The sFlt-1/PlGF ratio in fetal growth restriction, stillbirth and preterm birth

Introduction

Preeclampsia is a pregnancy complication characterized by high blood pressure and signs of damage to other organ systems, most often the liver and kidneys. It usually begins after 20 weeks of pregnancy in women whose blood pressure had been normal. Preeclampsia can lead to serious, even fatal, complications for both the mother and baby if not managed⁽¹⁾.

The sFlt-1/PlGF ratio is a biomarker used in the prediction, diagnosis and management of hypertensive disorders of pregnancy, particularly preeclampsia. Soluble Fms-like tyrosine kinase-1 (sFlt-1) is a circulating anti-angiogenic factor that binds to and sequestrates placental growth factor (PlGF) and vascular endothelial growth factor (VEGF). PlGF, on the other hand, is a pro-angiogenic factor secreted by the placenta. The balance between these factors is crucial for maintaining normal placental vascularization and endothelial function during pregnancy⁽²⁾.

In preeclampsia, there is an imbalance between angiogenic and anti-angiogenic factors, leading to decreased levels of PlGF and increased levels of sFlt-1. The sFlt-1/PlGF ratio is calculated by dividing the concentration of sFlt-1 by the concentration of PlGF in maternal serum. An elevated sFlt-1/PlGF ratio is associated with an increased risk of developing preeclampsia within the subsequent four weeks⁽²⁾.

Clinically, the sFlt-1/PlGF ratio has shown promise as a predictive tool for the onset of preeclampsia and as a prognostic indicator for disease severity. It is used to stratify patients into low-, intermediate- and high-risk categories, and in guiding clinical decision-making regarding the need for further monitoring, intervention or delivery⁽³⁾.

Maternal plasma concentrations of angiogenic/antiangiogenic factors in the third trimester of pregnancy can help identify patients at risk for stillbirth at or near term and severe late preeclampsia⁽⁴⁾.

The sFlt-1/PlGF ratio and fetal growth restriction

Fetal growth restriction (FGR) refers to a condition in which an unborn baby is smaller than expected for the number of weeks of pregnancy. It is often caused by problems with the placenta, leading to reduced oxygen and nutrient supply to the fetus. FGR increases the risk of complications during pregnancy and childbirth, and it can have long-term health implications for the baby⁽⁵⁾.

It is important to highlight the fact that the sFlt-1/PlGF ratio has implications not only in preeclampsia (PE), but also in other conditions such as FGR and, more importantly, in pregnancies which have an overlap of the two clinical entities^(6-8). In a systematic review and meta-analysis done by Chen et al., covering eight studies, it has been shown that the sFlt-1/PlGF ratio was notably increased in the FGR+PE group compared to the control patients. The combined findings revealed that the ratio exhibited high sensitivity and specificity in predicting FGR and FGR+PE. Furthermore, it showed a notably higher positive likelihood ratio and a high positive predictive value for identifying cases with FGR and FGR+PE⁽⁹⁾.

A very interesting finding was noted in a study done by Gaccioli et al., published in 2023, that highlighted the fact that placental sFlt-1 and PlGF differ in their contribution to the increased maternal serum sFlt-1/PlGF ratio encountered in pregnancies complicated by FGR and PE. In preeclampsia, the ratio is mainly influenced by raised levels of placental sFlt-1, in contrast with FGR cases in which the ratio is driven by decreased levels of placental PlGF. This shows that, even though the two clinical entities may exhibit overlapping clinical features, the underlying mechanisms behind them are frequently different⁽¹⁰⁾.

Herraiz et al. reported the fact that a value higher than 655 of the sFlt-1/PlGF ratio can be determined 48 hours prior to delivery in 65% of the cases which present both FGR and PE; however, the same finding can only be identified in 8% of the pregnancies which are diagnosed only with fetal growth restriction^(6,11).

Research performed by Papastefanou et al. showed the advantages of a new competing-risks model for the prediction of small-for-gestational-age (SGA) babies with the use of maternal characteristics, medical history, measurement of blood pressure, uterine artery Doppler Pi (UtA Pi) and maternal serum PAPP-A and PlGF at the first-trimester ultrasound (11-13 weeks). Therefore, cases who would be deemed as high risk would benefit from the prophylactic use of aspirin in order to prevent preterm preeclampsia and early-onset SGA. PlGF has been shown to also be a better predictor of SGA than PAPP-A particularly in cases where preeclampsia would be also diagnosed^(6,12-15).

