Association of Placental Jets and Mega-Jets With Reduced Villous Density

Unveiling the human fetal-maternal interface during the first trimester: biophysical knowledge and gaps

The intricate interplay between the developing placenta and fetal-maternal interactions is critical for pregnancy outcomes. Despite advancements, gaps persist in understanding biomechanics, transport processes, and blood circulation parameters, all of which are crucial for safe pregnancies. Moreover, the complexity of fetal-maternal interactions led to conflicting data and methodological variations. This review presents a comprehensive overview of current knowledge on fetal-maternal interface structures, with a particular focus on the first trimester. More in detail, the embryological development, structural characteristics, and physiological functions of placental chorionic plate and villi, fetal membranes and umbilical cord are discussed. Furthermore, a description of the main structures and features of maternal and fetal fluid dynamic exchanges is provided. However, ethical constraints and technological limitations pose still challenges to studying early placental development directly, which calls for sophisticated in vitro, microfluidic organotypic models for advancing our understanding. For this, knowledge about key in vivo parameters are necessary for their design. In this scenario, the integration of data from later gestational stages and mathematical/computational simulations have proven to be useful tools. Notwithstanding, further research into cellular and molecular mechanisms at the fetal-maternal interface is essential for enhancing prenatal care and improving maternal and fetal health outcomes.

Shear stress in the intervillous space promotes syncytial formation of iPS cells-derived trophoblasts†

The intervillous space of human placenta is filled with maternal blood, and villous trophoblasts are constantly exposed to the shear stress generated by maternal blood pressure and flow throughout the entire gestation period. However, the effects of shear stress on villous trophoblasts and their biological significance remain unknown. Here, using our recently established naïve human pluripotent stem cells-derived cytotrophoblast stem cells (nCTs) and a device that can apply arbitrary shear stress to cells, we investigated the impact of shear stress on early-stage trophoblasts. After 72 h of exposure to 10 dyn/cm2 shear stress, nCTs became fused and multinuclear, and mRNA expression of the syncytiotrophoblast (ST) markers, such as glial cell missing 1, endogenous retrovirus group W member 1 envelope, chorionic gonadotropin subunit beta 3, syndecan 1, pregnancy specific beta-1-glycoprotein 3, placental growth factor, and solute carrier family 2 member 1 were significantly upregulated compared to static conditions. Immunohistochemistry showed that shear stress increased fusion index, human chorionic gonadotropin secretion, and human placental lactogen secretion. Increased microvilli formation on the surface of nCTs under flow conditions was detected using scanning electron microscopy. Intracellular cyclic adenosine monophosphate significantly increased under flow conditions. Moreover, transcriptome analysis of nCTs subjected to shear stress revealed that shear stress upregulated ST-specific genes and downregulated CT-specific genes. Collectively, these findings indicate that shear stress promotes the differentiation of nCTs into ST.

Spiral, uterine artery doppler and placental ultrasound in relation to preeclampsia

Preeclampsia (PE) is a multiorgan disorder that complicates around 2-8% of pregnancies and is a major cause of perinatal and maternal morbidity and mortality. PE is a clinical syndrome characterized by hypertension secondary to systemic inflammation, endothelial dysfunction, and syncytiotrophoblast stress leading to hypertension and multiorgan dysfunction. The uterine arteries are the main blood vessels that supply blood to the uterus. They give off branches and plays an important role in maintaining blood supply during pregnancy. The arcuate artery originates from the uterine artery and runs medially through the myometrium. The arcuate arteries divide almost directly into anterior and posterior branches, from which the radial artery leads directly to the uterine cavity during their course. Near the endometrium-myometrium junction, the radial artery generates spiral arteries within the basal layer and functional endometrium. The walls of radial and spiral arteries are rich in smooth muscle, which is lost when trophoblast cells invade and become large-caliber vessels. This physiological transformation of uteroplacental spiral arteries is critical for successful placental implantation and normal placental function. In normal pregnancy, the luminal diameter of the spiral arteries is greatly increased, and the vascular smooth muscle is replaced by trophoblast cells. This process and changes in the spiral arteries are called spiral artery remodeling. In PE, this genetically and immunologically governed process is deficient and therefore there is decreased vascular capacitance and increased resistance in the uteroplacental circulation. Furthermore, this defect in uteroplacental spiral artery remodeling is not only associated with early onset PE, but also with fetal growth restriction, placental abruption, and spontaneous premature rupture of membranes. Doppler ultrasound allows non-invasive assessment of placentation, while the flow impedance decreases as the pregnancy progresses in normal pregnancies, in those destined to develop preeclampsia the impedance is increased.

