[Epidemiology and risk factors of preterm birth]

OBJECTIVE

To synthesize the available evidence regarding the incidence and several risk factors of preterm birth. To describe neonatal outcomes according to gestational age and to the context of delivery.

MATERIALS AND METHODS

Consultation of the Medline database.

RESULTS

In 2010, 11% of live births (15 million babies) occurred before 37 completed weeks of gestation worldwide. About 85% of these births were moderate to late preterm babies (32-36 weeks), 10% were very preterm babies (28-31 weeks) and 5% were extremely preterm babies (<28 weeks). In France, premature birth concerns 60,000 neonates every year, 12,000 of whom are born before 32 completed weeks of gestation. Half of them are delivered after spontaneous onset of labor or preterm premature rupture of the membranes, and the other half are provider-initiated preterm births. Several maternal factors are associated with preterm birth, including sociodemographic, obstetrical, psychological, and genetic factors; paternal and environmental factors are also involved. Gestational age is highly associated with neonatal mortality and with short- and long-term morbidities. Pregnancy complications and the context of delivery also have an impact on neonatal outcomes.

CONCLUSION

Preterm birth is one of the leading cause of the under-five mortality and of neurodevelopmental impairment worldwide; it remains a major public health issue.

Human thioredoxin-1 attenuates the rate of lipopolysaccharide-induced preterm delivery in mice in association with its anti-inflammatory effect

BACKGROUND

Maternal intrauterine infection/inflammation represents the major etiology of preterm delivery and the leading cause of neonatal mortality and morbidity. The aim of this study was to investigate the anti-inflammatory properties of thioredoxin-1 in vivo and its potential ability to attenuate the rate of inflammation-induced preterm delivery.

METHODS

Two intraperitoneal injections of lipopolysaccharide from Escherichia coli were administered in pregnant mice on gestational day 15, with a 3-h interval between the injections. From either 1 h before or 1 h after the first lipopolysaccharide injection, mice received three intravenous injections of either recombinant human thioredoxin-1, ovalbumin, or vehicle, with a 3-h interval between injections.

RESULTS

Intraperitoneal injection of lipopolysaccharide induced a rise of tumor necrosis factor-α, interferon-γ, monocyte chemotactic protein 1, and interleukin-6 in maternal serum levels and provoked preterm delivery. Recombinant human thoredoxin-1 prevented the rise in these proinflammatory cytokine levels. After the inflammatory challenge, placentas exhibited severe maternal vascular dilatation and congestion and a marked decidual neutrophil activation. These placental pathological findings were ameliorated by recombinant human thioredoxin-1, and the rate of inflammation-induced preterm delivery was attenuated.

CONCLUSION

Thioredoxin-1 may thus represent a novel effective treatment to delay inflammation-induced preterm delivery.

Cervical etiology of spontaneous preterm birth

The cervix functions as a barrier between the uterus and vagina and keeps the uterus closed until term so that the fetus can develop. For delivery the cervix must soften and dilate, and finally reconstitute to close the uterus. This complex process involves precisely timed activation of molecular and microstructural events. Spontaneous preterm birth (sPTB) can result from aberrant timing of these events in the cervix. Unfortunately, the pathophysiology of sPTB due to cervical causes remains unclear and thus our treatment options remain limited - even if all appropriate candidates were identified and correctly treated with currently available interventions, the rate of sPTB would only be reduced by 5%. Very recent molecular and microstructural investigation is challenging prevailing concepts about cervical remodeling in pregnancy. We believe that progress toward novel, targeted solutions for the diverse pathways to sPTB entails a paradigm shift in which the overlapping and complex interactions between the cervix, uterus, membranes, fetus, placenta, and surrounding (structural and molecular) environment are suitably honored.

Type I Interferon Regulates the Placental Inflammatory Response to Bacteria and is Targeted by Virus: Mechanism of Polymicrobial Infection-Induced Preterm Birth

PROBLEM

Preterm birth (PTB) affects approximately 12% of pregnancies and at least 50% of cases have no known risk factors. We hypothesize that subclinical viral infections of the placenta are a factor sensitizing women to intrauterine bacterial infection. Specifically, we propose that viral-induced placental IFN-β inhibition results in a robust inflammatory response to low concentrations of bacteria.

