Christophe Blockeel examines the impact of overweight and obesity on reproductive outcomes — a topic often reduced in clinic to "lose weight," though the data demand a far more nuanced approach.
The first and perhaps most important thesis is a J-shaped curve: both excess and insufficient body weight lower fertility. The extremes on either side are equally undesirable, so the conversation with a patient must cover not only "extra kilos" but also low body mass.
The male factor cannot be set aside. A man's waist circumference correlates with worse semen parameters and higher sperm DNA fragmentation, so metabolic health is a shared task for the couple, not the woman's alone.
The scale of the problem is shown by large SART data on 239,000 cycles: as BMI rises, implantation, pregnancy and live-birth rates fall consistently. These are not isolated observations but statistics one can rely on.
The donor-oocyte argument is especially telling: in donor-egg recipients the negative effect of excess weight persists. So it is not only about oocyte quality — the endometrium, the maternal side of implantation, is involved too.
The paradox of interventions: weight-loss programmes (diet, physical activity) in trials did not improve IVF outcomes as such, yet they did raise the rate of spontaneous conception in the same patients.
This is an important nuance for honest counselling. Lifestyle correction is certainly valuable — for general health and for natural fertility — but promising a better outcome for a specific IVF cycle through it would be incorrect.
The speaker's main practical message is to move from BMI to body-composition assessment: bioimpedance, calorimetry. BMI does not distinguish muscle from fat and therefore poorly predicts true metabolic and reproductive risk.
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