Longitudinal assessment of spiral and uterine arteries in normal pregnancy using novel ultrasound tool

OBJECTIVES

To use superb microvascular imaging (SMI) to evaluate longitudinally spiral artery (SA) and uterine artery (UtA) vascular adaptation in normal human pregnancy, and to develop reference ranges for use at various gestational ages throughout pregnancy.

METHODS

The data for this study were obtained from the National Institutes of Health (NIH)-funded Human Placenta Project. Women aged 18-35 years, with a body mass index < 30 kg/m2 , without comorbidities, with a singleton gestation conceived spontaneously, and gestational age at or less than 13 + 6 weeks were eligible for inclusion. The current analysis was restricted to uncomplicated pregnancies carried to term. Exclusion criteria included maternal or neonatal complications, fetal or umbilical cord anomalies, abnormal placental implantation or delivery < 37 weeks. Women who fulfilled the inclusion criteria formed the reference population of the Human Placenta Project study. Each participant underwent eight ultrasound examinations during pregnancy. The pulsatility index (PI) of both the left and right UtA were obtained twice for each artery and the presence or absence of a notch was noted. Using SMI technology, the total number of SA imaged was recorded in a sagittal placental section at the level of cord insertion. The PI and peak systolic velocity (PSV) were also measured in a total of six SA, including two in the central portion of the placenta, two peripherally towards the uterine fundal portion, and two peripherally towards the lower uterine segment.

RESULTS

A total of 90 women fulfilled the study criteria. Maternal UtA-PI decreased throughout the first half of pregnancy from a mean ± SD of 1.39 ± 0.50 at 12-13 weeks' gestation to 0.88 ± 0.24 at 20-21 weeks' gestation. The mean number of SA visualized in a sagittal plane of the placenta increased from 8.83 ± 2.37 in the first trimester to 16.99 ± 3.31 in the late-third trimester. The mean SA-PI was 0.57 ± 0.12 in the first trimester and decreased progressively during the second trimester, reaching a nadir of 0.40 ± 0.10 at 24-25 weeks, and remaining constant until the end of pregnancy. SA-PSV was highest in early pregnancy with a mean of 57.16 ± 14.84 cm/s at 12-13 weeks' gestation, declined to a mean of 49.38 ± 17.88 cm/s at 20-21 weeks' gestation and continued to trend downward for the remainder of pregnancy, reaching a nadir of 34.50 ± 15.08 cm/s at 36-37 weeks' gestation. A statistically significant correlation was noted between SA-PI and UtA-PI (r = 0.5633; P < 0.001). Multilevel regression models with natural cubic splines were used to create reference ranges of SA-PSV and SA-PI for given gestational ages.

CONCLUSION

From early gestation, we have demonstrated the ability to image and quantify SA blood flow in normal pregnancy, and have developed reference ranges for use at various gestational ages throughout pregnancy. © 2023 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

The human placenta project: Funded studies, imaging technologies, and future directions

The placenta plays a critical role in fetal development. It serves as a multi-functional organ that protects and nurtures the fetus during pregnancy. However, despite its importance, the intricacies of placental structure and function in normal and diseased states have remained largely unexplored. Thus, in 2014, the National Institute of Child Health and Human Development launched the Human Placenta Project (HPP). As of May 2023, the HPP has awarded over $101 million in research funds, resulting in 41 funded studies and 459 publications. We conducted a comprehensive review of these studies and publications to identify areas of funded research, advances in those areas, limitations of current research, and continued areas of need. This paper will specifically review the funded studies by the HPP, followed by an in-depth discussion on advances and gaps within placental-focused imaging. We highlight the progress within magnetic reasonance imaging and ultrasound, including development of tools for the assessment of placental function and structure.

