically significant at р<0.05.

Results

From January 2006 to April 2025, the clinical diagnosis of placental abruption was histologically confirmed in 109 puerperant women. PA combined with ascending amniotic fluid infection was diagnosed in 40 (36.7%) women in group I, and PA without the signs of ascending amniotic fluid infection was diua in 69 (63.3%) women in group II.

In group I, maternal inflammatory response was defined as subchorionitis in 29/40 (72.5%), and as chorioamnionitis in 11/40 (27.5%) patients. Fetal inflammatory response was manifested by chorionic vasculitis with or without umbilical phlebitis in 6/44 (13.6%) infants, and umbilical phlebitis and arteritis or panvasculitis in 5/44 (11.4%) infants (Table 1).  Comparative analysis of the clinical and laboratory data of patients in groups I and II was performed to identify the role of ascending amniotic fluid infection in the genesis of placental abruption.

The age of the puerperant women in group I was 33.5 (5.9) years, and 32.9 (5.2) years in group II (р=0.57, Student’s t-test).

There were no differences found between the groups in the incidence of somatic pathologies, gynecological diseases, including gynecological infections, and intrauterine interventions in history. The incidence of infertility, uterine fibroids, and inflammatory diseases in the groups did not exceed the rates in the general population.

There was no difference between the groups in the frequency of pregnancies achieved after using assisted reproductive technologies, parity, the number of multiple pregnancies, as well as pregnancy complications, including preeclampsia.   In group I, pathogenic flora was most often identified by microbiological tests of vaginal secretion specimens. However, weak association with placental abruption was found (V=0.221) (Table 2). The prevailing opportunistic pathogens were Enterococcus faecalis and Escherichia сoli.

Despite the fact that in 80% of patients the infectious factor was thought to be in the genesis of isthmic-cervical insufficiency, no differences were found between the groups in the incidence of cervical incompetence.  The incidence of preterm birth, particularly extremely early preterm birth in group I (PA and ascending amniotic fluid infection) was17 times higher versus group II (PA), and probably indicated the infectious genesis of placental abruption. Preterm birth at <33 weeks of gestation was usually caused by infection. It could be suggested that the underlying factors of infectious genesis of PA were premature rupture of membranes and timing of water breaking >24 hours (for 5 days). However, association between these factors and PA was either absent (V=0.075) or weak (V=0.187), respectively.

In almost half of the patients in group I, infection was likely due to longer time gap between water breaking and the start of labor, opportunistic pathogenic vaginal flora, and high incidence of preterm birth. It is likely that the combination of these factors caused placental abruption.

It has been proven that one of the reasons for early ending of pregnancy is an infection, specific for extremely early (22⁰–27⁶ weeks) and very early (28⁰–31⁶ weeks) preterm birth [16].

In group I (PA and ascending amniotic fluid infecton), pregnancy length was shorter. Preterm delivery, extremely early preterm birth, and birth before 33 weeks of gestation prevailed (Table 2).

All patients in group I (PA and ascending amniotic fluid infection) had cesarean delivery. In 11/69 (15.9%) patients in group II (PA), placental abruption occurred at the end of the second stage of labor, and they had vaginal delivery (р=0.008). There were no differences between the groups in the intraoperative blood loss during cesarean section, the frequency of autohemotransfusion, and the length of postpartum hospital stay (Table. 2)

The patients in group I gave birth to 44 liveborn infants, and in group II – 71 liveborn infants. Health status of newborns was assessed taking into consideration gestational age (preterm or term babies) (Table 3).

There was no difference between the subgroups in the weight of preterm and term infants (Table 3). In the presence of ascending amniotic fluid infection, preterm infants were born with asphyxia 4.2 times more often (V=0.314). In the presence of ascending amniotic fluid infection, asphyxia in preterm infants continued at 5 minutes after birth 11.8 times more often, and strong association was identified (V=0.502) (Table 4).

