Introduction

Maternal pre-pregnancy obesity has emerged as a critical public health issue with profound implications for both maternal and neonatal health1. According to the World Health Organization (WHO), more than 890 million adults aged 18 years and older were living with obesity in 2022, with women exhibiting the highest prevalence globally2. The rising prevalence of obesity among women of reproductive age has been strongly associated with adverse pregnancy outcomes, including gestational diabetes mellitus, hypertensive disorder of pregnancy, and preterm birth3,4. These maternal complications resulting from obesity can negatively impact fetal health by creating an obesogenic intrauterine environment that disrupts normal fetal development and programming5,6. A correlation between maternal body mass index (BMI) before pregnancy and offspring BMI or obesity has been reported by many studies7,8. In a meta-analysis conducted by Heslehurst et al.9 it was found that for every 5 kg/m2 increase in maternal BMI, the odds of childhood overweight or obesity increased by 55% (odds ratio [OR] 1.55). Additionally, maternal obesity is widely recognized as an independent risk factor for both fetal macrosomia and birth weight exceeding the 90th percentile10.

Beyond growth and metabolic outcomes, maternal pre-pregnancy obesity has broader implications for neonatal respiratory function. Recent studies have reported its potential influence on both immediate and long-term respiratory outcomes in offspring11,12,13. These complications are thought to arise from the obesogenic intrauterine environment, which may impair fetal lung maturation through systemic inflammation, altered hormonal regulation, and metabolic dysregulation during pregnancy14. Furthermore, conditions linked to maternal obesity, such as preterm delivery and macrosomia, are significant contributors to neonatal respiratory morbidity. According to Kureshi et al.15, infants born to obese mothers were significantly more likely to require respiratory support and admission to neonatal intensive care units (NICU) than those born to mothers with normal BMI. Additionally, there was a markedly higher incidence of respiratory distress syndrome (RDS) and transient tachypnea of the newborn (TTN) among the infants of obese mothers.

Previous studies investigating this relationship were often small-scale or single-center designs, presenting certain limitations. The present study aimed to fill existing knowledge gaps by analyzing large-scale, population-based data to explore the dose–response relationship between pre-pregnancy BMI and neonatal respiratory outcomes including RDS, TTN, and bronchopulmonary dysplasia (BPD). By providing robust evidence that extends beyond prior research limitations, this study seeks to emphasize the importance of maternal weight management during pregnancy as a means of improving neonatal respiratory health outcomes.

Methods

Data source and study population

This study was performed on a nationwide scale using data obtained from the National Health Information Database. The National Health Insurance Service (NHIS) is a mandatory, single-payer health insurance system that provides healthcare coverage to all residents of South Korea. The NHIS database includes comprehensive information on all patients, including their demographic characteristics, diagnoses, prescription medications, and procedures. In addition, South Korea operates a nationwide follow-up program known as the Korean National Health Screening Program for Infants and Children (NHSPIC), which provides health screenings for children from 4 to 71 months of age. To obtain detailed health information on infants, only children who underwent NHSPIC screening were included in the study. Children born between 2014 and 2021 and their matched mothers were selected for the analysis. After excluding children diagnosed with chromosomal anomalies, a total of 779,091 children were included in this study.

Variables and outcomes

Demographic information pertaining both children and their mothers was extracted from the NHIS database. Diagnostic codes used to define exposure variables and outcomes are detailed in Supplementary Table S1. All diagnoses were based on the 6th edition of the Korean Standard Classification of Diseases (KCD-6), which aligns with the 10th version of the International Classification of Diseases (ICD-10).

Data were collected on maternal age, preterm birth, delivery mode, and pregnancy-related complications including pregnancy-induced hypertension, depression, and gestational diabetes mellitus. Chronic hypertension was identified using ICD-10 codes, anti-hypertensive prescriptions, or documented systolic and diastolic blood pressure readings exceeding 140/90 mmHg.

Maternal BMI data were derived from the NHIS health screening database, which records height and weight during routine biennial health examinations for all adult beneficiaries. For each individual, the most recent BMI measurement taken within the 3 years prior delivery, including during pregnancy, was selected as the exposure variable. This 3-year window was chosen to balance sample size and data availability given the biennial nature of the screening program, recognizing that BMI may fluctuate over time and measurements nearer to conception could better represent pre-pregnancy status. Maternal BMI was classified into five groups based on the Asia-Pacific criteria: underweight (< 18.5 kg/m2), normal weight (18.5–22.9 kg/m2), overweight (23–24.9 kg/m2), obese class I (25–29.9 kg/m2), and obese class II/III (≥ 30 kg/m2)16.