Synergistic regulation of uterine radial artery adaptation to pregnancy by paracrine and hemodynamic factors

Fetal growth throughout pregnancy relies on delivery of an increasing volume of maternal blood to the placenta. To facilitate this, the uterine vascular network adapts structurally and functionally, resulting in wider blood vessels with decreased flow-mediated reactivity. Impaired remodeling of the rate-limiting uterine radial arteries has been associated with fetal growth restriction. However, the mechanisms underlying normal or pathological radial artery remodeling are poorly understood. Here, we used pressure myography to determine the roles of hemodynamic (resistance, flow rate, shear stress) and paracrine [β-estradiol, progesterone, placental growth factor (PlGF), vascular endothelial growth factor] factors on rat radial artery reactivity. We show that β-estradiol, progesterone, and PlGF attenuate flow-mediated constriction of radial arteries from nonpregnant rats, allowing them to withstand higher flow rates in a similar manner to pregnant vessels. This effect was partly mediated by nitric oxide (NO) production. To better understand how the combination of paracrine factors and shear stress may impact human radial artery remodeling in the first half of gestation, computational models of uterine hemodynamics, incorporating physiological parameters for trophoblast plugging and spiral artery remodeling, were used to predict shear stress in the upstream radial arteries across the first half of pregnancy. Human microvascular endothelial cells subjected to these predicted shear stresses demonstrated higher NO production when paracrine factors were added. This suggests that synergistic effects of paracrine and hemodynamic factors induce uterine vascular remodeling and that alterations in this balance could impair radial artery adaptation, limiting blood flow to the placenta and negatively impacting fetal growth.NEW & NOTEWORTHY Placenta-specific paracrine factors β-estradiol, progesterone, and placental growth factor attenuate flow-mediated constriction of the rate-limiting uterine radial arteries, enabling higher flow rates in pregnancy. These paracrine factors induce their actions in part via nitric oxide mediated mechanisms. A synergistic combination of paracrine factors and shear stress is likely necessary to produce sufficient levels of nitric oxide during early human pregnancy to trigger adequate uterine vascular adaptation.

A review of feto-placental vasculature flow modelling

The placenta provides the vital nutrients and removal of waste products required for fetal growth and development. Understanding and quantifying the differences in structure and function between a normally functioning placenta compared to an abnormal placenta is vital to provide insights into the aetiology and treatment options for fetal growth restriction and other placental disorders. Computational modelling of blood flow in the placenta allows a new understanding of the placental circulation to be obtained. This structured review discusses multiple recent methods for placental vascular model development including analysis of the appearance of the placental vasculature and how placental haemodynamics may be simulated at multiple length scales.

Ferumoxytol-enhanced MR demonstration of changes to internal placental structure in placenta accreta spectrum: Preliminary findings

INTRODUCTION

The goal of this pilot study is to determine if ferumoxytol-enhanced MR might provide a new approach to the diagnosis of placenta accreta spectrum (PAS), and if so, to identify signs of PAS.

METHODS

Ten pregnant women were referred for MRI evaluation for PAS. MR studies consisted of pre-contrast SSFSE, SSFP, DWI, and ferumoxytol-enhanced sequences. Post-contrast images were rendered as MIP and MinIP images to separately display the maternal and fetal circulations respectively. Two readers examined the images for architectural changes to placentone (fetal cotyledon) that might distinguish PAS cases from normal. Attention was given to the size and morphology of the placentone, villous tree, and vascularity. In addition, the images were examined for evidence of fibrin/fibrinoid, intervillous thrombus, basal and chorionic plate bulges. Interobserver agreement was characterized with kappa coefficients and levels of confidence for feature identification was recorded on a 10-point scale.

