Underweight is often overlooked as a risk factor for poor pregnancy outcomes.At the high and low extremes, BMI is associated with increased early miscarriage and reduced pregnancy success among patients undergoing assisted reproductive technology (ART), according to data presented at ESHRE 2026.
Studies have consistently linked a BMI of 25 kg/m² to poorer outcomes, but whether the risk is linear or U-shaped remains unclear. This prompted researchers to compare baseline characteristics and pregnancy outcomes in a retrospective cohort of 16,152 in vitro fertilization cycles, stratified into four BMI groups: underweight (n=644), normal weight (n=6788), overweight (n=6002), and obesity (n=2718). Data were collected from January 2020 to December 2024 via a centralized system. The cohort comprised women attending infertility treatment clinics. [ESHRE 2026, abstract L26/P-458]
BMI increased with age in women (p<0.001), and a similar pattern was observed in men. Anti-Müllerian hormone (AMH) varied slightly across BMI groups (3.6-3.7 ng/mL; p=0.021), with minimal clinical significance. Endometrial thickness also varied slightly (8.6-8.86 mm; p=0.008). Pregnancy rates were similar across BMI groups (69.3 to 71.4 percent; p=0.648). Implantation rates showed no directional trend.
By contrast, pregnancy loss followed a U-shaped pattern (p< 0.001). It was highest among underweight individuals (35.9 percent) and those with obesity (23.1 percent), and was sharply lower among the normal-weight (3.9 percent) and overweight (6.3 percent) groups.
Pregnancy rates differed significantly (p<0.001). Normal-weight (68.6 percent) and overweight (66.9 percent) patients had the highest rates. The pregnancy rate among underweight patients was 44.4 percent, and among those with obesity was 54.4 percent.
The researchers said this suggests that early miscarriage in patients with an extreme BMI category is unlikely to be driven by AMH or by small variations in endometrial thickness but may be more strongly influenced by metabolic or inflammatory factors.
Counselling beyond conception
“BMI appears to have a greater impact on pregnancy continuation than on achieving initial implantation or a biochemical pregnancy,” said the researchers. “This suggests that counselling should extend beyond simply achieving conception. Physicians should discuss how body weight may influence the likelihood of sustaining a pregnancy after implantation.”
They added that underweight is more often overlooked than obesity as a factor that may prevent optimal outcomes. In the current study, underweight women had the highest miscarriage rate (35.9 percent vs roughly 4 to 6 percent in normal-weight and overweight women) and the lowest pregnancy rate (44.4 percent).
“That’s a larger swing than we observed in those with obesity,” said study author Dr Manju Parmar from Indira IVF Hospital Limited, Reproductive Medicine, in Indore, India. She added that being underweight is often considered low-risk or protective. “However, our data suggest it warrants the same proactive counselling as obesity.”