At what Body Mass Index (BMI) does the risk increase for obstetric patients?

Obstetric Risk Increases at BMI ≥30 kg/m²

Maternal obesity, defined as BMI ≥30 kg/m², is recognized as one of the most commonly occurring risk factors in obstetrics, with significantly poorer outcomes for both mother and baby compared to the general population. 1

Risk Stratification by BMI Thresholds

BMI ≥30 kg/m² (Obesity Class I and Above)

• All women with BMI ≥30 should have active management of the third stage of labor due to increased risk of postpartum hemorrhage. 1
• Increased risk of gestational diabetes and pre-eclampsia begins at this threshold. 1
• Obesity increases the risk of developing hypertension (OR 2.5-7.0) and hyperglycemic disturbances (OR 5.5). 2, 3
• Higher rates of labor induction and instrumental delivery compared to non-obese women. 1
• Increased operative and postoperative complications including prolonged operative times, endometritis, and wound infection. 1

BMI >35 kg/m² (Obesity Class II and Above)

• Stillbirth rates are twice as high as the national stillbirth rate in women with BMI >35 kg/m². 1
• Substantially elevated risk of preterm delivery. 1
• Fetal risks include shoulder dystocia, brachial plexus lesions, fractured clavicle, and congenital birth defects such as neural tube defects. 1

BMI ≥40 kg/m² (Obesity Class III)

• Vascular access should be established early in labor for women with BMI ≥40 kg/m². 1
• Early establishment of venous access during labor is recommended. 1
• Antenatal referral to an anesthesiologist is specifically recommended for women with BMI >40 kg/m². 1
• Increased difficulties with general anesthesia and central neuraxial blockade, leading to increased decision-to-delivery intervals for category-1 or -2 cesarean sections. 1
• Particularly vulnerable to aorto-caval compression. 1

Dose-Response Relationship

The risk increases progressively with higher BMI categories:

• BMI 22-27.2: +21% increased incidence of endometrial cancer (as a proxy for metabolic risk). 1
• BMI 27.5-29.5: +43% increased incidence. 1
• BMI >30: +273% increased incidence, demonstrating exponential risk escalation. 1

Critical Clinical Implications

Anesthetic Considerations

• Women with BMI >40 kg/m² require individualized anesthetic assessment before the day of surgery. 1
• Discussion of limitations and risks of anesthesia during delivery should occur during antenatal consultation. 1

Thromboembolism Risk

• Obesity and pregnancy are both significant independent risk factors for venous thromboembolism (VTE). 1
• Weight-based dosing of pharmacologic thromboprophylaxis may be more effective than BMI-stratified strategies in Class III obesity after cesarean section. 1

Fetal Monitoring

• Obstetric care providers should consider BMI when arranging second-trimester fetal anatomic assessment, with evaluation at 20-22 weeks potentially optimal for obese patients. 4
• Appropriate screening for congenital abnormalities is essential given increased fetal risk. 4

Common Pitfalls to Avoid

• Do not delay establishing IV access in laboring women with BMI ≥40 kg/m²—this should be done early and proactively. 1
• Obesity alone is not an indication for elective cesarean section or induction of labor, though informed discussion about mode of delivery is appropriate given higher emergency operative risks. 1
• Do not assume standard anatomic ultrasound timing will be adequate—technical limitations increase with obesity. 4
• Recognize that even moderate overweight (BMI 25-29.9) carries increased risk for gestational diabetes and hypertensive disorders, though the threshold for "high-risk" designation is BMI ≥30. 5, 4