Objective To investigate the relationship between the leptin level and neonatal weight and the expression of leptin in placenta. Methods The concentrations of leptin in 100 maternal blood and umbilical blood of the term pregnant women were examined by radioimmunoassay (RIA). According to the neonatal weight to divide into the large for gestational age (LGA) group 19 cases, the appropriate for gestational age (AGA) group 65 cases, the small for gestational age (SGA) 16 cases. The level of leptin mRNA in 41 placental tissue was examined by Reverse transcription polymerase chain reaction (RT PCR). Results (1) The expression level of leptin mRNA in placenta was 0 97?0 04, which was positively related to the neonatal weight significantly ( r =0 43, P 0 05).(3)The concentration of leptin in umbilical blood was (7 58? 5 15) ?g/L, which was positively related to the neonatal weight ( r =0 57, P