Optimizing Gestational Weight Gain in Obese Pregnant Women: A Nationwide Real-World Study Using the U.S. Natality Cohort

Detailed Description This is a population-based retrospective cohort study that leverages the U.S. National Vital Statistics System (NVSS) natality files for the years 2023 through 2025. The NVSS is a comprehensive, publicly available database compiled by the National Center for Health Statistics (NCHS) at the U.S. Centers for Disease Control and Prevention (CDC), capturing nearly all births occurring in the United States. The dataset contains de-identified maternal demographic characteristics, medical history, prenatal care utilization, labor and delivery procedures, and infant health outcomes.

Study Cohort The target population consists of nulliparous and multiparous individuals aged 18 to 45 years with singleton, live-born, term pregnancies (37 to 41 completed weeks of gestation) and pre-pregnancy obesity, defined as a Body Mass Index (BMI) of 30.0 kg/m² or higher. To minimize confounding by late or absent prenatal care, only individuals who initiated prenatal care during the first trimester (months 1 through 3 of pregnancy) are included. After applying all inclusion criteria and excluding records with missing or implausible values for key variables (gestational weight gain below -10 kg or above 50 kg, unknown delivery mode, or documented prior cesarean section), the analytic cohort is anticipated to include approximately 3 million birth records.

The cohort is stratified into three World Health Organization (WHO) obesity classes for all primary analyses: Class I (BMI 30.0-34.9 kg/m²), Class II (BMI 35.0-39.9 kg/m²), and Class III (BMI ≥ 40.0 kg/m²). This stratification allows for the assessment of effect measure modification by obesity severity and the derivation of class-specific optimal gestational weight gain targets.

Exposure and Outcome Measurements The primary exposure is total gestational weight gain (GWG), calculated as the exact arithmetic difference between maternal weight at delivery and pre-pregnancy weight as recorded on the birth certificate. Continuous GWG is categorized into eight prespecified clinical intervals: (1) < 0 kg (weight loss), (2) 0 to 2.9 kg, (3) 3.0 to 4.9 kg, (4) 5.0 to 9.0 kg (the 2009 Institute of Medicine recommended range, inclusive of both endpoints, serving as the reference category), (5) 9.1 to 10.9 kg, (6) 11.0 to 12.9 kg, (7) 13.0 to 15.9 kg, and (8) ≥ 16.0 kg.

The primary endpoints are two comprehensive composite indicators. The maternal composite (M1_equal) captures any of the following: intensive care unit (ICU) admission, blood transfusion, eclampsia (Tier 1, severe); or gestational hypertension, gestational diabetes mellitus, primary cesarean delivery, or severe perineal laceration in the absence of Tier 1 events (Tier 2, moderate). The neonatal composite (M2_equal) captures: neonatal ICU admission, prolonged assisted ventilation (> 6 hours), or neonatal seizures (Tier 1, severe); or small for gestational age (birth weight < 10th percentile using the 2017 U.S. reference), large for gestational age (> 90th percentile), immediate assisted ventilation, or 5-minute Apgar score < 7 in the absence of Tier 1 events (Tier 2, moderate). These gradings align with CDC Severe Maternal Morbidity (SMM) indices, the WHO Maternal Near-Miss framework, and the WHO Neonatal Near-Miss criteria, respectively, ensuring clinical relevance and comparability across studies.