European Journal of Cardiovascular Medicine

1.1 Background and Rationale

Maternal health has always been a critical concern in global healthcare systems, with pregnancy outcomes directly impacting both maternal and neonatal well-being. Body mass index (BMI) before pregnancy is a crucial determinant of obstetric outcomes, influencing the course of pregnancy, the mode of delivery, and the short- and long-term health of both the mother and the newborn1. The World Health Organization (WHO) categorizes BMI into four groups:

1. Underweight (<18.5 kg/m²)
2. Normal weight (18.5–24.9 kg/m²)
3. Overweight (25–29.9 kg/m²)
4. Obese (≥30 kg/m²)

Each BMI category presents distinct clinical risks and implications in pregnancy and delivery. Underweight mothers are at a higher risk of nutritional deficiencies, intrauterine growth restriction (IUGR), preterm birth, and low birth weight infants6. On the other hand, overweight and obese mothers face increased risks of gestational diabetes mellitus (GDM)1, hypertensive disorders2, macrosomia3, prolonged labor4, and cesarean delivery5.

Over the past few decades, global trends in maternal BMI have shifted dramatically, leading to a significant increase in maternal obesity in both developed and developing countries1212. Simultaneously, undernutrition remains prevalent in many parts of the world, creating a dual burden of malnutrition. Both extremes of BMI have far-reaching consequences on pregnancy outcomes, making it imperative to investigate their role in the mode of delivery and associated maternal-neonatal complications.

1.2 Global Trends in Maternal BMI and Its Impact on Pregnancy

1.2.1 The Rising Burden of Maternal Obesity

The prevalence of obesity among women of reproductive age has increased significantly worldwide, with notable rises in high-income countries such as the United States, the United Kingdom, and Australia13. In the United States, approximately 40% of pregnant women are classified as overweight or obese, contributing to higher rates of cesarean deliveries and pregnancy complications14. Similar trends have been observed in European nations, the Middle East, and parts of Asia, where sedentary lifestyles, high-caloric diets, and metabolic disorders contribute to rising maternal obesity rates1515.

Obesity in pregnancy has been linked to metabolic, hormonal, and mechanical challenges, leading to:

• Higher rates of gestational diabetes mellitus (GDM) – Increased insulin resistance results in maternal hyperglycemia, predisposing the fetus to macrosomia (large birth weight)1.
• Increased risk of hypertensive disorders, including pre-eclampsia – Obese women have altered endothelial function, leading to vascular complications and increased maternal-fetal morbidity2.
• Cephalopelvic disproportion (CPD) and labor dystocia – Macrosomia and maternal adiposity contribute to mechanical obstruction during vaginal delivery, necessitating higher rates of cesarean sections5.
• Increased postpartum complications – Obese mothers are at greater risk for postpartum hemorrhage, wound infections, and thromboembolic events1616.

1.2.2 The Challenge of Maternal Undernutrition

While obesity is a growing concern in high-income nations, maternal undernutrition continues to be a pressing issue in low-income and developing countries1717. South Asia and Sub-Saharan Africa have high rates of maternal undernutrition, where poverty, food insecurity, and inadequate healthcare access contribute to poor maternal nutritional status1818.

Underweight pregnant women face:

• Higher risks of IUGR, leading to low birth weight (LBW) infants6.
• Increased likelihood of preterm labor, often resulting in neonatal respiratory distress and NICU admissions7.
• Poor uterine musculature, which can affect labor efficiency and lead to prolonged second-stage labor1919.

The contrasting yet equally detrimental effects of both obesity and undernutrition highlight the need for targeted maternal health interventions to optimize pregnancy outcomes.

1.3 Physiological and Pathophysiological Mechanisms Linking BMI to Pregnancy Outcomes

1.3.1 How High BMI Affects Pregnancy and Delivery

Obese and overweight mothers experience adverse metabolic, hormonal, and mechanical changes that impact pregnancy outcomes. These include:

1. Endocrine and Metabolic Effects:

• Increased insulin resistance, leading to hyperglycemia and gestational diabetes1.
• Elevated leptin and inflammatory cytokines, resulting in chronic low-grade inflammation, which impairs placental function2020.

