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Background: Carnitine is essential for fatty acid metabolism. Free carnitine (FCA) is excreted in the urine in the glomerulus, but is partly reabsorbed by a carnitine transporter. The mechanism underlying the decrease in serum carnitine level during pregnancy is unclear. Objective: To investigate whether low carnitine level is associated with increased renal excretion in pregnant women. Methods: We recruited 43 healthy pregnant and 25 non-pregnant women. Total carnitine (TCA) and FCA levels were measured using the enzymatic cycling method, and the acylcarnitine (ACA) level was calculated. Fractional excretion (FE) was calculated as carnitine clearance divided by creatinine clearance. Results: The mean TCA, FCA, and ACA levels were lower at 12 weeks of gestation in pregnant than non-pregnant women (P <.001); the levels decreased further at 36 weeks, reaching 39%, 36%, and 52% of those in non-pregnant women, respectively (P <.001). The FEs were 3–4-fold higher in pregnant women than non-pregnant women. Pregnant women had a lower serum FCA/TCA ratio than non-pregnant women (0.788 ± 0.098 vs 0.830 ± 0.074, respectively; P <.05), whereas the urine FCA/TCA ratio was similar between the groups. Conclusion: Low carnitine level is associated with increased renal excretion during late pregnancy. [ABSTRACT FROM AUTHOR]