The ratio leptin/soluble leptin receptor (sOB-r), free leptin index (FLI), is used as a marker of leptin sensitivity/resistance in different pathologies. The aim of this study was to evaluate FLI in healthy non-pregnant, healthy pregnant and mild preeclamptic women during pregnancy. We conducted a nested case-control study within a longitudinal observational prospective cohort study. Serum leptin (p=0.0001) and sOB-r (p=0.0000) levels rose significantly throughout pregnancy in healthy pregnant and preeclamptic women [leptin (p=0.0000); sOB-r (p=0.0380)]. Serum leptin levels were significantly higher in preeclamptic compared to healthy pregnant women at 2nd (p=0.0245) and 3rd trimesters of pregnancy (p=0.0016). Additionally, serum sOB-r levels were significantly lower in preeclamptic women during the 2nd (p=0.0236) and 3rd trimester (p=0.0024) of pregnancy compared to healthy pregnant women. Moreover, we found that FLI did not vary significantly during any of the three periods studied in healthy pregnant women (p=0.7640), whereas, increased throughout preeclamptic pregnancy (p=0.0037). Indeed, FLI was significantly higher at 2nd (p=0.0053) and 3rd (p=0.0003) trimesters of pregnancy in preeclamptic compared to healthy pregnant women. Additionally, FLI was significantly higher during luteal phase compared to the follicular phase (p=0.0039). These results demonstrate that FLI increases significantly in preeclamptic pregnant women towards the end of pregnancy.