A comprehensive study also looked into sFlt-1 and found that low values measured at 10-14 weeks of gestation were linked to a higher risk of SGA, preterm delivery and stillbirth developing in the pregnancy^(6,16,17). Furthermore, a study which included 5000 low-risk patients revealed that maternal levels of sFlt-1 and PAPP-A were lower in pregnancies diagnosed with SGA, but not complicated by maternal hypertension, while PAPP-A and PlGF were lower in pregnancies involving both findings^(6,16,18).

Sovio et al. discovered that maternal serum levels of PAPP-A, PlGF and sFlt-1 were lower in the first trimester in patients who would end up having adverse perinatal outcomes: fetal growth restriction, severe SGA, spontaneous and induced preterm birth, whereas higher levels of sFlt-1 were linked with a pregnancy loss before the fetus would become viable^(6,19).

The utility of sFlt-1 measured between 19 to 24 weeks of gestation as a biomarker for SGA fetuses is constrained by its limited variance in smaller babies and its temporal fluctuations until 32 weeks. Moreover, the incorporation of the sFlt-1/PlGF ratio diminishes the screening performance achieved by PlGF alone. Consequently, second-trimester measurement of sFlt-1, in either form, is unlikely to offer significant clinical utility in predicting SGA⁽²⁰⁾. However, using maternal risks factors, estimated fetal weight (EFW), UtA Pi and PlGF yields effective second-trimester prediction of SGA. Serum PlGF emerges as a valuable tool for anticipating SGA babies with a birthweight (BW) below the 3^rd centile born before 30 weeks, following a comprehensive evaluation encompassing maternal risk factors and biophysical markers⁽²¹⁾.

Findings in a paper published by Álvarez et al. indicate that applying a cutoff value of 38 for the sFlt-1/PlGF ratio in the second trimester has limited sensitivity and a low negative predictive value (NPV) for predicting overall placental complications and that lowering the cutoff values could enhance predictive values. For example, a cutoff of 10 in high-risk pregnancies would help to increase sensitivity from 32.1% to 69.8% and the NPV from 63.3% to 76.8%, at the expense of having a reduced specificity (84.1% instead of 98.4%) and a reduced positive predictive value (78.7% instead of 94.4%). Of course, the diagnostic accuracy varies with regards to the complication that is being studied, but the author suggests that lowering the cutoff point for predicting preeclampsia could achieve a 98.6% NPV, while at the same time being able to pick out a subgroup of pregnant women who are unlikely to experience complications and may not require rigorous monitoring⁽²²⁾.

Rodriguez-Calvo et al. established, in a study involving 210 cases fetuses, that combining EFW and maternal levels of PlGF can predict accurately perinatal survival in fetuses with early-onset FGR diagnosed before 28 weeks of gestation. A total of 185 fetuses out of 210 survived, and the PlGF levels in the cases who survived compared to the ones that demised were higher (41 versus 18 pg/mL) and, also, the sFlt-1/PlGf ratio was lower (129 versus 479). The group also mentioned that, compared with the STRIDER study, their results did not find a significant role of sFlt-1 in the prediction of early-onset fetal growth restriction, but highlighted that the marker is very useful in cases that overlap with preeclampsia^(23,24).

Herraiz et al. also stated that measuring the sFlt-1/PlGf ratio between 24 and 28 weeks in certain patients, selected based on risk factors and UtA Pi assessment, leads to precise prediction of PE and FGR, particularly in early-onset cases^(6,7).

Dröge et al. went even further and demonstrated that, by using the sFlt-1/PlGf ratio cutoffs below 38 (low risk), between 38 and 85 (intermediate) and above 85 (high risk), in a model with multiple markers, would help in determining populations who are more prone to develop preeclampsia-related adverse outcomes^(6,25).

