Body mass index has remained the default counselling tool in fertility clinics for decades — but it is increasingly clear that BMI conceals more clinical information than it reveals. The lecture argues for replacing it with body composition assessment as the new standard for pre-cycle workup, and for treating overweight, obesity and underweight as fundamentally different metabolic phenotypes.
BMI as an outdated criterion
Two patients with identical BMI can have fundamentally different fertility prospects. The same BMI can reflect normal composition in a constitutionally lean patient, high muscle mass in an athlete, or low fat-free mass in a malnourished patient. Using BMI as the sole criterion conflates these into a single category — losing the clinically relevant information.
Body composition as the new standard
A three-step workup operationalises this shift: bioelectrical impedance analysis (BIA) for fat and fat-free mass, indirect calorimetry for resting energy expenditure, and a structured food diary to identify under- or over-reporting. Together these tools allow targeted lifestyle recommendations rather than the generic «lose weight» message.
Couple-level fertility risk
Spontaneous fecundability shows a J-shaped relationship with BMI for both partners. Male obesity contributes through sperm DNA fragmentation and reduced concentration, motility and morphology — and the «obese woman with lean man» combination carries one of the worst fecundability ratios. Counselling that addresses only the female partner misses a substantial share of the risk.