METHODS

Human trophoblast SW.71, C57BL/6, and interferon (IFN) receptor knockout animals were used to determine IFN function. Illumina and Bio-Rad microarrays identified pathways.

RESULTS

Inhibiting the IFN-β pathway resulted in a significant increase in inflammatory cytokines such as IL-1B in response to LPS. Twist was positively correlated with IFN-β expression and STAT3 phosphorylation and overexpressing Twist reduced IL-1B. Treating IFNAR-/- mice with low-dose LPS at E15.5 caused preterm birth.

CONCLUSION

IFN-β was identified as a key modulator of placental inflammation and, importantly, is commonly affected by viruses. We propose dysregulation of IFN-β is a major determinant for preterm birth associated with polymicrobial infection.

Maternal microbiome - A pathway to preterm birth

Despite great medical advances in preventing maternal and infant mortality in the past century, one issue remains unresolved: why do so many women give birth prematurely? A major new field of human microbiome studies has begun to shed light on the impact of microbes (of both the commensal and pathogen varieties) on pregnancy outcomes. Recent advances in next-generation sequencing and metagenomic analysis have revealed that maternal microbiomes at a variety of niches including the oral, vaginal, gut, cervical, and even the placenta itself govern pregnancy outcomes. In this review, we describe how alterations in the microbial biomasses impact preterm birth and we discuss the major research questions concerning the cause and/or interdependent relationships between microbiome, infection, and preterm delivery.

Mechanisms of the Maternal Exposome and Implications for Health Outcomes

It is well established that the environment contributes to health. However, few studies have evaluated environmental exposures in women that may influence future health of their offspring. Knowledge gained may inform nursing how to better advocate for patients and families, and provide individualized interventions and education. Therefore, a more comprehensive investigation of the maternal exposome to uncover mechanistic insight into complex disease in offspring is warranted. To advance understanding of biologic mechanisms that contribute to high-risk birth outcomes and offspring predisposition to disease, it will be necessary to measure a range of exposures and biomarkers before and during pregnancy.

Global burden of prematurity

The global burden of preterm birth (PTB) includes the morbidity and mortality of babies born before 37 completed weeks of gestation. Prematurity has been the leading worldwide cause of neonatal mortality for at least a decade, but has now also become the leading cause of childhood mortality through age five years. Globally, each year, 15 million babies are born preterm, which is estimated to be about 11% of all deliveries. Preterm birth appears to be increasing in most countries. This review will address the epidemiology, rates, and etiology of PTB around the globe as well as survival by gestational age and interventions and preventative measures known to improve outcomes in high-burden countries.

Comparing human and macaque placental transcriptomes to disentangle preterm birth pathology from gestational age effects

INTRODUCTION

A major issue in the transcriptomic study of spontaneous preterm birth (sPTB) in humans is the inability to collect healthy control tissue at the same gestational age (GA) to compare with pathologic preterm tissue. Thus, gene expression differences identified after the standard comparison of sPTB and term tissues necessarily reflect differences in both sPTB pathology and GA. One potential solution is to use GA-matched controls from a closely related species to tease apart genes that are dysregulated during sPTB from genes that are expressed differently as a result of GA effects.

METHODS

To disentangle genes whose expression levels are associated with sPTB pathology from those linked to GA, we compared RNA sequencing data from human preterm placentas, human term placentas, and rhesus macaque placentas at 80% completed gestation (serving as healthy non-human primate GA-matched controls). We first compared sPTB and term human placental transcriptomes to identify significantly differentially expressed genes. We then overlaid the results of the comparison between human sPTB and macaque placental transcriptomes to identify sPTB-specific candidates. Finally, we overlaid the results of the comparison between human term and macaque placental transcriptomes to identify GA-specific candidates.