RESULTS

At delivery, there were five normal placentas and five with PAS (one accreta, two increta, two percreta). The ten changes of placental architecture in PAS included: focal/regional expansion of placentone(s); lateral displacement and compression of the villous tree; disruption of a regular pattern of normal placentones; bulging of the basal plate; bulging of the chorionic plate; transplacental stem villi; linear/nodular bands at basal plate; non-tapering villous branches; intervillous hemorrhage; and dilated subplacental vessels. All these changes were more common in PAS; the first five achieved statistical significance in this small sample. The interobserver agreement and confidence for the identification of these features was good to excellent except for dilated subplacental vessels.

DISCUSSION

Ferumoxytol-enhanced MR imaging appears to depict derangements of the internal architecture of placentas with PAS, thereby suggesting a promising new strategy to diagnose PAS.

Multi-scale Modelling of Shear Stress on the Syncytiotrophoblast: Could Maternal Blood Flow Impact Placental Function Across Gestation?

The placenta is a critical fetal exchange organ, with a complex branching tree-like structure. Its surface is covered by a single multinucleated cell, the syncytiotrophoblast, which bathes in maternal blood for most of pregnancy. Mechanosensing protein expression by the syncytiotrophoblast at term suggests that shear stress exerted by maternal blood flow may modulate placental development and function. However, it is not known how the mechanosensitive capacity of the syncytiotrophoblast, or the shear stress it experiences, change across gestation. Here, we show that the syncytiotrophoblast expresses both mechanosensitive ion channels (Piezo 1, Polycystin 2, TRPV6) and motor proteins associated with primary cilia (Dynein 1, IFT88, Kinesin 2), with higher staining for all these proteins seen in late first trimester placentae than at term. MicroCT imaging of placental tissue was then used to inform computational models of blood flow at the placentone scale (using a porous media model), and at the villous scale (using explicit flow simulations). These two models are then linked to produce a combined model that allows the variation of shear stress across both these scales simultaneously. This combined model predicts that the range of shear stress on the syncytiotrophoblast is higher in the first-trimester than at term (0.8 dyne/cm² median stress compared to 0.04 dyne/cm²) when considering both these scales. Together, this suggests that the nature of blood flow through the intervillous space, and the resulting shear stress on the syncytiotrophoblast have important influences on placental morphogenesis and function from early in pregnancy.

Impact of tissue porosity and asymmetry on the oxygen uptake of the human placenta: A numerical study

INTRODUCTION

This study proposes a computational fluid dynamics model of a human placenta's independent exchange unit (placentome) to assess the effect that the inner villi distribution and decidual veins (DVs) location and number, have on the oxygen uptake.

METHODS

The internal placentome porosity distribution was altered in symmetric morphology, while asymmetry was introduced by varying the location and number of DVs. The DV asymmetry was introduced by either displacing them circumferentially, thereby changing the angle between them, or by adding DVs in the inlet cross-section. The results were analyzed by the changes in the normalized oxygen mass fraction and the oxygen uptake.

RESULTS

Oxygenated blood was shown to be delivered deeper into the placentome when the area of non-homogeneous porosity was larger. The largest oxygen uptake was achieved in the asymmetric model with the smallest angle distance between the DVs, where a 10% decrease relative to the farthest case was obtained. Placing DVs adjacent to the spiral artery opening enhanced the drainage of oxygenated blood.

DISCUSSION

This study demonstrates the importance of the local porosity distribution for the proper perfusion of the intervillous space and proposes a novel approach to improve our understanding of the role of the DVs in placental oxygen uptake.

Longitudinal assessment of spiral artery and intravillous arteriole blood flow and adverse pregnancy outcome

OBJECTIVE

Superb microvascular imaging (SMI) has been shown to improve visualization of small vessels by suppressing global motions while preserving low-flow components, such as the microvessels in the placenta. We sought to determine if SMI-aided visualization of flow velocity waveforms in the spiral arteries (SA) and intravillous fetal arterioles (IVA) could predict fetal growth restriction (FGR), gestational hypertension (GH) and/or pre-eclampsia (PE).

