Introduction

The consensus among reproductive biologists is that when extravillous trophoblast cells fail to invade maternal spiral arteries early in pregnancy, it reduces maternal blood flow to the placenta. This failure induces functional and pathological changes in the placenta, often observed in individuals with hypertensive disorders of pregnancy, Small-for-Gestational-Age (SGA) neonates, and stillbirths. Many of these complications result in medically indicated Preterm Births (PTBs). Moreover, evidence suggests that inadequate trophoblast invasion into maternal uterine vasculature early in pregnancy can lead to placental dysfunction, resulting in spontaneous PTB, such as preterm labor or Preterm Premature Rupture of Membranes (PPROM). However, the study of placental function by reproductive biologists has faced obstacles due to the absence of animal models that accurately mimic the human placenta and the inability to access placental tissue during human pregnancy. Consequently, the ability to detect placental dysfunction before adverse outcomes occur has been limited. The human placenta is characterized by a villous structure that facilitates gas and nutrient exchange. Placental products are mainly secreted from the villous surface of the syncytiotrophoblast into the intervillous space and maternal circulation. To investigate placental physiology related to Adverse Pregnancy Outcomes (APOs) and discover new biomarkers, we analyzed levels of placental proteins in the maternal circulation of a large group of first-time pregnant women during the first two trimesters. The nine proteins we focused on can be categorized into three main physiological groups: angiogenesis (Vascular Endothelial Growth Factor (VEGF), Placental Growth Factor (PlGF), Endoglin (Eng), and soluble fms-like tyrosine kinase-1); placental implantation and development (A Disintegrin and Metalloproteinase domaincontaining protein 12 (ADAM12) and Pregnancy-Associated Plasma Protein A (PAPP-A)); and established clinical markers of fetal aneuploidy (free beta-Human Chorionic Gonadotropin (β- Hcg), inhibin A, and Alpha-Fetoprotein (AFP)). Although AFP is not produced by the placenta, elevated levels in maternal serum during the second trimester have been linked to APOs, likely due to increased placental permeability.

Placental analytes

During the initial and subsequent stages of pregnancy, blood samples were collected from maternal participants in the nuMoM2b study. This investigation aimed to assess the utility of placental substances present in maternal blood to anticipate various adverse pregnancy outcomes, including preterm birth (both medically indicated and spontaneous), preeclampsia, the birth of small-for-gestational-age infants, and stillbirth. The study enrolled 10,040 eligible pregnant women in the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be cohort. A nested case-control analysis was conducted, comparing 900 instances of preterm birth, 600 cases of preeclampsia, 400 cases of small-for-gestational-age birth, and 50 cases of stillbirth with 915 controls who delivered at term without complications. While levels of the analyzed substances generally differed between cases and controls at one or two visits, the odds ratios indicated less than a twofold difference between cases and controls in all comparisons. Receiver operating characteristic curves were constructed to evaluate the association between substance levels and preterm birth and other adverse pregnancy outcomes. The resulting areas under the curves were relatively low for each substance. Logistic regression models revealed that incorporating baseline clinical characteristics and combinations of substances improved the predictive accuracy of adverse pregnancy outcomes compared to baseline characteristics alone, although the areas under the curves remained relatively low for each outcome. Successful pregnancy establishment heavily relies on placental function. Aberrant placentation has been associated with obstetric complications such as preeclampsia, placental abruption, fetal growth restriction, stillbirth, and postpartum hemorrhage. Women experiencing recurrent pregnancy losses after their first childbirth display a higher incidence of these complications during their initial pregnancy, suggesting altered placentation in these cases. The european society of human reproduction and embryology defines recurrent pregnancy loss as experiencing two pregnancy losses, affecting 1%–3% of women attempting conception.

Prerequisite placentation

A fundamental requirement for proper placental development involves the precise transformation of the endometrial lining, known as decidualization, driven by estrogen and progesterone. This process triggers a significant change in endometrial stromal cells. Successful implantation and formation of the placenta hinge upon a series of intricate steps, including the formation of new blood vessels leading to the modification of spiral arteries. This modification involves the removal of vascular smooth muscle and the deposition of specific cells, ensuring an optimal blood flow pattern crucial for placental function. Angiopoietin-1 (Ang-1) plays a key role in supporting angiogenesis by stabilizing blood vessel walls and guiding trophoblast cells. During early pregnancy, Angiopoietin-2 (Ang-2) is upregulated, potentially aiding in the separation of smooth muscle cells during artery remodeling, particularly in hypoxic conditions. Additionally, the presence of angiogenic factors like Vascular Endothelial Growth Factor (VEGF), secreted by uterine NK-cells, further enhances Ang-2 secretion by the placenta, promoting trophoblast growth and nitric oxide release. The placenta also produces soluble fmslike tyrosine kinase-1 (sFlt-1), which counters the effects of VEGF and is elevated in preeclampsia. Dysregulation of angiogenic factors in early pregnancy may lead to inadequate placental artery remodeling, potentially contributing to conditions like preeclampsia and intrauterine growth restriction. Abnormal activation of the complement system, an integral part of the innate immune response, has also been linked to faulty placental development. This system comprises three pathways classical, alternative, and lectin where lectin pathway activation involves pattern recognition molecules like ficolins, Mannose-Binding Lectin (MBL), and Pentraxin 3 (PTX3), which recognize and facilitate the clearance of pathogens or damaged cells