Review questions

1. Which socio-economic determinants and routinely collected clinical determinants are used to classify a high-risk pregnancy?
2. What is the relationship between the high-risk pregnancy status and its complications?

Study design

This protocol follows PRISMA-P guidelines (PRISMA-P) [21] and the PRISMA statement, including the flow chart, to track search records. It is registered with PROSPERO [registration number: CRD420251026327].

Search strategy

The Cochrane Library, Web of Science, CNKI, SCOPUS, and PubMed (MEDLINE) will be searched for observational studies from 2014 onwards. Studies must describe risk-factor classification systems, the risk factors defining them, and their association with adverse birth outcomes. The search strategy will follow the POE framework: Population/Problem (high-risk pregnancy classification), Outcome (pregnancy complications and adverse outcomes), and Exposure (risk factors Search strategies will use Mesh-indexed keywords combined with Boolean operators (AND/OR), including terms like “risk factor,” “high-risk pregnancy,” “pregnancy complication,” and “adverse birth outcome.” The draft search strings for all databases are provided in S1 File. Data management will involve Excel and EndNote software to identify and remove duplicates electronically and manually.

All non-English studies meeting eligibility criteria will be manually translated by bilingual team members, with subsequent grammar and clarity checks performed using Grammarly (Premium version) to ensure linguistic accuracy. No automated translation tools will be used without human verification. The search will be constructed using keywords and limiters appropriate to the database:

• (high-risk pregnan* OR at-risk pregnan* OR high risk prenan*) AND
• (risk factor* OR predictor* OR determinant*) AND

(pregnan* complication OR adverse pregnan* outcome OR adverse obstetric outcome OR birth outcome OR maternal outcome)

Study selection

The review will investigate and explore the factors proposed to define high-risk pregnancies, with a focus on the information routinely collected during antenatal appointments. These include maternal age, BMI, low education, low household income, historical obstetric and gynaecological complications, surgical history, and fetal biometry identified through ultrasound, such as fetal heart, femur length, abdominal circumference, and biparietal diameter.

Adverse birth outcomes will encompass pregnancy, labour, and delivery complications affecting newborn health, ranging from mild to severe and with potential long-term impacts (Fig 1 and Table 1). The review will focus on prevalent maternal and neonatal outcomes:

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Table 1. Descriptions of routinely collected clinical determinants/data.

https://doi.org/10.1371/journal.pone.0334326.t001

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Fig 1. Logic model and conceptual framework (Descriptions of variables are detailed in Table 1).

https://doi.org/10.1371/journal.pone.0334326.g001

1. (i). maternal and neonatal mortality (defined as death status),
2. (ii). abnormal birth weight (macrosomia defined as birth weight >4 kg and low birth weight defined as <2500g), and
3. (iii). neonatal intensive care unit (NICU) admission within 24 hours post-delivery
4. (iv). preterm delivery (<37 weeks)
5. (v). postpartum hemorrhage (defined as bleeding volume >500ml after vaginal delivery or >1000ml after cesarean section)
6. (vi). Cesarean section
7. (vii). Fetal death/stillbirth (defined as intrauterine death occurring at ≥20 weeks of gestation, acknowledging that the specific gestational age threshold may vary across included studies)
8. (viii). Maternal near miss (defined according to the WHO criteria as a woman who nearly died but survived a complication that occurred during pregnancy, childbirth, or within 42 days of termination of pregnancy)

The review will include observational studies (cross-sectional, case-control, and cohort studies) from 2014 to 2024 that recruit mothers with high-risk pregnancy status and contain information on maternal and neonatal outcomes. Eligible studies must report identified determinants in pregnancy (exposures of high-risk pregnancy) and conclude with maternal and neonatal outcomes (pregnancy complications and adverse outcomes). We will exclude studies with no maternal or neonatal outcomes reported at the delivery stage, and those with unavailable full-text manuscripts despite attempts to contact the corresponding authors. Protocols, ongoing studies, and studies without ethical approval will be listed but not included. The language of publication will be inclusive through the use of translation applications.

The literature search and record screening have not yet started. Record screening is expected to begin in August 2025, followed by data extraction in October 2025. Results synthesis and reporting are expected to be finished by 1 December 2025.

Data extraction and analysis

A two-stage screening process will be carried out using EndNote software: the initial stage will involve title and abstract screening by a single reviewer (YX), while the subsequent stage will include full-text screening by two independent reviewers. Discrepancies will be resolved by a third reviewer. Data extraction will adhere to a structured protocol, with YX conducting the data extraction and CXW verifying its accuracy; PB will settle disputes. A pre-tested Microsoft Excel spreadsheet will be used to extract predefined details: authors, study settings (e.g., country, location, and duration), respondent numbers, study design, risk factors, pregnancy complications, and adverse outcomes. Primary outcomes will comprise delivery complications and adverse birth outcomes, along with the identified high-risk pregnancy determinants.

Methodological quality assessment

Two reviewers will independently evaluate study quality using the Newcastle-Ottawa scale for non-randomised studies [22–23]. A third reviewer will resolve any discrepancies in the assessments.

Data synthesis

Descriptive synthesis will summarise the included studies’ population characteristics, context, methods used, and the results reported, including any statistical descriptors in a tabular format. All eligible studies will be reported. Data will be interpreted in the context of the identified risks of bias, selective reporting of outcomes, and the precision of the findings. To strengthen the narrative synthesis, we will thematically categorise risk-factor classification systems using the PEO framework. Finally, we will develop a comprehensive conceptual framework to visualise risk factor-outcome relationships across studies.