Leptin is an adipocytokine that is also synthesized by the placenta. Leptin and its receptor, which is also expressed by the placenta, are believed to play an auto- and paracrine role in trophoblast invasion and placental development. The leptin concentration in first trimester maternal serum and its relation to fetal growth disturbances were examined in this study. The study is a case-control study with 36 small-for-gestational-age (SGA) (<5th percentile) pregnancies and 108 appropriate-for-gestational-age (AGA) (≥5th percentile) pregnancies. The groups were matched by maternal age, gestational age and body mass index (BMI). All were non-smokers. Leptin was measured in maternal serum in weeks 8-13 and was normalized for BMI with concentrations expressed as multiples of the median for the actual BMI. It was found that maternal serum leptin increased strongly (r = 0.7, P < 10-4) with maternal BMI. There was no significant difference in maternal serum leptin concentrations between SGA and AGA pregnancies. In conclusion, SGA pregnancies are not associated with a lower maternal serum leptin concentration in first trimester. The maternal serum leptin concentration is largely determined by maternal BMI. Variation in the leptin concentration in maternal serum in first trimester does not seem to be associated with impaired fetal growth.