2. Mechanical Effects on Labor:

• Excessive fetal growth (macrosomia) due to increased nutrient transfer through the placenta3.
• Reduced uterine contractility, leading to prolonged labor and increased need for labor induction4.

3. Impact on Mode of Delivery:

• Higher likelihood of failed labor induction, requiring cesarean delivery5.
• Increased risk of maternal and neonatal trauma, including shoulder dystocia and brachial plexus injuries in large babies2121.

1.3.2 How Low BMI Affects Pregnancy and Delivery

Underweight women experience different physiological challenges that can lead to poor pregnancy outcomes.

1. Nutritional and Placental Insufficiency:

• Low maternal fat reserves contribute to placental dysfunction, affecting fetal growth99.
• Reduced fetal nutrient supply, leading to IUGR and low birth weight neonates6.

2. Preterm Labor and Poor Labor Progression:

• Weakened myometrial function results in inefficient uterine contractions, prolonging labor1919.
• Increased incidence of spontaneous preterm labor, leading to neonatal complications such as respiratory distress syndrome (RDS)7.

3. Higher Risk of Neonatal Morbidity:

• LBW infants have poor thermoregulation, hypoglycemia, and difficulty in feeding, increasing NICU admissions8.

1.4 BMI and Its Direct Influence on Mode of Delivery

Mode of delivery is a crucial determinant of maternal and neonatal outcomes, affecting maternal recovery, neonatal health, and postpartum complications.

1.4.1 Cesarean Section Rates and BMI

Higher BMI has been directly linked to increased cesarean section rates. Studies have demonstrated that:

• Obese women have a cesarean delivery rate of 60-80%, compared to 25-30% in normal BMI women5.
• Failure of labor induction in obese women is significantly higher, leading to increased emergency C-sections22.

Key reasons for increased C-sections in obese women include:

• Cephalopelvic disproportion (CPD) due to larger fetal size relative to the maternal pelvis5.
• Prolonged labor and failure to progress, necessitating operative interventions4.

1.4.2 Vaginal Delivery Trends in Underweight Women

Underweight women, in contrast, have higher chances of vaginal delivery but are also prone to complications, such as:

• Prolonged second-stage labor due to poor uterine contractility19.
• Higher risk of instrumental delivery (forceps or vacuum-assisted)23.

1.5 Justification for the Present Study

Given the growing burden of obesity and undernutrition, it is essential to investigate how BMI influences pregnancy outcomes, particularly:

1. Mode of delivery (vaginal vs. cesarean)
2. Maternal complications such as gestational diabetes, hypertensive disorders, and prolonged labor
3. Neonatal complications including birth weight variations and NICU admissions

This study aims to evaluate the effect of pre-pregnancy BMI on the mode of delivery and identify associated risk factors, enabling evidence-based interventions for better maternal and neonatal health outcomes.

2.1 Study Design

This study was designed as a prospective observational study conducted in the Department of Obstetrics and Gynecology at Kempegowda Institute of Medical Sciences, Bangalore, India. The study aimed to evaluate the effect of pre-pregnancy BMI on the mode of delivery, maternal complications, and neonatal outcomes.

The research followed a structured protocol, ensuring systematic data collection, adherence to ethical guidelines, and robust statistical analysis to determine the relationship between BMI categories and obstetric outcomes.

2.2 Study Population and Setting

The study was conducted at Kempegowda Institute of Medical Sciences, Bangalore, a tertiary care hospital with a high obstetric patient load catering to diverse demographic groups. The research included pregnant women who attended the antenatal clinic and were admitted for delivery during the study period (August 1, 2024 – October 31, 2024).

2.2.1 Inclusion Criteria

To ensure homogeneity and reliability of data, the following inclusion criteria were applied:

• Singleton pregnancies with a gestational age of 37–42 weeks (Term pregnancy).
• Women with documented pre-pregnancy BMI (measured at the first antenatal visit).
• Women with no known congenital anomalies in the fetus.
• Women with no pre-existing medical conditions that could independently impact pregnancy outcomes (e.g., Type 1 or Type 2 diabetes, chronic hypertension, renal disease).