Neonatal data included gestational age, birth weight, NICU admission, and presence of major congenital anomalies (ICD-10 codes Q00-Q98.4).

The primary outcome was respiratory morbidity, defined as the presence of RDS, TTN, and BPD. The need for ventilator support or supplemental oxygen therapy was also assessed.

Statistical analysis

We performed all analyses using SAS version 9.4 (SAS Institute, Inc., Cary, NC, USA). Categorical variables are reported as frequencies and percentages (%), and continuous variables as means and standard deviations. We compared maternal and neonatal characteristics across BMI categories using analysis of variance (ANOVA) for continuous variables and chi-square tests for categorial variables.

To address potential confounding, we applied inverse probability of treatment weighting (IPTW) using propensity scores estimated from a multinomial logistic regression model. We derived stabilized weights by dividing the overall probability of each exposure level by the corresponding individual propensity score, truncated the weights at the 1st and 99th percentiles, and normalized them to average 1. Mothers with normal BMI (18.5–22.9 kg/m2) served as the reference group.

Relative risks (RRs) and 95% confidence intervals (CIs) for respiratory outcomes were then estimated using generalized linear models incorporating IPTW weights and adjusting for additional covariates, including maternal age, maternal comorbidities, multiple birth, small for gestational age status, birth weight, sex, and vaginal delivery. These variables were included because they are important clinical predictors of neonatal respiratory outcomes.

Absolute risk differences (RDs) were obtained after IPTW as the difference in weighted outcome probabilities between each BMI category and the reference (18.5–22.9 kg/m2), expressed per 100 births, with 95% CIs.

Covariate balance was assessed using absolute standardized differences (ASDs) before and after weighting. Because the exposure comprised five BMI categories, we computed ASDs for all 10 pairwise comparisons and, for each covariate, summarized imbalance as the maximum |ASD| across comparisons. The complete set of ASDs before and after weighting for all covariates is provided in Supplementary Table S2.

Ethics approval

This study was performed in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of the Konkuk University Medical Center (IRB No. 2025-06-023). Informed consent from patients was not required because of the retrospective and de-identified nature of the study.

Results

From the national birth registry comprising 2,285,943 live births in South Korea between 2014 and 2021, a total of 779,091 mother-infant dyads who participated in the Korean NHSPIC were selected for inclusion in the final analysis (Fig. 1).

Fig. 1
figure 1

Flow chart of the study population.

Full size image

Maternal characteristics stratified by pre-pregnancy BMI are presented in Table 1. Obese mothers, compared with their normal BMI counterparts, tended to be older and more frequently experienced pregnancy-related complications, such as gestational diabetes and hypertensive disorders of pregnancy. Approximately, one-third of mothers with a BMI of 30 or higher received a diagnosis of gestational diabetes, highlighting the metabolic risks associated with obesity. Interestingly, mothers with a normal BMI value reported the lowest incidence of depression compared with underweight, overweight, or obese groups.

Table 1 Demographic and clinical characteristics of mothers according to pre-pregnancy body mass index (BMI).
Full size table

Table 2 summarizes the demographic and clinical characteristics of the offspring stratified by maternal BMI classification. Infants of mothers with a BMI of 25 or greater were more likely to be born before 35 weeks gestation and to be delivered via cesarean section. Additionally, the prevalence of macrosomia was greater among infants of overweight and obese mothers. No statistically significant differences were observed in the distribution of male sex across BMI categories. Approximately 10% of newborns born to mothers with obesity required admission to NICU and a greater proportion had major congenital anomalies.

Table 2 Demographic and clinical characteristics of offspring according to maternal pre-pregnancy body mass index (BMI).
Full size table

Before weighting, several covariates showed meaningful imbalance. After IPTW, all covariates met the balance criterion (maximum |ASD| < 0.10), indicating adequate balance (Supplementary Table S2).

Figure 2 displays the associations between maternal pre-pregnancy BMI and neonatal respiratory outcomes. After adjustment for relevant covariates, offspring of overweight and obese mothers demonstrated elevated risks of receiving ventilator support (Fig. 2A; RR 1.603; 95% CI 1.558–1.649; P < 0.0001) and oxygen therapy (Fig. 2B; RR 1.102; 95% CI 1.089–1.115; P < 0.0001).