RESULTS

Examination of relative expression for all human genes with macaque orthologs identified 267 candidate genes that were significantly differentially expressed between preterm and term human placentas. 29 genes were identified as sPTB-specific candidates and 37 as GA-specific candidates. Altogether, the 267 differentially expressed genes were significantly enriched for a variety of developmental, metabolic, reproductive, immune, and inflammatory functions. Although there were no notable differences between the functions of the 29 sPTB-specific and 37 GA-specific candidate genes, many of these candidates have been previously shown to be dysregulated in diverse pregnancy-associated pathologies.

DISCUSSION

By comparing human sPTB and term transcriptomes with GA-matched control transcriptomes from a closely related species, this study disentangled the confounding effects of sPTB pathology and GA, leading to the identification of 29 promising sPTB-specific candidate genes and 37 genes potentially related to GA effects. The apparent similarity in functions of the sPTB and GA candidates may suggest that the effects of sPTB and GA do not correspond to biologically distinct processes. Alternatively, it may reflect the poor state of knowledge of the transcriptional landscape underlying placental development and disease.

The role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding

BACKGROUND

Human pregnancy requires robust hemostasis to prevent hemorrhage during extravillous trophoblast (EVT) invasion of the decidualized endometrium, modification of spiral arteries and post-partum processes. However, decidual hemorrhage (abruption) can occur throughout pregnancy from poorly transformed spiral arteries, causing fetal death or spontaneous preterm birth (PTB), or it can promote the aberrant placentation observed in intrauterine growth restriction (IUGR) and pre-eclampsia; all leading causes of perinatal or maternal morbidity and mortality. In non-fertile cycles, the decidua undergoes controlled menstrual bleeding. Abnormal uterine bleeding (AUB) accompanying progestin-only, long-acting, reversible contraception (pLARC) accounts for most discontinuations of these safe and highly effective agents, thereby contributing to unwanted pregnancies and abortion. The aim of this study was to investigate the role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding.

METHODS

We conducted a critical review of the literature arising from PubMed searches up to December 2015, regarding in situ and in vitro expression and regulation of several specific proteins involved in uterine hemostasis in decidua and cycling endometrium. In addition, we discussed clinical and molecular mechanisms associated with pLARC-induced AUB and pregnancy complications with abruptions, chorioamnionitis or pre-eclampsia.

RESULTS

Progestin-induced decidualization of estradiol-primed human endometrial stromal cells (HESCs) increases in vivo and in vitro expression of tissue factor (TF) and type-1 plasminogen activator inhibitor (PAI-1) while inhibiting plasminogen activators (PAs), matrix metalloproteinases (MMPs), and the vasoconstrictor, endothelin-1 (ET-1). These changes in decidual cell-derived regulators of hemostasis, fibrinolysis, extracellular matrix (ECM) turnover, and vascular tone prevent hemorrhage during EVT invasion and vascular remodeling. In non-fertile cycles, progesterone withdrawal reduces TF and PAI-1 while increasing PA, MMPs and ET-1, causing menstrual-associated bleeding, fibrinolysis, ECM degradation and ischemia. First trimester decidual hemorrhage elicits later adverse outcomes including pregnancy loss, pre-eclampsia, abruption, IUGR and PTB. Decidual hemorrhage generates excess thrombin that binds to decidual cell-expressed protease-activated receptors (PARs) to induce chemokines promoting shallow placentation; such bleeding later in pregnancy generates thrombin to down-regulate decidual cell progesterone receptors and up-regulate cytokines and MMPs linked to PTB. Endometria of pLARC users display ischemia-induced excess vasculogenesis and progestin inhibition of spiral artery vascular smooth muscle cell proliferation and migration leading to dilated fragile vessels prone to bleeding. Moreover, aberrant TF-derived thrombin signaling also contributes to the pathogenesis of endometriosis via induction of angiogenesis, inflammation and cell survival.

CONCLUSION

Perivascular decidualized HESCs promote endometrial hemostasis during placentation yet facilitate menstruation through progestational regulation of hemostatic, proteolytic, and vasoactive proteins. Pathological endometrial hemorrhage elicits excess local thrombin generation, which contributes to pLARC associated AUB, endometriosis and adverse pregnancy outcomes through several biochemical mechanisms.

