Trends in the Incidence of Placenta Previa and Its Influencing Factors: A Multicenter Retrospective nested-case-control Study

Technical Summary: Trends in the Incidence of Placenta Previa and Its Influencing Factors

Problem Statement
Placenta previa (PP), defined as the attachment of the placenta to the lower uterine segment covering or reaching the internal cervical os after 28 weeks of gestation, remains a leading cause of antenatal hemorrhage and a serious third-trimester complication. While global incidence rates range from 0.30% to 0.50%, data from China indicates higher rates (0.70%–1.24%) with significant regional variations. Despite established risk factors such as advanced maternal age, multiple cesarean sections, and high gravidity, there is a need for updated, large-scale epidemiological data to understand current incidence trends and specific risk stratifications, particularly concerning the "floating population" (migrant workers) and urban-rural disparities in a rapidly developing Chinese context.

Methodology
This study employed a multicenter retrospective nested case-control design utilizing data from three tertiary hospitals in Hangzhou, China (Hangzhou Women's Hospital, Linping Maternal and Child Health Hospital, and the Affiliated Hospital of Hangzhou Normal University).

• Cohort: The study analyzed 195,440 pregnant women with singleton pregnancies and a gestational age >28 weeks delivered between January 2014 and December 2023.
• Sampling: From this cohort, 1,939 confirmed PP cases formed the case group. A control group of 1,982 non-PP pregnant women was randomly selected at a 1:1 ratio.
• Data Collection: Diagnostic criteria followed clinical guidelines, confirmed by transabdominal ultrasound at 28 weeks and intraoperative findings. Variables included demographic data (age, household registration, floating status), obstetric history (gravidity, parity, scarred uterus), pregnancy complications (HDP, GDM), and delivery outcomes.
• Statistical Analysis: Univariate comparisons were performed using Chi-square tests. Multivariate logistic regression was utilized to calculate adjusted odds ratios (aOR) and 95% confidence intervals (CI) to identify independent risk factors, controlling for confounders.

Key Results

• Incidence Trends: The overall incidence of PP in the study population was 0.99%. A distinct downward trend was observed over the decade, decreasing from 2.71% in 2014 to 0.60% in 2023, subsequently stabilizing between 0.60% and 1.00%.
• Independent Risk Factors: Multivariate analysis identified several significant predictors for PP:
  • Floating Population: Individuals without permanent Hangzhou residence or with residence <6 months showed a drastically elevated risk (aOR = 85.66).
  • Rural vs. Urban: Rural residents had a higher risk compared to urban residents (aOR = 1.36).
  • Advanced Maternal Age (AMA): Women aged ≥35 years had an increased risk (aOR = 1.47).
  • Gravidity and Parity: Risk increased with the number of pregnancies (e.g., ≥4 pregnancies: aOR = 2.37) and deliveries (≥2 deliveries: aOR = 2.36).
  • Gestational Age: Preterm delivery (<35 weeks) was strongly associated with PP (aOR = 6.08).
• Outcomes: The presence of PP was significantly associated with a higher likelihood of cesarean section (aOR = 83.54) and prolonged hospital stays (aOR = 1.38).
• Non-Significant Factors: No statistically significant association was found between PP and in vitro fertilization (IVF) or hypertensive disorders of pregnancy (HDP) in the multivariate model.

Significance and Claims
The authors claim that this study provides robust epidemiological evidence based on a large, 10-year, multi-center dataset. The primary contributions include:

1. Trend Analysis: Documenting a significant decline in PP incidence in Hangzhou, stabilizing at lower rates in recent years.
2. Health Inequality Insight: The study highlights a stark "household registration-related incidence gradient," identifying the floating population and rural residents as high-risk groups. The authors attribute this to disparities in healthcare resource availability, screening rates, and health literacy, noting that migrant women often have fewer prenatal visits and miss early intervention windows.
3. Clinical Implications: The findings support the implementation of early monitoring and targeted management strategies, particularly for high-risk demographics (elderly, multiparous, and floating populations), to mitigate adverse pregnancy outcomes such as hemorrhage and preterm birth.

Limitations
The authors acknowledge that the study is limited to the Hangzhou region and does not stratify PP by severity subtypes. Additionally, the potential impact of the COVID-19 pandemic (2020–2023) on data collection and incidence rates was not explicitly accounted for.