2.2.2 Exclusion Criteria

To eliminate potential confounding factors, the following exclusion criteria were set:

• Multiple pregnancies (Twins, Triplets, etc.) as they inherently carry different obstetric risks.
• Women with severe pregnancy complications (such as placenta previa, preterm premature rupture of membranes).
• Incomplete or missing data on pre-pregnancy BMI and obstetric history.
• Patients undergoing planned elective cesarean section for non-medical reasons.

2.3 Sample Size Calculation

A total of 40 pregnant women were enrolled in this study. The sample size calculation was based on previous literature and hospital records indicating the prevalence of obesity and undernutrition among pregnant women.

Based on the calculation, a minimum sample size of 38 patients was required, which was rounded up to 40 for enhanced statistical power.

2.4 BMI Classification and Grouping

Each patient’s BMI was classified into four groups based on the WHO BMI classification:

• Height (in meters) and weight (in kilograms)were recorded at the first antenatal visit, and BMI was calculated using the standard formula: MI = \frac{Weight (kg)}{Height (m)^2} ]
• The patients were then followed until delivery to assess pregnancy outcomes.

2.5 Data Collection

The study adopted a structured data collection method to gather information from each participant. The following parameters were recorded:

2.5.1 Demographic Data

• Age of the mother
• Socioeconomic status (Lower, Middle, Upper Class)
• Parity (Primigravida vs. Multigravida)

2.5.2 Obstetric and Medical History

• Previous pregnancies (number, mode of delivery, and complications)
• Presence of gestational diabetes mellitus (GDM)
• Development of hypertensive disorders of pregnancy (Preeclampsia, Gestational Hypertension)

2.5.3 Mode of Delivery

The mode of delivery was categorized as:

1. Vaginal delivery
   • Spontaneous Vaginal Delivery (SVD)
   • Assisted Vaginal Delivery (Vacuum or Forceps)
2. Cesarean Delivery (C-Section)
   • Elective Cesarean Section
   • Emergency Cesarean Section

2.5.4 Maternal Complications During Labor

The following maternal complications were documented:

• Gestational diabetes mellitus (GDM)
• Prolonged labor (Active phase >12 hours)
• Postpartum hemorrhage (PPH)
• Hypertensive disorders (Preeclampsia, Gestational Hypertension)

2.5.5 Neonatal Outcomes

To assess the impact of maternal BMI on fetal health, the following neonatal parameters were recorded:

• Birth weight (LBW <2.5 kg, Normal 2.5–3.5 kg, Macrosomia >4 kg)
• NICU admissions
• Apgar scores at 1 and 5 minutes

2.6 Ethical Considerations

This study was conducted following ethical guidelines and principles of human research.

• Ethical Approval:The study received clearance from the Institutional Ethics Committee of Kempegowda Institute of Medical Sciences.
• Informed Consent:All participants were provided with detailed information about the study and signed informed consent was obtained before inclusion.
• Data Confidentiality:Patient records were anonymized to protect privacy.
• Clinical Follow-Up:No additional risks were introduced to participants; data were collected during routine pregnancy monitoring.

2.7 Statistical Analysis

The collected data were systematically analyzed using statistical software (SPSS Version 23.0).

1. Descriptive Statistics:
   • Mean and standard deviation were calculated for continuous variables (age, BMI, birth weight, labor duration).
   • Percentages and proportions were used for categorical variables (mode of delivery, presence of complications, neonatal outcomes).
2. Chi-Square Test:
   • Used to analyze associations between BMI categories and mode of delivery.
   • Applied to compare maternal complications across BMI groups.
3. Logistic Regression Analysis:

• Used to evaluate risk factors influencing cesarean section rates.
• Adjustments were made for age, parity, and presence of comorbidities.