An elevated amniotic fluid prostaglandin F2α concentration is associated with intra-amniotic inflammation/infection, and clinical and histologic chorioamnionitis, as well as impending preterm delivery in patients with preterm labor and intact membranes

OBJECTIVE

To determine whether an elevated amniotic fluid concentration of prostaglandin F2α (PGF2α) is associated with intra-amniotic inflammation/infection and adverse pregnancy outcomes in patients with preterm labor and intact membranes.

MATERIALS AND METHODS

The retrospective cohort study included 132 patients who had singleton pregnancies with preterm labor (< 35 weeks of gestation) and intact membranes. Amniotic fluid was cultured for aerobic and anaerobic bacteria as well as for genital mycoplasmas. Intra-amniotic inflammation was defined by an elevated amniotic fluid matrix metalloproteinase-8 (MMP-8) concentration (>23 ng/mL). PGF2α was measured with a sensitive and specific immunoassay. The amniotic fluid PGF2α concentration was considered elevated when it was above the 95th percentile among pregnant women at 15-36 weeks of gestation who were not in labor (≥170 pg/mL).

RESULTS

(1) The prevalence of an elevated amniotic fluid PGF2α concentration was 40.2% (53/132) in patients with preterm labor and intact membranes; (2) patients with an elevated amniotic fluid PGF2α concentration had a significantly higher rate of positive amniotic fluid culture [19% (10/53) versus 5% (4/79); p = 0.019], intra-amniotic inflammation/infection [49% (26/53) versus 20% (16/79); p = 0.001], spontaneous preterm delivery, clinical and histologic chorioamnionitis, and funisitis, as well as a higher median amniotic fluid MMP-8 concentration and amniotic fluid white blood cell count and a shorter amniocentesis-to-delivery interval than those without an elevated concentration of amniotic fluid PGF2α (p < 0.05 for each); and (3) an elevated amniotic fluid PGF2α concentration was associated with a shorter amniocentesis-to-delivery interval after adjustment for the presence of intra-amniotic inflammation/infection [hazard ratio 2.1, 95% confidence interval (CI) 1.4-3.1; p = 0.001].

CONCLUSION

The concentration of PGF2α was elevated in the amniotic fluid of 40.2% of patients with preterm labor and intact membranes and is an independent risk factor for intra-amniotic inflammation/infection, impending preterm delivery, chorioamnionitis, and funisitis.

The genetics of preterm birth: Progress and promise

Preterm birth is the single leading cause of mortality for neonates and children less than 5 years of age. Compared to other childhood diseases, such as infections, less progress in prevention of prematurity has been made. In large part, the continued high burden of prematurity results from the limited understanding of the mechanisms controlling normal birth timing in humans, and how individual genetic variation and environmental exposures disrupt these mechanisms to cause preterm birth. In this review, we summarize the outcomes and limitations from studies in model organisms for birth timing in humans, the evidence that genetic factors contribute to birth timing and risk for preterm birth, and recent genetic and genomic studies in women and infants that implicate specific genes and pathways. We conclude with discussing areas of potential high impact in understanding human parturition and preterm birth in the future.

Evidence of perturbations of the cytokine network in preterm labor

OBJECTIVE

Intraamniotic inflammation/infection is the only mechanism of disease with persuasive evidence of causality for spontaneous preterm labor/delivery. Previous studies about the behavior of cytokines in preterm labor have been largely based on the analysis of the behavior of each protein independently. Emerging evidence indicates that the study of biologic networks can provide insight into the pathobiology of disease and improve biomarker discovery. The goal of this study was to characterize the inflammatory-related protein network in the amniotic fluid of patients with preterm labor.