Fig. 2
figure 2figure 2

Association between maternal pregnancy body mass index (BMI) and neonatal respiratory outcomes. Relative risks are adjusted for maternal and neonatal covariates. Outcomes: (A) ventilator use; (B) oxygen therapy; (C) respiratory distress syndrome (RDS); (D) transient tachypnea of the newborn (TTN); (E) all cases of bronchopulmonary dysplasia (BPD); (F) moderate-to-severe BPD.

Full size image

Neonates of underweight mothers exhibited a reduced risk of moderate-to-severe bronchopulmonary dysplasia (BPD) (RR 0.387; 95% CI 0.163–0.918; P = 0.0313), whereas those born to obese mothers had a markedly higher likelihood of developing BPD (RR 6.070; 95% CI 3.687–9.994; P < 0.0001) (Fig. 2F). Comparable trends were observed for other respiratory diagnoses, including RDS and TTN. Elevated maternal pre-pregnancy BMI was associated with increased risks of neonatal RDS (RR 2.598; 95% CI 2.523–2.686; P < 0.0001) and TTN (RR 1.154; 95% CI 1.126–1.182; P < 0.0001). In contrast, infants born to underweight mothers demonstrated decreased risks of RDS (RR 0.873; 95% CI 0.842–0.906; P < 0.0001), TTN (RR 0.951; 95% CI 0.927–0.975; P < 0.0001), and BPD (RR 0.371; 95% CI 0.263–0.523; P < 0.0001).

Stratified analyses by delivery mode (vaginal vs. cesarean) and maternal age (< 35 vs. ≥ 35 years) demonstrated that maternal obesity was consistently associated with an increased risk of neonatal respiratory morbidities, irrespective of delivery mode or maternal age. Effect sizes were larger among infants delivered by cesarean section and among mothers aged ≥ 35 years. These stratified results were broadly consistent with the overall results, supporting the robustness and clinical relevance of our conclusions (Supplementary Tables S3–S4).

In absolute terms, underweight was associated with small but consistent risk reduction across outcomes (RD range − 0.371 to − 0.002 per 100 births), whereas BMI ≥ 30 showed higher absolute risks (+ 0.025 to + 2.766 per 100 births). Full RD estimates are shown in Supplementary Table S5.

Discussion

Maternal pre-pregnancy BMI was strongly linked to increased rates of neonatal respiratory conditions, such as RDS, TTN, and BPD. After IPTW and multivariable adjustment, overweight and obesity were associated with higher risks of ventilator support, oxygen therapy, RDS, TTN, and BPD. These associations persisted across a range of clinically relevant covariates, supporting maternal BMI as a critical risk factor for early respiratory morbidity in newborns.

From a clinical perspective, absolute risks associated with maternal BMI are modest at the individual level, but follow a consistent, graded pattern. Being underweight leads to a small reduction in risk, while having BMI ≥ 30 is associated with higher absolute risks. These absolute estimates complement the RR findings and illustrate potential population impact.

Previous investigations have examined how maternal BMI influences respiratory outcomes in neonates. For example, Carmichael and colleagues reported that both elevated and low maternal BMI were related with higher rates of BPD among preterm infants born before 30 weeks of gestation13. In contrast, a more recent study involving extremely low birth weight infants did not find a significant link between maternal obesity and a composite outcome of mortality or BPD17. While mothers of infants with BPD in that study tended to have higher BMI, the analysis did not identify BMI as an independent predictor. A large cohort study conducted by Vincent et al. showed a strong connection between elevated BMI and adverse neonatal outcomes, including longer durations of assisted ventilation and higher NICU admission rates18. However, the scope of that study was limited to elective cesarean deliveries, which may have introduced selection bias and skewed the estimates of risk. Unlike these prior studies, the current analysis demonstrated a clear dose–response relationship between maternal BMI and multiple neonatal respiratory outcomes, and included a broader neonatal population, not limited to preterm births.

The observed associations between maternal pre-pregnancy BMI and neonatal respiratory conditions may be mediated by multiple interconnected biological pathways14. Obesity during pregnancy is characterized by chronic low-grade systemic inflammation, which is driven by elevated circulating levels of pro-inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α)19,20,21. Experimental animal studies have demonstrated that these inflammatory mediators can cross the placenta, disrupting normal fetal lung development and surfactant production, thereby contributing to conditions such as RDS17,22. In addition, maternal obesity often involves hormonal imbalances, including hyperinsulinemia and leptin resistance, which may delay fetal lung maturation23. The intrauterine environment shaped by maternal obesity may also trigger metabolic abnormalities that interfere with the neonatal respiratory transition and increase susceptibility to TTN15. Clinically, comorbidities such as gestational diabetes and preeclampsia—more common in obese pregnancies—are also known to impair fetal pulmonary development. Taken together, these factors—chronic inflammation, endocrine disruption, metabolic imbalance, and associated pregnancy complications—provide plausible biological explanations for the observed associations.