An important finding regarding the management of SGA and FGR pregnancies was suggested by Bonacina et al. in a study that evaluated 134 singleton pregnancies with an EFW<10^th centile and positive end-diastolic flow (EDF) in the umbilical artery between 20+0 and 31+6 weeks of gestation. Their group concluded that, in FGR cases with an sFlt-1/PlGF ratio exceeding 85, the doctors should undergo close monitoring for the early detection of either fetal or maternal complications. In pregnancies with a ratio below 38, it may be feasible to extend the interval between the ultrasound scans to up to three weeks for the SGA fetuses and up to two weeks for the FGR cases with positive EDF in the umbilical artery, which ultimately could lead to fewer ultrasound scans required⁽²⁶⁾.

Another group, led by Quezada et al., analyzed the time of delivery for fetuses diagnosed with early-onset FGR with positive EDF and without the presence of structural abnormalities. They wanted to establish the usefulness of the sFlt-1/PlGF ratio in the prediction of a suitable timeframe for the delivery of the patients. Thus, 75% of the patients had a ratio over the cutoff of 85, and 36% of these had to be delivered within one week, whereas 72.4% of the cases with a ratio value of less than 85 could continue the pregnancy for at least one month^(6,27,28).

In a study led by Garcia-Manau et al., it has been shown that 126 out of 207 cases diagnosed with FGR in any stage had higher levels of sFlt-1/PlGF ratio compared with the ones measured in the SGA and control group, and they exhibited notably worse outcomes. Further­more, median values showed significant variation across different FGR stages (9.76 for stage I, 284.3 for stage II and 625.02 for stage III), increasing with the severity of FGR, along with a rise in the occurrence of adverse pregnancy outcomes^(6,29).

In a prospective study, with a population of 287 patients, Kumar et al. observed that 54 cases (18.8%) experienced severe adverse outcomes (SAO): severe pre­eclampsia (21; 7.3%), fetal growth restriction with absent or reversed EDF Doppler (23; 8%), and intrauterine or neonatal death (10, 3.5%). The sFlt-1/PlGF ratio measured at 20 weeks with a cutoff higher than 38 proved to be the most accurate test (accuracy of 97.6%) for detecting complications up until 30 weeks of gestation, followed by mean arterial pressure (MAP) and UtA Pi. Furthermore, for the detection of complications until 34 weeks, combining the sFlt-1/PlGF ratio with the UtA Pi resulted in a good prediction (accuracy of 80.4%); however, the predictive value was notably higher for complications occurring before 34 weeks compared to the ones that appeared after that period⁽³⁰⁾.

An increased sFlt-1/PlGF ratio signals swifter fetal deterioration and an earlier onset of delivery or fetal demise, particularly in cases of early fetal growth restriction. This association remains strong even after excluding deliveries prompted by maternal indications and adjusting for potential confounders, such as the occurrence of preeclampsia, gestational age at the time of the ratio measurement, maternal age and smoking during pregnancy. Notably, when evaluating the cutoff specified in the existing literature for ratio positivity, a positive ratio reveals a significant connection with expedited delivery/fetal demise and reduced latency to these events. Furthermore, Palma Dos Reis et al. underline that the sFlt-1/PlGF ratio serves as a promising predictor of impending delivery due to fetal deterioration within the subsequent week⁽³¹⁾.

Between 30 and 34 weeks of gestation, reduced levels of PlGF and increased levels of sFlt-1 were in correlation with SGA delivering <37 weeks of gestation (detection rate of 51%), and ≥37 weeks of gestation (detection rate of 88%), at a 10% false positive rate. The prediction was established by associating the biomarkers with the ultrasound markers: EFW, UtA Pi, umbilical artery Pi, and fetal middle cerebral artery (MCA) Pi^(16,32).

In a similar study, at 35-37 weeks, the detection rate for SGA was 63%, with a 10% false positive rate, and was established using EFW, PlGF (characterized by lower values) and UtA Pi, which were the main contributors. The same study established the fact that the measurement of the placental markers of impaired placentation was not very effective in assessing the risk of perinatal morbidity, mainly due to the fact that ultrasound scan findings interfered with the obstetricians’ overall management plan^(16,33).

However, incorporating PlGF, sFlt-1, mean UtA Pi, umblical artery Pi and fetal MCA Pi alongside maternal factors and fetal biometry was proven by Ciobanu et al. to only slightly enhance the predictive accuracy for delivering SGA babies, compared to the predictive performance achieved by maternal factors and fetal biometry at 35-37 weeks of gestation^(6,34).