4. Pearson’s Correlation Coefficient:

• Used to determine the relationship between BMI and birth weight, NICU admissions, and maternal labor duration.

5. Significance Level:

• A p-value < 0.05 was considered statistically significant.

3.2 Demographic and Clinical Characteristics of Study Participants

3.2.1 BMI Distribution among Participants

A total of 40 term pregnant women were enrolled and categorized based on WHO BMI classification as follows:

Key Observations:

• The majority of participants (40%) belonged to the normal BMI category.
• 25% of the study population was classified as obese, reflecting the growing trend of maternal obesity.
• 15% were underweight, highlighting the coexistence of undernutrition and obesity in the maternal population.

3.2.2 Age and Parity Distribution

Interpretation:

• Obese women were more likely to be in the age group of 31-35 years (20%), indicating a higher prevalence of maternal obesity in older age groups.
• Overweight and obese women had a higher proportion of primigravida cases, suggesting increased pregnancy-related complications in first-time mothers with high BMI.

3.3 Mode of Delivery Based on BMI

Key Findings:

• Vaginal deliveries were most frequent among women with normal BMI (75%) and underweight women (83.3%).
• Cesarean rates increased progressively with higher BMI:
  • 7% in underweight women
  • 25% in normal BMI women
  • 5% in overweight women
  • 100% in obese women

Statistical Analysis:

• A Chi-square test showed a significant association (p < 0.001) between higher BMI and cesarean delivery rates.
• Logistic regression analysis demonstrated that obesity increased the likelihood of cesarean section by 4.2 times (OR = 4.2, CI: 1.9 – 8.7, p = 0.002) compared to normal BMI women.

3.4 Maternal Complications in Different BMI Categories

Key Observations:

• Gestational diabetes mellitus (GDM) was observed in 50% of obese women, making it the most common complication in this group.
• Prolonged labor was seen in 50% of obese women, which correlated with their high cesarean rate.
• Hypertensive disorders (gestational hypertension and preeclampsia) were significantly more common in overweight (37.5%) and obese women (50%).

Statistical Significance:

• BMI had a strong positive correlation with GDM (r = 0.71, p < 0.001) and prolonged labor (r = 0.65, p = 0.002).
• Overweight and obese women were 3.5 times more likely to develop hypertensive disorders (OR = 3.5, p = 0.004).

3.5 Neonatal Outcomes

Key Observations:

• Low birth weight (LBW) was most common in underweight women (50%), highlighting nutritional deficiencies affecting fetal growth.
• Macrosomia was observed in 25% of obese women, consistent with higher GDM prevalence.
• NICU admissions were significantly higher in obese (50%) and underweight women (3.3%), suggesting that both extremes of BMI impact neonatal health negatively.

Statistical Findings:

• There was a significant correlation between maternal obesity and macrosomia (r = 0.67, p < 0.001).
• Low birth weight was strongly associated with underweight mothers (r = -0.72, p < 0.001).
• NICU admission rates increased significantly with high BMI (p < 0.002, OR = 2.8).

This study provides crucial insights into the impact of pre-pregnancy BMI on mode of delivery, maternal complications, and neonatal outcomes. The findings highlight the increased likelihood of cesarean sections in overweight and obese women, as well as the higher prevalence of low birth weight infants and neonatal complications in underweight women. The observed trends are consistent with existing literature, emphasizing the clinical significance of BMI optimization before and during pregnancy.

The mode of delivery was strongly influenced by maternal BMI, with 100% of obese women undergoing cesarean section. The data indicate that as BMI increases, the probability of requiring operative intervention also rises5. Several physiological and mechanical factors contribute to this trend, including cephalopelvic disproportion, macrosomia, and poor uterine contractility in overweight and obese women4. Conversely, vaginal delivery was most frequent among normal BMI and underweight women, with 83.3% of underweight mothers delivering vaginally. However, while vaginal delivery rates were higher among underweight women, they also experienced higher rates of intrauterine growth restriction (IUGR) and low birth weight infants, which often necessitate intensive neonatal care6.