An unexpected finding in this study was the relatively lower incidence of RDS, TTN, and BPD among neonates born to underweight mothers when compared with those born to mothers of normal BMI. This finding diverges from much of the prevailing evidence, which generally associates maternal underweight status with negative neonatal outcomes24,25. For instance, a meta-analysis by Santos et al.26 revealed that underweight mothers with low gestational weight gain have the highest risk of delivering small for gestational age (SGA) infants. Other studies have documented a similar dose–response trend between lower pre-pregnancy BMI and adverse perinatal outcomes, including higher SGA rates27,28.

Despite these associations, emerging research suggests that fetal exposure to maternal undernutrition may, paradoxically, accelerate pulmonary development. This could be mediated through increased endogenous corticosteroid exposure29. Animal studies have reported that maternal undernutrition may not reduce surfactant production directly but may instead increase main pulmonary artery flow—a possible compensatory mechanism to prioritize nutrient delivery to vital fetal organs30. This physiologic adaptation could help explain the observed trend of reduced neonatal RDS incidence among infants of underweight mothers, as also suggested by Kim et al.31

Another contributing factor is vitamin D status32. Underweight women may have lower rates of vitamin D deficiency compared with overweight or obese counterparts. Sufficient vitamin D levels have been linked to enhanced surfactant synthesis and reduced inflammation, both of which may decrease the risk of neonatal respiratory disorders. Recent meta-analyses support this hypothesis, showing a beneficial role for vitamin D in fetal lung development and respiratory health33,34.

In our cohort, however, underweight mothers were generally younger, more likely to have vaginal deliveries, and had infants with fewer NICU admissions and congenital anomalies. Although some of these variables may lie on the causal pathway, our primary IPTW outcome models adjusted for them to enhance comparability. To assess potential over-adjustment, we conducted sensitivity analyses that stratified by delivery mode and maternal age. The direction of the associations was unchanged, with some evidence of effect modification. Importantly, higher maternal BMI remained associated with increased neonatal respiratory risks across maternal age and delivery mode strata, reinforcing the robustness and clinical relevance of our findings.

This study has limitations. First, its retrospective design and reliance on claims data from the NHIS may introduce inherent biases. Nevertheless, leveraging a nationwide, mandatory single-payer database provided comprehensive coverage, enhancing the robustness and generalizability of population-level inferences. Second, although we adjusted for a wide range of maternal and neonatal factors, the lack of detailed clinical information (e.g., smoking status, socioeconomic status, and nutritional factors) and the limited precision of gestational age among term infants—both of which may serve as confounders or mediators in the association with maternal obesity—constrained further stratified or mediation analyses. Nevertheless, the large sample size, inclusion of multiple clinically relevant covariates, and the application of IPTW enhanced the internal validity and minimized residual confounding. Third, the inclusion of certain pregnancy complications and delivery outcomes in the analytical models may have led to conservative estimates, as these variables could act as intermediates in the relationship between maternal BMI and neonatal respiratory morbidity. Nonetheless, including these variables allowed us to demonstrate associations independent of well-established clinical predictors, thereby reinforcing the clinical relevance of our findings. Fourth, the identification of respiratory conditions was based on diagnostic codes, which, while widely used in population-level research, may not fully reflect clinical diagnoses. Despite this limitation, the diagnostic code system (ICD-10) has been validated and is consistently applied across Korean hospitals, which improves reliability and comparability of outcomes in large-scale epidemiologic studies. Finally, maternal BMI was assessed up to three years before delivery, which may not have precisely reflected BMI at conception or captured gestational weight changes. However, this approach maximized sample size and data availability, and importantly, the majority of BMI measurements were obtained within one to two years before delivery, making them reasonably representative of pre-pregnancy status.

Despite these limitations, the present study adds to the growing body of evidence that maternal BMI is a modifiable risk factor for neonatal respiratory health35,36. These results emphasize the importance of preconception counseling aimed at weight optimization and nutritional support, including adequate micronutrient intake, as a potential strategy to improve neonatal outcomes. Additional research is warranted to clarify causal pathways underlying this association and to inform interventions that promote neonatal respiratory health.