RESUMO
The objective of this study was to determine the risk of preterm birth in relation to umbilical cord blood lead levels (UCL) among primiparous and multiparous women. A case-cohort study was performed in Mexico City during 1995. A total of 459 full-term births was compared with 161 preterm births (before 37 gestational weeks). Mothers were interviewed before the delivery about their reproductive histories and other related factors of preterm birth. Lead was determined by atomic absorption spectrophotometry. Lead levels were higher in primiparous women who had a preterm birth than in primiparous women with a full-term birth (9.77+/-2.0 microgram/dl vs 8.24+/-2.15 microgram/dl); this difference was marginally significant. After adjusting for other known preterm birth risk factors, the frequency of preterm birth was almost three times higher among women who had UCL levels greater or equal to 5.1 microgram/dl compared to those who had UCL levels lower than 5.1 microgram/dl. This difference was not observed among multiparous women. Our results suggest that intrauterine lead exposure may be associated with preterm birth in first deliveries but not in subsequent ones.