Despite the similar frequency of hospitalization of preterm infants in the ICU, mechanical ventilation (AVL) was required most often in the presence of ascending amniotic fluid infection. However, there was no difference in duration of ventilation. The laboratory markers of inflammation (leukocytosis, leukopenia, neutrophil ratio >0.21, high C-reactive protein level) and the need for antimicrobial therapy were higher in the subgroup of preterm infants with inflammatory changes in the placenta.

Intrauterine pneumonia was diagnosed in 4/29 (13.8%) preterm infants and  1/15 (6.7%) term infant in the presence of ascending amniotic fluid infection Neonatal death was only in the subgroups of preterm infants (Table 3).

Histological examination of placentas in both groups found that deviations from the normal placental weight were in the same number of patients. In patients in group II (placental abruption), maternal vascular malperfusion with infarctions, compensatory angiomatosis, syncytial kidneys, and deciduitis in the basal plate were found more often versus group I (placental abruption and ascending amniotic fluid infection). In group II, fetal vascular malperfusion included thrombosis in the chorionic plate (Table 1).

Discussion

Many risk factors were described, and using different combinations of these factors, the researchers tried to substantiate the pathogenesis underlying placental abruption.

Chronic process predisposing placental abruption is vasculopathy due to defective second wave of trophoblast cell invasion.  This is the most common mechanism for formation of retroplacental hematoma. Defective uterine spiral artery remodeling, maintaince of the structure of spiral arteries specific in non-pregnant women, plays a key role in the development of hydrodynamic shock due to pressure fluctuations. As a result, rupture of the vessel wall in the decidua occurs, and retroplacental hematoma is formed.

Another possible variant in the occurrence of placental abruption implies the development of acute inflammation associated with the infectious factor, and can be caused both by thrombin and bacterial endotoxin. Their combined action aggravates the severity of the pathological changes [17]. The local thrombin concentration increases not only due to intrauterine hemorrhage, but also due to bacterial infection. Intestinal endotoxins are lipopolysaccharides produced due the destruction of gram-negative intestinal microbiota. They play a leading role in activation of nuclear segmentation in neutrophils. Cell-surface toll-like receptors (TLRs) in neutrophils mediate innate immune responses. TLR-2 binds to proteins of gram-positive bacteria, TLR-3 binds to viral particles, and TLR-4 recognizes gram-negative microorganisms. Activation of nuclear segmentation in neutrophils results in the synthesis of inflammatory mediators and proinflammatory cytokines, and increased phagocytosis. Release of matrix metalloproteinases has a lytic effect on microorganisms and contributes to the destruction of the extracellular matrix, resulting in premature separation of the placenta from the uterine wall. This correlation is dangerous and leads to inevitable miscarriage.

Acute chorioamnionitis with or without fetal inflammatory response is defined as acute inflammation in the placenta, fetal membranes, and the umbilical cord [8]. 70% of placental and fetal infections are caused by ascending bacterial infection spreading from the vagina to the uterine cavity through the cervix). Maternal inflammatory response is associated with migration of neutrophils to the chorion and amnion from the decidual vessels, and from the intervillous space into the chorionic plate. Fetal inflammatory response indicates the presence of chorionic vasculitis, phlebitis, and umbilical arteritis [9].

Currently, the hypolthesis implying that the cause of placental abruption is an ascending amniotic fluid infection gains growing support [1, 12].

Despite the fact that hematogenous spread of viral or protozoan infections in the placenta are detected in 30% of cases, their influence on occurrence of retroplacental hematomas is not proved [18].

The clinical diagnosis of PA is not always confirmed by the results of pathomorphological examination, that is reported in modern literary sources [4, 19]. [4, 19]. According to Kovo M. et al. (2020), the morphological signs of placental abruption were registered only in one-third of patients. [10].

