The aim of this study was to evaluate the influence of maternal exposure to pesticides in the 1st and 2nd trimesters of pregnancy on infant birthweight in a population of Polish farmers. The subjects were women who delivered in 25 maternity hospitals in the region of Lódź (Central Poland), including 117 women who delivered infants with low birthweight (LBW) and 377 infants with birthweight > or = 2500 g delivered on randomly selected 70 days between 31 January 1998 and 30 June 2001. A questionnaire on maternal demographic and anthropometric characteristics as well as the occurrence of several occupational hazards, including pesticide use and involvement in heavy physical work on the farm in each of pregnancy trimesters, was administered by a physician 1-2 days after delivery. The pesticides used most frequently included: phenoxyacetic acid derivatives, organophosphates, ureas, triazines, synthetic pyrethroids and N-phenylamides (anilides). Infants born to women exposed to pesticides in 1st or 2nd trimester had birthweight lower by 189 g than that of infants of the non-exposed women. When adjusted for pregnancy duration, the women exposed to pesticides were found to deliver infants with birthweight lower by about 100 g (p = 0.067) than that of infants of the non-exposed women. After adjusting for the variables that may have impact on pregnancy duration, we noted that mothers exposed to pesticides, on average delivered half a week earlier than those non-exposed. Our results indicate that maternal exposure to pesticides may contribute to a slight reduction in the duration of pregnancy. A slower pace of fetal development, corresponding to the small-for-gestational-age effect, was observed, but the increment in the risk was of borderline significance.