Background: Leptin hormone is produced by adipocytes, and it is a putative regulator of energy metabolism in which there are substantial changes after the birth.

Aim of the study was to study whether there are changes in circulating leptin concentrations during the early postnatal period.

Subjects: 38 healthy newborn infants (20 male, 18 female, gestational age 36.3 - 41.9 weeks, weight 3675±509 g males and 3243±519 g females).

Methods: Plasma leptin concentrations were measured from umbilical cord and from venous plasma at three days of age when also the thickness of the subcutaneous fat of the arm was estimated with ultrasound and the circumference of the arm was measured.

Results: Cord P-leptin concentration was 9.7±5.2 μg/L(mean±SD) with no gender difference (p>0.05). In male newborns, cord P-leptin concentration correlated with the circumference of the arm (r = 0.48, P>0.01), and in female with BMI (r = 0.62, P<0.01), thickness of the subcutaneous fat (r = 0.54 P<0.05) and the circumference of the arm (r = 0.72, P<0.01). By the third postnatal day the P-leptin had decreased (male 1.8±0.4 and female 2.3±0.8μg/L; both P<0.001) and there was a significant gender difference in leptin levels (P=0.01). At three days of age P-leptin concentration correlated with BMI (r = 0.47, P<0.05), weight (r = 0.49, P<0.05) and the circumference of the arm (r = 0.49, P<0.05) in female but not in male newborns.

Conclusions: Leptin decreases markedly in both genders by the third postnatal day. The gender difference with higher leptin levels in females develops by that time. Leptin levels correlate with adiposity already at birth. The postnatal decrease in P-leptin concentration is physiologically feasible, when the infant becomes dependent on external food intake.