METHODS

This was a prospective longitudinal study of singleton pregnancies without fetal anomaly, receiving prenatal care in one of two medical centers over a 5-year period. Using SMI-aided color Doppler, SA and IVA flow velocity was measured at three timepoints: 11 + 0 to 14 + 0, 18 + 0 to 22 + 6 and 28 + 0 to 34 + 6 weeks of gestation. SA and IVA flow velocity waveforms were reported as resistance indices (RI). RI values were analyzed using multilevel modeling; individual regression curves were estimated and combined to obtain the reference intervals for SA-RI and IVA-RI in uncomplicated pregnancies. The primary clinical outcome was FGR and secondary outcomes were PE and GH. FGR was defined as estimated fetal weight < 10^th percentile. Student's t-test was used to compare deviation from expected RI between normal and complicated pregnancies.

RESULTS

Among 540 pregnancies included in the analysis, 18 (3.3%) had FGR, 31 (5.7%) PE and 61 (11.3%) GH. In uncomplicated pregnancies, the SA-RI decreased progressively with advancing gestation, whereas the IVA-RI increased with gestational age. In the third trimester, the mean SA-RI and IVA-RI values were significantly higher in the FGR group compared with pregnancies that did not develop FGR, while the mean SA-RI was significantly higher in PE compared with non-PE pregnancies. There was no significant difference in mean SA-RI or IVA-RI between pregnancies with vs those without GH at any gestational age. When all three adverse outcomes were combined, SA-RI was significantly higher in pregnancies with these outcomes when compared to uncomplicated pregnancies in the third trimester (mean ± SD, 0.29 ± 0.12 vs 0.26 ± 0.12; P = 0.02). In screening for FGR using SA-RI, the areas under the receiver-operating-characteristics curves (AUC) were 0.68, 0.73 and 0.73 in the first, second and third trimesters, respectively. The respective AUCs for IVA-RI were 0.72, 0.72 and 0.73 for each trimester.

CONCLUSIONS

SA-RI and IVA-RI, measured using SMI technology, were significantly higher in pregnancies at risk for FGR in late gestation. Larger studies are needed to determine if SA and IVA flow are reliable predictors of adverse pregnancy outcome. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

From stem cells to spiral arteries: A journey through early placental development

Early placental development lays the foundation of a healthy pregnancy, and numerous tightly regulated processes must occur for the placenta to meet the increasing nutrient and oxygen exchange requirements of the growing fetus later in gestation. Inadequacies in early placental development can result in disorders such as fetal growth restriction that do not present clinically until the second half of gestation. Indeed, growth restricted placentae exhibit impaired placental development and function, including reduced overall placental size, decreased branching of villi and the blood vessels within them, altered trophoblast function, and impaired uterine vascular remodelling, which together combine to reduce placental exchange capacity. This review explores the importance of early placental development across multiple anatomical aspects of placentation, from the stem cells and lineage hierarchies from which villous core cells and trophoblasts arise, through extravillous trophoblast invasion and spiral artery remodelling, and finally remodelling of the larger uterine vessels.

The Role of the 3Rs for Understanding and Modeling the Human Placenta

Modeling the physiology of the human placenta is still a challenge, despite the great number of scientific advancements made in the field. Animal models cannot fully replicate the structure and function of the human placenta and pose ethical and financial hurdles. In addition, increasingly stricter animal welfare legislation worldwide is incentivizing the use of 3R (reduction, refinement, replacement) practices. What efforts have been made to develop alternative models for the placenta so far? How effective are they? How can we improve them to make them more predictive of human pathophysiology? To address these questions, this review aims at presenting and discussing the current models used to study phenomena at the placenta level: in vivo, ex vivo, in vitro and in silico. We describe the main achievements and opportunities for improvement of each type of model and critically assess their individual and collective impact on the pursuit of predictive studies of the placenta in line with the 3Rs and European legislation.

