Venous thromboembolism (VTE) is a common condition with an average incidence of 1 to 2 cases per 1000 pregnancies, including pulmonary embolism (PE) and deep vein thrombosis (DVT) [1, 2]. Physiological and hormonal changes during pregnancy and the postpartum period can increase the risk of VTE. Compared with non-pregnant women, the incidence of VTE in antepartum and postpartum women is 3–10 times and 12–35 times higher, respectively [3, 4]. Although it is potentially preventable, the incidence of obstetric VTE has been increasing yearly in China [5]. Some women still develop VTE despite receiving prophylaxis, with the underlying reasons remaining unclear. Currently, significant controversy surrounds the use of thromboprophylaxis in postpartum women, and a suitable VTE risk assessment model (RAM) has yet to be established.

In recent years, various RAMs and guidelines for the prevention of VTE during pregnancy and the puerperium have been developed. These include the Royal College of Obstetricians and Gynecologists (RCOG) guidelines [6], the American College of Obstetricians and Gynecologists (ACOG) Practice Bulletins [7], the Society of Obstetricians and Gynecologists of Canada (SOGC) risk assessment [8], the Swedish Guidelines (Swedish method) [9] and the Queensland Clinical Guideline (QLD guideline) (Supplementary Fig. 1) [10]. However, the risk factors included in these RAMs are not consistent, and their predictive value for puerperal VTE varies significantly. Moreover, several studies have suggested that VTE risk assessment tools developed in Western countries may not be directly applicable to Chinese populations [11, 12, 13]. To address this, the Chinese Expert Consensus on the Prevention and Treatment of VTE during Pregnancy and Puerperium (Chinese Consensus) was developed in 2021 and subsequently promoted across many hospitals in China (Supplementary Table 1) [14].

In our hospital, the QLD guideline was widely applied in previous years, but was replaced by the Chinese Consensus in 2021. The QLD guideline incorporates relevant risk factors and provides recommendations for the duration of anticoagulation maintenance. However, it does not specify the timing for the initiation of postnatal anticoagulation treatment. On the other hand, the Chinese Consensus does not include certain risk factors, such as premature birth, diabetes mellitus, assisted reproductive technology (ART), vaginal delivery transfer cesarean section, cesarean hysterectomy, and stillbirth. Additionally, it specifies the timing of anticoagulation initiation postpartum and the duration of treatment according to different risk scores. Therefore, the predictive ability of these two models requires further validation.

The primary objectives of this study were to analyze the relationship between various risk factors and VTE, and to compare the predictive value of the QLD guideline and the Chinese Consensus in assessing VTE risk.

A total of 140 patients were included in the VTE group, comprising 78 cases of PE and 62 cases of DVT. A further 420 women without VTE were recruited as the control group (Fig. 1).

The baseline characteristics and risk factors for the VTE and control groups are summarized in Table 1. The length of hospital stay, D-dimer and PLT were significantly higher in the VTE group (p 0.05) compared to the control group, while the gestational age was shorter (p 0.001) and the total cholesterol level was lower (p 0.05) (Table 1).

Significant differences were also observed between the VTE and control groups for BMI $\ge$25 kg/m², PPH, premature birth and cesarean delivery (p 0.05). Specifically, the proportion of women undergoing elective cesarean delivery and vaginal delivery transfer cesarean were all higher in the VTE group (p 0.05), while the proportion of women undergoing emergency cesarean delivery was higher in the control group (p 0.05). Additionally, the VTE group showed a higher proportion of anti-phospholipid antibody (+) compared to the control group (p = 0.054), highlighting its potential clinical relevance. Conversely, the proportion of ART use was higher in the control group (p = 0.001).

No significant differences were found between the two groups for the other risk factors analyzed, including age $\ge$35 years, BMI $\ge$30 kg/m², triglycerides, multiple pregnancy, preeclampsia, parity $\ge$3, stillbirth, pre-existing diabetes, maternal comorbidities, and other evaluated factors.

Interestingly, scores for the Chinese Consensus were not significantly different between the VTE and control groups, either before or after delivery. In contrast, scores for the QLD guideline were significantly higher after delivery in the VTE group compared to the control group (p 0.001).

Trigger frequency of different risk factors was shown in Table 2. Cesarean delivery was identified as the most frequent risk factor for VTE, followed by age $\ge$35 years, ART, premature birth, multiple pregnancy, vaginal delivery transfer cesarean, BMI $\ge$30 kg/m², preeclampsia, PPH, parity $\ge$3, pre-existing diabetes, maternal comorbidities and stillbirth.

Fig. 2 visually presents the proportion of VTE cases amongst the different risk factors. The highest incidence of VTE was observed in cases of stillbirth (66.7%), followed by anti-phospholipid antibody (+) (52.9%), PPH (52%), maternal comorbidities (42.9%), vaginal delivery transfer cesarean (41.2%), premature birth (36.2%), preeclampsia (33.3%), BMI $\ge$30 kg/m² (30%), multiple pregnancy (29.8%), cesarean delivery (24.8%), age $\ge$35 years (22%), pre-existing diabetes (21.4%), parity $\ge$3 (17.4%) and ART (17.3%).

Table 3 shows the relationship between the scores from the two RAMs and the incidence of VTE. As the scores increased, the incidence of VTE also rose, with the highest occurrence observed in patients scoring 3 or above (p 0.05). However, VTE still occurred in the low-risk group (score of 0 or 1), with an incidence of 20.6% using the QLD guideline and 24.6% using the Chinese Consensus. Notably, two cases of VTE were observed in patients scored as 0 with both RAMs. Moreover, with both RAMs the proportion of VTE in patients with a score of 1 was higher than in those with a score of 2, although this difference was not statistically significant. The QLD guideline identified a slightly higher proportion of high-risk patients (34.6%) than the Chinese Consensus (31.4%).