STUDY DESIGN

A retrospective cohort study was conducted that included women with singleton pregnancies who had spontaneous preterm labor and intact membranes (n = 135). These patients were classified according to the results of amniotic fluid culture, broad-range polymerase chain reaction coupled with electrospray ionization mass spectrometry, and amniotic fluid concentration of interleukin (IL)-6 into the following groups: (1) those without intraamniotic inflammation (n = 85), (2) those with microbial-associated intraamniotic inflammation (n = 15), and (3) those with intraamniotic inflammation without detectable bacteria (n = 35). Amniotic fluid concentrations of 33 inflammatory-related proteins were determined with the use of a multiplex bead array assay.

RESULTS

Patients with preterm labor and intact membranes who had microbial-associated intraamniotic inflammation had a higher amniotic fluid inflammatory-related protein concentration correlation than those without intraamniotic inflammation (113 perturbed correlations). IL-1β, IL-6, macrophage inflammatory protein (MIP)-1α, and IL-1α were the most connected nodes (highest degree) in this differential correlation network (degrees of 20, 16, 12, and 12, respectively). Patients with sterile intraamniotic inflammation had correlation patterns of inflammatory-related proteins, both increased and decreased, when compared to those without intraamniotic inflammation (50 perturbed correlations). IL-1α, MIP-1α, and IL-1β were the most connected nodes in this differential correlation network (degrees of 12, 10, and 7, respectively). There were more coordinated inflammatory-related protein concentrations in the amniotic fluid of women with microbial-associated intraamniotic inflammation than in those with sterile intraamniotic inflammation (60 perturbed correlations), with IL-4 and IL-33 having the largest number of perturbed correlations (degrees of 15 and 13, respectively).

CONCLUSIONS

We report for the first time an analysis of the inflammatory-related protein network in spontaneous preterm labor. Patients with preterm labor and microbial-associated intraamniotic inflammation had more coordinated amniotic fluid inflammatory-related proteins than either those with sterile intraamniotic inflammation or those without intraamniotic inflammation. The correlations were also stronger in patients with sterile intraamniotic inflammation than in those without intraamniotic inflammation. The findings herein could be of value in the development of biomarkers of preterm labor.

Placenta: the forgotten organ

The placenta sits at the interface between the maternal and fetal vascular beds where it mediates nutrient and waste exchange to enable in utero existence. Placental cells (trophoblasts) accomplish this via invading and remodeling the uterine vasculature. Amazingly, despite being of fetal origin, trophoblasts do not trigger a significant maternal immune response. Additionally, they maintain a highly reliable hemostasis in this extremely vascular interface. Decades of research into how the placenta differentiates itself from embryonic tissues to accomplish these and other feats have revealed a previously unappreciated level of complexity with respect to the placenta's cellular composition. Additionally, novel insights with respect to roles played by the placenta in guiding fetal development and metabolism have sparked a renewed interest in understanding the interrelationship between fetal and placental well-being. Here, we present an overview of emerging research in placental biology that highlights these themes and the importance of the placenta to fetal and adult health.

Is preterm birth a human-specific syndrome?

Human preterm birth (PTB), a multifactorial syndrome affecting offspring born before 37 completed weeks of gestation, is the leading cause of newborn death worldwide. Remarkably, the degree to which early parturition contributes to mortality in other placental mammals remains unclear. To gain insights on whether PTB is a human-specific syndrome, we examined within- and between-species variation in gestation length across placental mammals and the impact of early parturition on offspring fitness. Within species, gestation length is normally distributed, and all species appear to occasionally give birth before the 'optimal' time. Furthermore, human gestation length, like that of many mammalian species, scales proportionally to body mass, suggesting that this trait, like many others, is constrained by body size. Premature humans suffer from numerous cognitive impairments, but little is known of cognitive impairments in other placental mammals. Human gestation differs in the timing of the 'brain growth spurt', where unlike many mammals, including closely related primates, the trajectory of human brain growth directly overlaps with the parturition time window. Thus, although all mammals experience early parturition, the fitness costs imposed by the cognitive impairments may be unique to our species. Describing PTB broadly in mammals opens avenues for comparative studies on the physiological and genetic regulators of birth timing as well as the development of new mammalian models of the disease.