(P27-043-25) Urine Osmolality and Fluid Intake in Pregnancy: Impact of Pre-Pregnancy BMI on Fluid Balance and the Need for Optimized Hydration Guidelines

Objectives: Maintaining adequate fluid intake during pregnancy is crucial for maternal and fetal health, yet limited research suggests a risk of hypohydration. This study assessed longitudinal urine osmolality (UOsm) and total water intake (TWI) across all trimesters, examining pre-pregnancy BMI as a potential modifier.

Methods: Data from the GLOWING trial (NCT01131117) were analyzed, including 79 healthy, second-parity women enrolled before 10 weeks gestation. Participants were categorized by BMI (normal weight: 18.5–24.9 kg/m²; overweight/obesity: ≥25 kg/m²) and provided spot urine samples and TWI questionnaires each trimester. Linear mixed-effect models evaluated hydration markers and BMI-related differences.

Results: UOsm significantly changed over pregnancy, F(2,113.88)=4.419, p=.014, increasing in the second trimester (497.7±23.5 mOsm/kg, mean difference 72.7±26.5 mOsm/kg, p=.020). Pre-pregnancy BMI also influenced UOsm (F(1,80.57)=4.863, p=.030), with higher values in women with overweight/obesity (493.4±23.7 mOsm/kg, mean difference 75.2±34.1 mOsm/kg, p=.030), though no interaction effect was observed (p=.660). TWI increased across pregnancy (F(2,106.54)=6.472, p=.002), rising from baseline (2209.7±82.6 mL/d) to the second (2383.5±82.6 mL/d, p=.023) and third trimester (2471.3±83.3 mL/d, p=.002), with no significant effects of BMI (p=.816) or BMI*trimester (p=.142).

Conclusions: UOsm increased during pregnancy, peaking in the second trimester, and was higher in women with overweight/obesity despite increasing TWI across gestation. The lack of a BMI effect on TWI suggests fluid intake may be inadequate, particularly in BMI ≥25 groups. Given the absence of defined UOsm hypohydration thresholds in pregnancy, these findings underscore the need to optimize hydration recommendations for maternal and fetal health.

Funding Sources: USDA