Finally, as detailed in Table 4 and Fig. 3, the QLD guideline demonstrated an AUC of 0.624 for the prediction of VTE, with a sensitivity of 0.81, specificity of 0.64, positive predictive value (PPV) of 0.43 and negative predictive value (NPV) of 0.91. In comparison, the Chinese Consensus showed an AUC of 0.538, with a sensitivity of 0.69, specificity of 0.83, PPV of 0.58, and NPV of 0.89.

VTE is a severe maternal complication during the antenatal or postpartum period [1, 16, 17]. Despite the improved preventive measures for obstetric VTE in recent years, its incidence continues to rise. Many women still experience VTE even after prophylactic measures have been implemented, leading to increased treatment costs and prolonged hospital stays [18, 19]. This retrospective study analyzed obstetric women admitted to our hospital over the past five years, focusing on the relationship between various risk factors and the incidence of postpartum VTE. In particular, we compared the predictive accuracy of the QLD guideline and the Chinese Consensus for VTE.

The average age of pregnant women in China has increased in recent years, which may be related to the adjustment to China’s three-child policy. Although the BMI of pregnant women has also risen, no significant differences in age or BMI $\ge$30 kg/m² were found between the VTE and control groups, consistent with previous reports [11, 12]. However, factors such as BMI $\ge$25 kg/m², PPH, preterm birth, and the vaginal delivery transfer cesarean section were found to have significant associations with VTE. Additionally, anti-phospholipid antibodies (+) showed a potential closely association with VTE. Despite these findings, neither the QLD guideline nor the Chinese Consensus fully incorporates these factors. For example, preterm birth and vaginal delivery transfer cesarean are not included in the Chinese Consensus. These omissions may partly explain the limited predictive value of both models.

Preeclampsia plays an important role in the occurrence of postpartum VTE, due to its association with increased levels of coagulation factors and reduced fibrinolytic activity [20, 21]. However, the present study did not find a significant relationship between preeclampsia and postpartum VTE. A similar result was also found for ART, consistent with findings from previous studies [12, 22, 23]. In addition, the study may indicate a potential relationship between stillbirth and VTE, but the small sample size is insufficient to draw definitive conclusions regarding their association. Further research is required to evaluate the impact of these factors on the risk of postpartum VTE.

Two patients who were classified as low-risk (0 points) in this study by both the QLD guideline and the Chinese Consensus nevertheless experienced VTE. Neither patient had associated risk factors from the QLD guideline and Chinese Consensus, but their D-dimer level was elevated (4.17 mg/L and 12.97 mg/L). Interestingly, the median D-dimer level in the VTE group was significantly higher than in the control group, suggesting that elevated levels of this marker indicate thrombosis, even in patients classified as low-risk. This finding was similar to that of other previous reports [22, 23]. The incidence of VTE in the low-risk group was unacceptably high (20%–25%), highlighting the weak predictive accuracy of both RAMs.

Effective prevention strategies for VTE rely on accurate risk assessment, but in this study both the QLD guideline and the Chinese Consensus showed suboptimal accuracy. Potential reasons for this are firstly the underestimation of VTE risk due to the inclusion of inappropriate or incomplete risk factors, leading to missed opportunities for standard thromboprophylaxis. For patients identified as high-risk by the RAMs who developed VTE, the reasons may include a lack of standardized thromboprophylaxis, or delays in initiating prophylaxis after delivery.

Several limitations should be considered when interpreting the results of this retrospective study. First, the characteristics of the control group are critical and may vary depending on the definition used, thus potentially introducing bias. Second, the sample size for certain subgroups, such as PE and DVT cases, was relatively small, thereby reducing the statistical power to detect differences. Third, the study was conducted at a single center, which may limit the generalizability of the findings to other healthcare settings.

The Chinese Consensus and the QLD guideline show limited predictive ability for VTE, highlighting the need to further refine the risk factors included in the RAMs. Additional studies are required to develop more effective RAMs for puerperal VTE, with the aim of better preventing postpartum VTE and thus reducing maternal mortality.

QLD, Queensland clinical; VTE, Venous thromboembolism; PE, Pulmonary embolism; DVT, deep vein thrombosis; RAMs, risk assessment models; AUC, area under the curve; ROC, receiver operating characteristic; CTPA, computed tomography pulmonary angiography; BMI, body mass index; ART, assisted reproductive technology; APS, antiphospholipid syndrome; PPH, postpartum hemorrhage; PLT, platelet; SLE, systemic lupus erythematosus; IMVT, intermuscular venous thrombosis; SVT, superficial venous thrombosis; PPV, positive predictive value; NPV, negative predictive value.

The data used and analyzed during the current study are available from the corresponding author on reasonable request via email.

FX: study design and results interpretation. YX: data collection, data cleaning, data analysis and manuscript writing. SZ: data collection. JH: provided partial advices and results interpretation. XX: led protocol development and results interpretation. All authors contributed to editorial changes in the manuscript. All authors critically reviewed and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Medical Ethics Committee of the West China Women’s and Children’s Hospital of Sichuan University (approval number: 2024195).

This research received no external funding.

The authors declare no conflict of interest.

During the preparation of this work, the authors used ChatGPT-4o in order to check spelling and grammar. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/CEOG31322.