In contrast, a study done by Gaccioli et. al suggested that adding ultrasound EFW and sFlt-1/PlGF ratio at 28 weeks of gestation resulted in a higher positive likelihood ratio (41.1 versus 5.9) and greater specificity (99.1% versus 92.2%) in predicting preterm delivery of an SGA baby compared to using only EFW. However, this combined approach showed lower sensitivity (38.5% versus 46.2%)^(6,35).

A group from Poland led by Dymara-Konopka et al. observed a positive correlation between the levels of PlGF and the ultrasound EFW not only in all the cases that were diagnosed with preeclampsia, but also in pregnancies that had fetal growth restriction, either isolated, or accompanied by preeclampsia. On the other hand, sFlt-1 levels and the measurements of the MCA and cerebroplacental ratio (CPR) were discovered to be interrelated in the isolated FGR group, suggesting the fact that a quick or elevated rise in the levels of sFlt-1 might be associated with fetal risk⁽³⁶⁾.

Maternal plasma PlGF levels below the 5^th centile for gestational age demonstrated excellent sensitivity (98.2%), reasonable specificity (75.1%), high negative predictive value (99.2%), and modest positive predictive value (58.5%) in the detection of fetuses with placental fetal growth restriction. These statistics, alongside the low negative likelihood ratio (0.02), suggest that a concentration of PlGF below the 5^th centile for gestational age could serve as valuable “rule-out test” for placental FGR, especially among patients presenting before 35 weeks of gestation. Notably, reduced levels of PlGF outperformed gestational age, abdominal circumference and umbilical artery resistance index in the prediction of placental fetal growth restriction⁽³⁷⁾.

In a retrospective study with a population of 233 singleton pregnancies, Rajiv et al. found that the sFlt-1/PlGF ratio exhibited significantly higher levels in SGA fetuses with FGR compared to SGA fetuses without FGR, regardless of the presence of preeclampsia. Moreover, there was a negative correlation between the sFlt-1/PlGF ratio and FGR BW percentiles, indicating its potential as a clinical marker for assessing the severity of fetal growth restriction⁽³⁸⁾.

A study by Griffin et al., involving 274 women who were seen between 20+0 and 34+6 gestational weeks, with a clinical suspicion of preeclampsia, has highlighted the high sensitivity (93%) and NPV 90% of the PlGF measurement (<100 pg/mL) in the prediction of SGA-3 babies (<3^rd centile) in comparison to the use of ultrasound (71% sensitivity and 79% NPV). Moreover, the group reported similar results for 123 cases which were seen between 35+0 and 36+6 gestational weeks^(6,39).

The sFlt-1/PlGF ratio and stillbirth

Stillbirth is defined as the death of a fetus in the womb after 20 weeks of gestation but before delivery. It is a devastating event for expectant parents, and it has significant emotional, psychological and sometimes physical implications. The causes of stillbirth can be multifactorial and may include placental abnormalities, fetal genetic disorders, maternal health conditions, infections, umbilical cord accidents, and lifestyle factors such as smoking or drug use during pregnancy. The diagnosis of stillbirth is typically confirmed through ultrasound or other imaging modalities, and the management involves supportive care for the parents and evaluation to determine the cause whenever possible. Understanding the risk factors and implementing appropriate prenatal care measures can help reduce the incidence of stillbirth^(40-42).

There is a bit of controversy in the literature regarding the relationship between the level of maternal serum PlGF and sFlt-1, and their role in predicting the risk of stillbirth. PlGF was highlighted as the most precise marker at predicting stillbirth, having a diagnostic odds ratio of 49.2 (95% CI; 12.7-191), in a large systematic review of 21 studies with a total of 740 stillbirths. Another finding by the group was that ultrasound EFW was the most accurate test to determine SGA, with a diagnostic odds ratio of 21.3 (95% CI 13.1-34.6), and that hPL (human placental lactogen) was the most precise biomarker, with a diagnostic odds ratio of 4.78 (95% CI; 3.21-7.13) in a dataset of 15,471 SGA babies^(6,43).

In contrast, a group led by Jacobs et al. stated the fact that there was no conclusive evidence regarding the levels of sFlt-1 measured in the first trimester and the prediction of adverse pregnancy outcomes, including stillbirth^(6,44). Moreover, in another systematic review, maternal age, parity and previous adverse pregnancy history were shown to be superior predictors of stillbirth, compared to any of the other biomarkers measured, including PlGF^(6,45).

