Optimized Cutoff Maternal Age for Adverse Obstetrical Outcomes

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Optimized Cutoff Maternal Age for Adverse Obstetrical Outcomes: A Multicenter Retrospective Cohort Study in Urban China During 2011 to 2012

Introduction

China’s implementation of the two-child policy in 2016 catalyzed a demographic shift toward delayed childbearing, amplifying concerns about advanced maternal age and its association with adverse pregnancy outcomes. Globally, increasing maternal age is linked to higher risks of cesarean sections, gestational diabetes, hypertensive disorders, and neonatal complications. In China, the average maternal age for first-time mothers rose from 26.3 years in 2000 to 28.2 years in 2010, with projections suggesting further increases. This study addresses the critical need to define age-related risk thresholds for adverse outcomes in urban Chinese populations, providing evidence-based guidance for clinical practice and policy decisions.

Methods

Study Design and Population
This secondary analysis utilized data from a multicenter retrospective cohort study conducted across 39 hospitals in 14 Chinese cities from 2011 to 2012. The study included 108,059 women aged 20–50 years with singleton pregnancies, excluding cases with missing parity or maternal age data. Participants were stratified into primiparae (81.2%) and multiparae (18.8%).

Data Collection and Variables
Maternal age at delivery was recorded as an integer. Outcomes assessed included:

  • Maternal complications: Gestational diabetes, hypertensive disorders, placenta previa, post-partum hemorrhage, cesarean section.
  • Perinatal outcomes: Pre-term birth, low birth weight (4000 g), neonatal asphyxia (1-minute Apgar score ≤7), NICU admission.

Adjustments were made for hospital location, maternal education level, and residence status (local vs. migrant). Low-prevalence variables (<1% incidence) and factors like body mass index (BMI) or abortion history were excluded from multivariable analysis due to insufficient data.

Statistical Analysis
Logistic regression calculated adjusted odds ratios (aOR) for outcomes across maternal age groups (reference: 20–24 years). Clinically significant cutoff age was defined as the youngest age where aOR exceeded 2.0 with sustained statistical significance (P < 0.05). Stratified analyses for parity and post-hoc subgroup analyses examined interactions between maternal age and BMI for gestational diabetes mellitus (GDM).

Results

Participant Characteristics
Primiparae were younger (27.6 ± 4.2 vs. 31.1 ± 5.1 years), more educated (54.7% with bachelor’s degrees vs. 29.3%), and predominantly local residents (71.4% vs. 61.6%). Multiparae exhibited higher rates of anemia (8.5% vs. 5.6%), hypertensive disorders (6.4% vs. 4.9%), and neonatal complications such as macrosomia (6.3% vs. 5.1%) and NICU admission (3.8% vs. 2.3%).

Maternal Outcomes
Primiparae:

  • Gestational diabetes: Cutoff age at 27 years (aOR: 2.136; 95% CI: 1.856–2.458).
  • Cesarean section: Cutoff at 33 years (aOR: 2.511; 95% CI: 2.341–2.694).
  • Hypertensive disorders: Cutoff at 37 years (aOR: 2.122; 95% CI: 1.753–2.569).
  • Placenta previa: Cutoff at 31 years (aOR: 2.400; 95% CI: 1.863–3.090).

Multiparae:

  • Gestational diabetes: Cutoff at 29 years (aOR: 2.977; 95% CI: 1.808–4.904).
  • Hypertensive disorders: Cutoff at 31 years (aOR: 2.555; 95% CI: 1.836–3.554).
  • Post-partum hemorrhage: Cutoff at 35 years (aOR: 2.140; 95% CI: 1.472–3.110).

Neonatal Outcomes
Primiparae:

  • Low birth weight: Risk escalated at 41 years (aOR: 2.174; 95% CI: 1.615–2.927).
  • Neonatal asphyxia: No clinically significant cutoff until 41 years (aOR: 1.831; 95% CI: 1.183–2.834).

Multiparae:

  • Macrosomia: Cutoff at 41 years (aOR: 2.215; 95% CI: 1.552–3.161).
  • Neonatal asphyxia: Cutoff at 41 years (aOR: 2.132; 95% CI: 1.461–3.110).

Age-Related Risk Trends

  • Cesarean section: Linear increase with age in both groups, doubling by 33 years.
  • Placenta previa: Linear rise in primiparae vs. J-shaped curve in multiparae (lowest risk at 27–28 years).
  • GDM: Synergistic interaction between maternal age and BMI. Overweight women (BMI ≥24 kg/m²) exhibited steeper risk increases (Figure 3).

Discussion

Key Findings
This study identifies parity-specific age thresholds for adverse outcomes, challenging the traditional “advanced maternal age” benchmark of 35 years. Early cutoffs for GDM (27–29 years) and cesarean section (33 years) emphasize the need for pre-35 interventions. Neonatal risks, however, became clinically significant only beyond 41 years, suggesting distinct biological mechanisms.

Clinical Implications

  • GDM Screening: Women ≥27 (primiparae) and ≥29 (multiparae) require intensified monitoring.
  • Cesarean Counseling: Age 33 marks a critical threshold for shared decision-making on delivery mode.
  • Hypertensive Disorders: Multiparae face elevated risks earlier (31 years), necessitating early blood pressure management.

Methodological Considerations
The large sample size enabled detection of modest risk increments, though low-incidence outcomes (e.g., placental abruption) were excluded. Unadjusted confounders like BMI and assisted reproductive technology use may partially explain residual variability.

Comparison With Prior Research
The cesarean section cutoff aligns with Chinese studies showing rising rates with age, yet contrasts with Western reports where thresholds were higher (>35 years). Disparities in healthcare practices and BMI distributions may underlie these differences. For hypertensive disorders, multiparae’s earlier cutoff (31 vs. 37 years in primiparae) highlights parity’s role in vascular adaptation.

Public Health Relevance
With China’s median maternal age approaching 28.2 years, pre-30 interventions for GDM and cesarean risks could mitigate health system burdens. Policymakers must prioritize antenatal resources for women nearing these thresholds.

Limitations

  • Retrospective design limited causal inference.
  • Missing data on BMI and lifestyle factors (e.g., smoking) may bias estimates.
  • Hospital-based sampling underrepresents rural populations.

Conclusions

This study redefines age-related risk stratification for obstetric outcomes in urban China. Clinically actionable thresholds for GDM (27–29 years) and cesarean delivery (33 years) precede traditional benchmarks, urging revised clinical guidelines. While neonatal risks surge post-41 years, maternal complications demand earlier vigilance. Tailored interventions based on parity and age-specific risks will enhance maternal and neonatal health outcomes amid China’s evolving demographic landscape.

doi.org/10.1097/CM9.0000000000000626

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