Our study was based only on observations, where the diagnosis of placental abruption was histologically verified. We analyzed maternal and perinatal outcomes in patients with PA, who were divided into groups based on the presence or absence of ascending amniotic fluid infection of the placenta. Histological tests showed that in most patients in group I (placental abruption and ascending amniotic infection), placental inflammation was manifested by stage I of maternal inflammatory response (leukocyte infiltration into the subchorionic space). Only one-quarter of placentas in group I exhibited fetal inflammatory response (chorionic vasculitis with or without umbilical phlebitis, as well as umbilical phlebitis and arteritis, or panvasculitis). In placentas of patients with placental abruption in the absence of chorioamnionitis, impaired maternal and fetal blood flow was found most often, as evidenced by placental infarctions in 28.2% of cases. Spasms of the placental vessels are followed by dilation, leading to abnormal filling of the intervillous space with maternal blood. In the absence of adequate venous drainage, blood flow velocity in certain areas of intervillous space significantly decreases. This leads to progressive hypercoagulation with loss of fibrin strands and formation of intervillous thrombi. Dense fibrinoid deposites surround villi in a form of a "belt" along the periphery of the thrombi. Dystrophic changes and necrosis of placental villi, occurrence of ischemic infarction zone and compromised fetal–placental circulation are explained by abnormal blood supply and functioning of this placental zone.

All patients with placental abruption and infection of the placenta had cesarean deliveries. The length of pregnancy varied from 25 to 39 weeks. In this group of patients, opportunistic pathogens in vaginal flora were identified by microbiological tests of vaginal secretion specimens. Among them, the greater number of women (by 2.54 times) had preterm birth 3.3 times more often, birth before 33 weeks of pregnancy 2.8 times more often, and extremely early preterm birth by 17 times.

 Placental abruption in the presence of ascending amniotic fluid infection was characterized by moderate association with asphyxia in preterm babies at 1 minute, and strong association with asphyxia at 5 minute after birth.

Analysis of the data in our study confirms the importance of microbiological monitoring of patients with the clinical signs of threatened miscarriage and preterm birth, particularly at 37 weeks of gestation and premature rupture of membranes. Close clinical and laboratory monitoring, accompanying watch-and-wait strategy in cases of long time between water breaking and the start of labor will help to take timely measures to prevent premature rupture of membranes. Histological examination of the placenta is undoubtedly mandatory in all cases of  placental abruption.

It should be noted that the limitations of this retrospective study include the use of data from only one medical facility, incomplete medical documentation, and inadequate microbiological diagnostics in the earlier period, and the lack of virologic testing of pregnant and parturient women. The objectives of the study did not include detailed analysis of the dynamics of newborns' condition during the first months of life or the causes of death of preterm infants depending on the presence or absence of ascending amniotic fluid infection in cases of placental abruption.

Conclusion

Ascending amniotic fluid infection is a risk factor for premature separation of the normally implanted placenta, especially in the presence of opportunistic pathogenic flora in the female genital tract and a long time gap between water breaking and the start of labor in pregnancies ending before the due date. The combination of infectious lesion of the placenta and placental abruption is associated with a higher risk of preterm birth and adverse perinatal outcomes.

Currently, due attention is not given to the role of infectious agents in the genesis of placental abruption, and therefore, prevention and treatment methods are not developed. The researchers mainly focus attention on vascular impairment and hypercoagulation in hypertension disorders (preeclampsia, chronic arterial hypertension, chronic glomerulonephritis). The obtained data confirmed that ascending amniotic fluid infection had significant importance in the development of placental abruption due to bacterial inflammation. It occurred in one-third of patients due to preexisting impairment of uteroplacental blood flow. Most patients (72.5%) exhibited stage 1 maternal inflammatory response, manifested as subchorionitis. Fetal response was in 25% of observations.

Therefore, the diagnostic measures aimed at identifying the infectious factor in patients with suspected placental abruption, should be included in the algorithm for management of this category of patients.