A predictive model incorporating maternal factors, PlGF, UtA PI and fetal ductus venosus pulsatility index for veins (DV-PIV) at 11-13 weeks of gestation was demonstrated to have promising potential in anticipating approximately 60% of stillbirths attributed to impaired placentation, while maintaining a 10% FPR. The model’s screening efficacy is notably higher for stillbirths occurring before 32 weeks of gestation (71%) compared to those at term (46%)⁽⁴⁶⁾.

A study by Chaiworapongsa et al. examined 11 fetal deaths cases, with three occurring before 28 weeks of gestation. The results showed that a maternal plasma angiogenic index-1 value (PlGF/sVEGFR-1 or sFlt-1 ratio) below the 2.5^th centile between 24 and 28 weeks was associated with a 29-fold increase in the risk of subsequent fetal death, identifying 55% of at-risk patients, with a low false positive rate of 3.7%. Additionally, 61% of women with false positive results experienced adverse pregnancy outcomes like preeclampsia or SGA babies, underlining the fact that the maternal plasma angiogenic index-1 measured during 24-28 weeks of gestation emerged as a biomarker to detect women at risk of fetal death or other obstetrical complications⁽⁴⁷⁾.

The sFlt-1/PlGF ratio and preterm birth

Several studies have sought to determine the relationship between PlGF levels and the sFlt-1/PlGF ratio in order to assess the risk of preterm birth for FGR fetuses in comparison with those with a normal predicted weight. Hong et al. showed that the early FGR population exhibited lower median of PlGF levels (54 versus 229 ng/L; p<0.001), a higher median of sFlt-1 levels (2774 ng/L versus 2096 ng/L; p<0.001), and correspondingly elevated sFlt-1/PlGF ratio (35 versus 10; p<0.001). Both a PlGF level below 100 ng/L and an elevated sFlt-1/PlGF ratio were robust predictors of preterm birth (PTB), including PTB within one, two or three weeks of diagnosis. In both the FGR and AGA (appropriate for gestational age) populations, a PlGF level below 100 ng/L or an increased sFlt-1/PlGF ratio significantly correlated with a heightened risk of medically indicated PTB⁽⁴⁸⁾.

Maternal levels of PlGF were more reduced, and the maternal levels of AFP were elevated at 19-24 weeks of gestation in the patients who would later deliver an SGA baby prematurely. In contrast, lower levels of PlGF, sFlt-1 and PAPP-A were linked with the cases of SGA babies delivered at term^(16,49).

Barton et al. described the relationship between PlGF values and preterm birth in a study with a population of 753 patients, seen between 20+0 and 35+0 gestational weeks. A PlGF measurement below 100 pg/ml is heavily associated with a spontaneous or induced preterm delivery, regardless of whether the patient will be eventually diagnosed with preeclampsia or the gestational age at which the condition may be identified. In contrast, a measurement above 100 pg/ml means that the pregnancy can continue, even though preeclampsia will be diagnosed at a later stage. In the patients who delivered after two weeks from the assessment, 89% received a diagnosis of preeclampsia, with 92% having low PlGF levels and being more at risk of adverse perinatal outcomes. However, only 8% of the women who were diagnosed with preeclampsia within two weeks of joining the study had normal PlGF measurements, and these specific cases had no perinatal deaths and a low incidence of SGA babies. These findings would lead to the conclusion that low levels of PlGF in patients suspected of having preeclampsia are indicative of a high risk for adverse pregnancy outcomes, regardless of the final diagnosis of preeclampsia^(6,50).

Conclusions

Although the clinical utility of sFlt-1, PlGF and the sFlt-1/PlGF ratio is high, and it has been known to be valuable in pregnancies complicated by preeclampsia, further research with a higher number of cases needs to be done in order to implement the biomarkers in guidelines for other issues, such as fetal growth restriction in singleton and in twin pregnancies, stillbirth, and other adverse perinatal outcomes.   n

Corresponding author: Anca-Maria Panaitescu, e-mail: anca.panaitescu@umfcd.ro

Conflict of interests: none declared.

Financial support: none declared.

This work is permanently accessible online free of charge and published under the CC-BY.