BMI at high and low extremes was associated with a higher biochemical (early miscarriage) loss and lower clinical pregnancy rates for patients using assisted reproductive technology (ART), while normal weight and overweight were linked to the best outcomes, according to new data presented in a poster at the European Society of Human Reproduction and Embryology (ESHRE) 2026 Annual Meeting in London.

More Than 16,000 Cycles Included

Studies consistently link a BMI of at least 25 with poorer ART outcomes, but whether the risk is linear or U-shaped has been unclear, wrote researchers led by Manju Parmar, MBBS, with Indira IVF Hospital Limited, Reproductive Medicine in Indore, India.

The team compared baseline characteristics and pregnancy outcomes in a retrospective cohort of 16,152 in vitro fertilization cycles stratified across 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 from a centralized system, and the cohort included women visiting clinics for infertility treatment across various centers in the region.

The researchers found that some factors were linear: BMI rose progressively with age for women (P < .001), and that pattern was mirrored in men. Anti-Müllerian hormone (AMH) differed slightly across BMI groups (3.6-3.7 ng/mL; P = .021), with little clinical effect. Endometrial thickness differed slightly (8.6-8.86 mm; P = .008). Biochemical pregnancy rates were similar across BMI groups (69.3%-71.4%; P = .648), and implantation rates demonstrated no directional trend.

U-Shape for Early Loss

In comparison, biochemical pregnancy loss followed a U-shape (P < .001). It was highest in underweight (35.9%) and obese (23.1%) groups and sharply lower in the normal-weight (3.9%) and overweight (6.3%) groups. Clinical pregnancy rates differed significantly (P < .001): Normal-weight (68.6%) and overweight (66.9%) patients had the highest rates; underweight patient pregnancy rate was 44.4%, and the rate for obese patients was 54.4%.

The data suggest that early miscarriage when a patient is in an extreme BMI category is not likely driven by AMH or small endometrial thickness variances, the authors wrote, but may be influenced more heavily by metabolic or inflammatory factors.

“The key message is that BMI appears to have a greater impact on pregnancy continuation rather than on achieving the initial implantation or biochemical pregnancy,” Parmar told Medscape Medical News. “This suggests that counseling should extend beyond simply achieving conception. Physicians should discuss how body weight may influence the likelihood of sustaining a pregnancy after implantation.”

Optimizing BMI Important Before Attempting Pregnancy

“These data really underscore the importance of health optimization before pregnancy, and it’s something we can’t stress enough — that getting into the best shape before you attempt pregnancy is going to lead to the healthiest outcomes,” reproductive endocrinologist Eve Feinberg, MD, professor in the Obstetrics and Gynecology Department at Northwestern University in Chicago, told Medscape Medical News.

Underweight is overlooked more often than obesity as a factor that may prevent optimal outcomes, Parmar and Feinberg said.

“In our cohort, underweight women had the highest biochemical loss rate of any group (35.9% versus roughly 4%-6% in normal-weight and overweight women) and the lowest clinical pregnancy rate (44.4%),” Parmar said. “That’s a bigger swing than we saw in the obese group.” She noted that being underweight is frequently treated as a low-risk or even thought of as protective, “but our data suggest it deserves the same proactive counseling as obesity, not less.”

Feinberg said she recommends a Mediterranean diet with healthy fats and healthy oils to her patients. “There’s a lot of data showing it helps lead to healthier pregnancy outcomes,” she said.

Parmar and Feinberg reported no relevant financial relationships.