Maternal leptin levels in serum and urine, their relations to maternal weight and body mass index, were examined in 9 healthy pregnant women from the 12th week of gestation until term. Serum leptin concentration was found to increase progressively during the first two trimesters followed by a slight decline thereafter. The peak value of 27.6 +/- 15.3 ng/ml (mean +/- SD) concentration was reached at the 28th week. Serum leptin levels during the first two trimesters correlated significantly with maternal weight (p = 0.002) and body mass index (p = 0.002) but such a relationship was absent during the third trimester. Leptin could be detected only in about half of urine samples; its concentrations proved to be independent of serum values. No correlation was found between maternal serum leptin levels and the birth weight of neonates. Maternal leptin levels appear to refer to alterations in maternal fat tissue mass that occur during pregnancy.