Hidden Fat, Hidden Risk in Pregnancy: Rethinking Insulin Resistance beyond BMI

Dr Shivani Sidana, Associate Professor, Department of Endocrinology and Metabolism, AIIMS, Bathinda

Insulin resistance in pregnancy is driven more by fat distribution than BMI alone, but no single fat measure fully explains metabolic changes.

Obesity and insulin resistance are major drivers of adverse maternal and fetal outcomes, including gestational diabetes and pregnancy complications. During pregnancy, insulin sensitivity naturally declines—even in healthy-weight women—but the drop is highly variable and not fully explained by body mass index (BMI). Interestingly, some studies show that insulin sensitivity falls even more in normal-weight pregnancies than in those with obesity, suggesting that BMI alone does not capture the full metabolic story.

A key limitation of BMI is that it cannot distinguish where fat is stored or how it behaves metabolically. Evidence from non-pregnant populations shows that visceral fat and liver fat are far more strongly linked to insulin resistance than total body weight. Liver fat, in particular, appears to be a stronger predictor of metabolic dysfunction than visceral fat itself. Yet, how these fat depots change during pregnancy—and how they influence insulin sensitivity—has remained largely unclear.

To address this gap, a prospective study used advanced imaging (MRI and spectroscopy) and metabolic clamps to track fat distribution and insulin sensitivity from early to late pregnancy. Participants ranged from normal weight to obesity. The study found that insulin sensitivity declined significantly across pregnancy, while visceral fat increased markedly. However, associations between fat depots and insulin resistance were inconsistent and often weak, especially in late pregnancy.

Overall, findings suggest that pregnancy-related insulin resistance is complex and not fully explained by BMI or even regional fat stores alone, highlighting the need for deeper metabolic profiling beyond traditional weight measures.

(Reference: Purnell JQ, Marshall N, Francisco M, Leo M, Rooney WD, Baetscher E, Frias A, Valent A, Catalano P, King J, Vesco KK. Relationships between regional and ectopic adiposity and insulin sensitivity in early and late pregnancy. Diabetes care. 2026 Feb 26:dc252081.)