Being too thin or too heavy hurts IVF success — but the relationship isn't a straight line. A large study of over 16,000 IVF cycles found that underweight women had the highest early pregnancy loss rate (35.9%) and the lowest clinical pregnancy rate (44.4%), even worse than obese patients. Experts say BMI optimization before treatment is critical — and underweight is just as much of a concern as obesity.
A new study presented at the ESHRE 2026 Annual Meeting in London reveals that BMI's impact on assisted reproductive technology (ART) outcomes follows a U-shaped curve — not a straight line. Analyzing over 16,000 IVF cycles from 2020–2024, researchers found that normal-weight and overweight women had the best clinical pregnancy rates, while underweight and obese women fared significantly worse. Crucially, BMI appeared to affect pregnancy continuation more than initial implantation, pointing to metabolic and inflammatory factors rather than egg quality or uterine receptivity alone.
Perhaps the most striking finding: underweight women were actually worse off than obese women in several key metrics — a group that often flies under the radar in fertility counseling.
By the Numbers:
Why it matters: These findings push clinicians to expand pre-treatment counseling beyond obesity — underweight patients deserve equal attention. Optimizing BMI before starting ART could meaningfully improve the odds of not just getting pregnant, but staying pregnant.