Male fetal sex affects uteroplacental angiogenesis in growth restriction mouse model†

Abnormally increased angiotensin II activity related to maternal angiotensinogen (AGT) genetic variants, or aberrant receptor activation, is associated with small-for-gestational-age babies and abnormal uterine spiral artery remodeling in humans. Our group studies a murine AGT gene titration transgenic (TG; 3-copies of the AGT gene) model, which has a 20% increase in AGT expression mimicking a common human AGT genetic variant (A[-6]G) associated with intrauterine growth restriction (IUGR) and spiral artery pathology. We hypothesized that aberrant maternal AGT expression impacts pregnancy-induced uterine spiral artery angiogenesis in this mouse model leading to IUGR. We controlled for fetal sex and fetal genotype (e.g., only 2-copy wild-type [WT] progeny from WT and TG dams were included). Uteroplacental samples from WT and TG dams from early (days 6.5 and 8.5), mid (d12.5), and late (d16.5) gestation were studied to assess uterine natural killer (uNK) cell phenotypes, decidual metrial triangle angiogenic factors, placental growth and capillary density, placental transcriptomics, and placental nutrient transport. Spiral artery architecture was evaluated at day 16.5 by contrast-perfused three-dimensional microcomputed tomography (3D microCT). Our results suggest that uteroplacental angiogenesis is significantly reduced in TG dams at day 16.5. Males from TG dams are associated with significantly reduced uteroplacental angiogenesis from early to late gestation compared with their female littermates and WT controls. Angiogenesis was not different between fetal sexes from WT dams. We conclude that male fetal sex compounds the pathologic impact of maternal genotype in this mouse model of growth restriction.

Hemodynamic consequences of incomplete uterine spiral artery transformation in human pregnancy, with implications for placental dysfunction and preeclampsia

Normal human pregnancy requires a dramatic increase in uteroplacental blood flow, which is achieved by a transformation in the geometry of uterine spiral arteries, a key element in this blood supply system. The transformation is mediated by trophoblast invasion directed at converting a portion of the spiral artery into an open funnel, thereby greatly reducing resistance to flow. The converted portion lies within the depth of the decidua and part of the myometrium. Insufficient depth of trophoblast invasion in early pregnancy predisposes to inadequate perfusion of the developing placenta and fetus and may lead to preeclampsia, fetal growth restriction, and preterm delivery, sometimes referred to as the "Great Obstetrical Syndromes." We examine the hemodynamic consequences of spiral artery transformation in human pregnancy and the relationship between the degree of transformation and the corresponding change in flow rate and resistance to flow. We identify two key variables in determining the hemodynamic change: the longitudinal converted fraction of the spiral artery and the relative downstream diameter of the open funnel. Our results indicate that there is a critical threshold in the value of the converted fraction required to achieve the marked increase in uteroplacental blood flow in normal pregnancy. This finding validates common clinical observations that the depth of trophoblast invasion reflects the "adequacy" of the increase in uteroplacental blood supply required in normal human pregnancy. Our results provide a quantitative measure of that adequacy and may serve as a future diagnostic marker for high-risk pregnancy.NEW & NOTEWORTHY Human pregnancy requires dramatic increase in uteroplacental blood supply achieved by geometric transformation of uterine spiral arteries and facilitated by trophoblast invasion of these arteries to greatly reduce resistance to flow. Incomplete transformation has been associated with failed pregnancies, preeclampsia, and other pathologies, but a quantitative measure of "incompleteness" has been unavailable so far. We use a mathematical model to obtain a numerical threshold for this measure which may serve as a future diagnostic marker.

Spiral artery blood flow during pregnancy: a systematic review and meta-analysis

Background

Downstream remodeling of the spiral arteries (SpA) decreases utero-placental resistance drastically, allowing sustained and increased blood flow to the placenta under all circumstances. We systematically evaluated available reports to visualize adaptation of spiral arteries throughout pregnancy by ultra-sonographic measurements and evaluated when this process is completed.