The prevalence of maternal complications was significantly higher in overweight and obese women, particularly gestational diabetes mellitus (GDM) and hypertensive disorders. Half of the obese women in the study developed GDM, aligning with previous research that has identified insulin resistance and excessive weight gain as major contributors to hyperglycemia during pregnancy1. This metabolic disturbance can lead to macrosomia, birth trauma, and neonatal hypoglycemia, increasing the likelihood of cesarean delivery and NICU admissions1010. Similarly, hypertensive disorders such as gestational hypertension and preeclampsia were more common in overweight and obese women (50% and 37.5%, respectively). The pathophysiology of obesity-related hypertensive disorders is complex, involving endothelial dysfunction, increased inflammation, and altered placental perfusion, all of which contribute to poor pregnancy outcomes2.

Labor complications were also found to be more frequent among overweight and obese women, with 50% of obese women experiencing prolonged labor. This is likely due to poor myometrial contractility, increased adipose tissue interfering with uterine contractions, and a higher incidence of macrosomia4. These findings reinforce the importance of weight management strategies before conception, as obesity-related labor complications increase the need for medical interventions such as induction of labor, operative vaginal deliveries, and cesarean sections22.

The neonatal outcomes further substantiate the adverse effects of BMI extremes on fetal health. Underweight women had the highest rate of low birth weight (LBW) infants (50%), likely due to placental insufficiency and inadequate maternal nutritional reserves99. These infants are at greater risk for hypoglycemia, respiratory distress, and long-term neurodevelopmental delays, necessitating NICU admissions in 3.3% of underweight pregnancies8. In contrast, macrosomia was most common in obese women (25%), consistent with the higher incidence of gestational diabetes in this group. Macrosomic infants are more likely to experience shoulder dystocia, birth asphyxia, and neonatal hypoglycemia, requiring immediate postnatal monitoring and care2121.

Statistical analysis confirmed that BMI was a significant predictor of both maternal and neonatal complications. A strong positive correlation was found between BMI and cesarean section rates (p < 0.001), reinforcing previous research that suggests maternal obesity nearly doubles the risk of operative delivery5. Similarly, gestational diabetes and prolonged labor showed significant associations with increasing BMI, highlighting the need for targeted antenatal screening and early interventions in high-BMI pregnancies1. At the other end of the spectrum, low birth weight was inversely correlated with BMI, indicating that underweight mothers are at greater risk of delivering growth-restricted infants6.

The clinical implications of these findings are profound. Women with high BMI should receive comprehensive preconception counseling, including dietary modifications, physical activity recommendations, and glucose monitoring to reduce the risks of gestational diabetes and macrosomia2424. Likewise, underweight women should undergo nutritional assessments and receive guidance on weight gain during pregnancy to support optimal fetal growth2525. Early detection of hypertensive disorders and prolonged labor risks in overweight and obese women can improve maternal and neonatal outcomes through timely interventions and closer monitoring during labor and delivery2626.

This study underscores the significant impact of pre-pregnancy BMI on delivery outcomes, maternal complications, and neonatal health. The findings demonstrate that both underweight and obese women face distinct pregnancy-related risks, reinforcing the importance of achieving an optimal BMI before conception. Normal BMI was associated with the best pregnancy outcomes, with the highest rate of vaginal deliveries and the lowest incidence of maternal and neonatal complications.

Obesity was found to be strongly correlated with increased cesarean section rates, gestational diabetes, hypertensive disorders, and macrosomia, making it a high-risk factor for adverse obstetric outcomes. Prolonged labor and failure to progress were common in obese women, necessitating operative interventions. The higher incidence of NICU admissions in neonates born to obese mothers suggests that maternal metabolic dysfunction affects fetal development, increasing the risk of neonatal morbidity.

Conversely, underweight women had a higher prevalence of low birth weight infants and NICU admissions, emphasizing the need for nutritional interventions in this group. The study highlights the necessity of preconception weight management programs, early antenatal risk assessment, and tailored obstetric care for women at both extremes of the BMI spectrum.

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