BMI (Body Mass Index) is used cautiously in pregnancy because, as you noted, weight gain is expected and necessary for a healthy pregnancy. However, BMI can still be relevant for assessing obesity and associated risks, particularly at the start of pregnancy. Here’s how it’s applied and the considerations involved:

1. Pre-Pregnancy BMI as a Baseline:
   • BMI is most commonly calculated before pregnancy or early in the first trimester to categorize a woman’s weight status (e.g., normal weight: 18.5–24.9, overweight: 25–29.9, obese: ≥30).
   • This baseline helps identify potential risks associated with obesity, such as gestational diabetes, preeclampsia, cesarean delivery, or macrosomia (large baby).
   • For example, women with a pre-pregnancy BMI ≥30 may be monitored more closely for these complications.
2. Weight Gain Guidelines Based on BMI:
   • The Institute of Medicine (IOM) provides weight gain recommendations during pregnancy based on pre-pregnancy BMI:
     • Normal weight (BMI 18.5–24.9): 25–35 lbs (11.5–16 kg)
     • Overweight (BMI 25–29.9): 15–25 lbs (7–11.5 kg)
     • Obese (BMI ≥30): 11–20 lbs (5–9 kg)
   • These guidelines aim to balance the need for healthy fetal growth with minimizing risks like excessive weight gain, which can exacerbate obesity-related complications.
3. Limitations of BMI During Pregnancy:
   • Dynamic Changes: BMI becomes less reliable as pregnancy progresses because weight gain includes the fetus, placenta, amniotic fluid, increased blood volume, and maternal fat stores. Calculating BMI mid-pregnancy doesn’t accurately reflect maternal obesity.
   • Not a Direct Measure: BMI doesn’t distinguish between fat mass and other pregnancy-related weight, so it’s not used to “diagnose” obesity during pregnancy.
   • Individual Variation: Factors like fluid retention or ethnicity can affect BMI interpretation, and guidelines may need adjustment for specific populations.
4. Focus on Health Outcomes, Not BMI Alone:
   • Instead of tracking BMI during pregnancy, healthcare providers focus on:
     • Appropriate weight gain within IOM guidelines.
     • Monitoring for obesity-related complications (e.g., blood pressure, glucose levels).
     • Promoting healthy behaviors like balanced nutrition and physical activity (when safe).
   • For obese pregnant women, interventions may include dietary counseling or closer monitoring rather than weight loss, which is generally discouraged during pregnancy.
5. Challenges and Controversies:
   • Some argue BMI is too simplistic for pregnancy, as it doesn’t account for body composition or fat distribution, which can influence risks.
   • Women with high pre-pregnancy BMI may face stigma or overly restrictive advice, so care must be individualized.
   • Emerging research explores other metrics (e.g., waist circumference pre-pregnancy or visceral fat) but these aren’t yet standard in clinical practice.

Summary: BMI is primarily used before or early in pregnancy to assess obesity and guide weight gain recommendations. During pregnancy, the focus shifts to monitoring weight gain and health outcomes rather than BMI itself, as weight gain is essential for fetal development. Always consult a healthcare provider for personalized guidance, as obesity in pregnancy requires careful, individualized management.