Methods

A systematic review and meta-analysis of spiral artery flow (pulsatility index (PI), resistance index (RI) and peak systolic velocity (PSV)) was performed. English written articles were obtained from Pubmed, EMBASE and Cochrane Library and included articles were assessed on quality and risk of bias. Weighted means of Doppler indices were calculated using a random-effects model.

Results

In healthy pregnancies, PI and RI decreased from 0.80 (95% CI: 0.70–0.89) and 0.50 (95% CI: 0.47–0.54) in the first trimester to 0.50 (95% CI: 0.45–0.55, p < 0.001) and 0.39 (95% CI: 0.37–0.42, p < 0.001) in the second trimester and to 0.49 (95% CI: 0.44–0.53, p = 0.752) and 0.36 (95% CI: 0.35–0.38, p = 0.037) in the third trimester, respectively. In parallel, PSV altered from 0.22 m/s (95% CI: 0.13–0.30 m/s) to 0.28 m/s (95% CI: 0.17–0.40 m/s, p = 0.377) and to 0.25 m/s (95% CI: 0.20–0.30 m/s, p = 0.560) in the three trimesters. In absence of second and third trimester Doppler data in complicated gestation, only a difference in PI was observed between complicated and healthy pregnancies during the first trimester (1.49 vs 0.80, p < 0.001). Although individual studies have identified differences in PI between SpA located in the central part of the placental bed versus those located at its periphery, this meta-analysis could not confirm this (p = 0.349).

Conclusions

This review and meta-analysis concludes that an observed decrease of SpA PI and RI from the first towards the second trimester parallels the physiological trophoblast invasion converting SpA during early gestation, a process completed in the midst of the second trimester. Higher PI was found in SpA of complicated pregnancies compared to healthy pregnancies, possibly reflecting suboptimal utero-placental circulation. Longitudinal studies examining comprehensively the predictive value of spiral artery Doppler for complicated pregnancies are yet to be carried out.

Computational modeling of the interactions between the maternal and fetal circulations in human pregnancy

In pregnancy, fetal growth is supported by its placenta. In turn, the placenta is nourished by maternal blood, delivered from the uterus, in which the vasculature is dramatically transformed to deliver this blood an ever increasing volume throughout gestation. A healthy pregnancy is thus dependent on the development of both the placental and maternal circulations, but also the interface where these physically separate circulations come in close proximity to exchange gases and nutrients between mum and baby. As the system continually evolves during pregnancy, our understanding of normal vascular anatomy, and how this impacts placental exchange function is limited. Understanding this is key to improve our ability to understand, predict, and detect pregnancy pathologies, but presents a number of challenges, due to the inaccessibility of the pregnant uterus to invasive measurements, and limitations in the resolution of imaging modalities safe for use in pregnancy. Computational approaches provide an opportunity to gain new insights into normal and abnormal pregnancy, by connecting observed anatomical changes from high-resolution imaging to function, and providing metrics that can be observed by routine clinical ultrasound. Such advanced modeling brings with it challenges to scale detailed anatomical models to reflect organ level function. This suggests pathways for future research to provide models that provide both physiological insights into pregnancy health, but also are simple enough to guide clinical focus. We the review evolution of computational approaches to understanding the physiology and pathophysiology of pregnancy in the uterus, placenta, and beyond focusing on both opportunities and challenges. This article is categorized under: Reproductive System Diseases >Computational Models.

Blood flow and transport in the human placenta

The placenta is a multi-functional organ that exchanges blood gases and nutrients between a mother and her developing fetus. In humans, fetal blood flows through intricate networks of vessels confined within villous trees, the branches of which are bathed in pools of maternal blood. Fluid mechanics and transport processes play a central role in understanding how these elaborate structures contribute to the function of the placenta, and how their disorganization may lead to disease. Recent advances in imaging and computation have spurred significant advances in simulations of fetal and maternal flows within the placenta, across a range of lengthscales. Models describe jets of maternal blood emerging from spiral arteries into a disordered and deformable porous medium, and solute uptake by fetal blood flowing through elaborate three-dimensional capillary networks. We survey recent developments and emerging challenges in modeling flow and transport in this complex organ.

