What is the correct formula to calculate Body Mass Index (BMI) for a pregnant adult female patient?

How to Calculate BMI

BMI is calculated as weight in kilograms divided by height in meters squared (kg/m²). 1

Standard Calculation Formula

The formula can be expressed in three equivalent ways depending on your measurement system:

• Metric: BMI = weight (kg) / [height (m)]² 2
• Metric alternative: BMI = weight (kg) / height (cm) / height (cm) × 10,000 1
• Imperial: BMI = weight (lb) / height (in) / height (in) × 703 1

Practical Calculation Steps

For metric measurements:

1. Measure weight in kilograms 2
2. Measure height in centimeters and convert to meters by dividing by 100 2
3. Square the height in meters (multiply it by itself) 1
4. Divide weight by the squared height 1

Example: A person weighing 68 kg with height 157.48 cm (1.5748 m) would have BMI = 68 / (1.5748)² = 27.4 kg/m² 2

BMI Classification for Adults

Standard categories (for most populations):

• Underweight: BMI < 18.5 kg/m² 1
• Normal/Healthy weight: BMI 18.5-24.9 kg/m² 1, 2
• Overweight: BMI 25.0-29.9 kg/m² 1, 2
• Obesity Class I: BMI 30.0-34.9 kg/m² 1, 2
• Obesity Class II: BMI 35.0-39.9 kg/m² 1, 2
• Obesity Class III (severe): BMI ≥ 40 kg/m² 1, 2, 3

Special Considerations for Pregnancy

The same BMI calculation formula applies to pregnant women - weight (kg) divided by height (m) squared - but interpretation requires pre-pregnancy weight or early first-trimester weight for accurate classification. 1 The standard BMI categories remain the same for determining pre-pregnancy weight status, which guides gestational weight gain recommendations.

Important Limitations

BMI does not distinguish between lean mass and fat mass, which means muscular individuals may be misclassified as overweight or obese. 1, 4 The measure has only 50% sensitivity for detecting excess body fat despite 90% specificity, meaning half of individuals with excess adiposity may have a "normal" BMI. 1

For Asian populations, lower thresholds apply:

• Overweight: BMI ≥ 23 kg/m² 2
• Obesity: BMI ≥ 25 kg/m² 2

This adjustment reflects higher body fat percentages and visceral adiposity at lower BMI values in Asian populations compared to Caucasians. 2

Additional Assessment Recommended

Waist circumference should be measured alongside BMI for more complete cardiovascular risk assessment. 1, 2 Increased risk thresholds are:

• Men: waist > 102 cm (40 inches) 1
• Women: waist > 88 cm (35 inches) 1, 2
• Asian women: waist ≥ 80 cm 2

Central adiposity measured by waist circumference predicts cardiovascular mortality independent of BMI and may be more predictive than BMI alone, particularly in older adults. 1