Advances in Human Placental Biomechanics

Pregnancy complications are a major clinical concern due to the related maternal and fetal morbidity. Many are caused through defective placentation, but research into placental function is difficult, principally because of the ethical limitations associated with the in-vivo organ and the difficulty of extrapolating animal models. Perfused by two separate circulations, the maternal and fetal bloodstreams, the placenta has a unique structure and performs multiple complex functions. Three-dimensional imaging and computational modelling are becoming popular tools to investigate the morphology and physiology of this organ. These techniques bear the potential for better understanding the aetiology and development of placental pathologies, however, their full potential is yet to be exploited. This review aims to summarize the recent insights into placental structure and function by employing these novel techniques.

Geometric and hemodynamic characterization of uterine spiral arteries: The concept of resistance reserve

BACKGROUND

The coiled geometry of spiral arteries in the human uteroplacental circulation is a hemodynamic enigma because of added length of a spiral artery compared with that of a straight artery, as well as added complexity of the flow within the vessel because of the coiling curvature.

METHODS

We examined the geometric and hemodynamic characteristics of mathematically defined helical and spiral arteries and compared these with the corresponding characteristics of a straight artery traversing the same depth of tissue, with the aim of gaining some insight into the possible role of spiral geometry in uteroplacental perfusion.

RESULTS

The results indicate that the added length of a spiral artery provides the uteroplacental circulation with a reserve of high resistance to flow. The effect of coiling geometry on the flow within the artery is the development of churning vortices in planes normal (perpendicular) to the main flow direction.

CONCLUSIONS

In the early stages of pregnancy the reserve of high resistance is intact, thus keeping blood supply low. As pregnancy progresses, the reserve is gradually purged by trophoblast invasion and transformation of the distal portion of the spiral artery into an open funnel, thus providing the required high blood supply. The development of churning vortices within the spiral artery support earlier suggestions in the literature that the "spurts" of maternal blood emerging from these arteries may play a role in shaping the anatomy of the villous trees among placental lobules.

Understanding abnormal uterine artery Doppler waveforms: A novel computational model to explore potential causes within the utero-placental vasculature

INTRODUCTION

Uterine artery (UtA) Doppler indices are one of the most commonly employed screening tests for pre-eclampsia worldwide. Abnormal indices appear to result from increased uterine vascular resistance, but anatomical complexity and lack of appropriate animal models mean that little is known about the relative contribution of each of the components of the uterine vasculature to the overall UtA Doppler waveform. Previous computational models suggested that trophoblast-mediated spiral artery remodeling has a dominant effect on the UtA Doppler waveform. However, these models did not incorporate the myometrial arterio-venous anastomoses, which have significant potential to affect utero-placental haemodynamics.

METHODS

We present a more anatomically complete computational model, explicitly incorporating a structural description of each component of the uterine vasculature, and crucially including myometrial arterio-venous anastomoses as parallel pathways for blood-flow away from the placental bed. Wave transmission theory was applied to the network to predict UtA waveforms.

RESULTS

Our model shows that high UtA resistance indices, combined with notching, reflect an abnormal remodeling of the entire uterine vasculature. Incomplete spiral artery remodeling alone is unlikely to cause abnormal UtA Doppler waveforms as increased resistance in these arteries can be 'buffered' by upstream anastomoses. Critically, our results indicate that the radial arteries, may have a more important effect on utero-placental flow dynamics, and the UtA Doppler waveform than previously thought.

CONCLUSIONS

This model suggests that to appropriately interpret UtA Doppler waveforms they must be considered to be reflecting changes in the entire system, rather than just the spiral arteries.