Exposure to metals at the onset of life,both in the fetal period and during the first years,can be associated with negative health effects in later stages(Vahter 2008).Assessing the exposure to metals,particularly in the earliest stages of life,may provide the necessary knowledge for identifying public health problems and implementing prevention policies early on.However,only a few studies on prenatal exposure to trace metals have been published; most of which focused on a small number of these elements.Attending to these premises,a collection of urine samples taken both during the first and third trimesters of pregnancy from 489 mothers from Sabadell(Catalonia,Spain),a highly industrialized town,were analyzed for cobalt(Co),nickel(Ni),copper(Cu),zinc(Zn),selenium(Se),arsenic(As),molybdenum(Mo),cadmium(Cd),antimonium(Sb),cesium(Cs),thallium(Tl),and lead(Pb).An acid digestion method was developed and validated for inductively coupled plasma quadruple mass spectrometry(Q-ICP-MS)analysis of these 12 metals.The median concentrations of metals ranged from 0.13 to 290 μg/g creatinine,the highest levels were found for Zn and the lowest for Tl.Concentrations of most metals were approximately on the same range than those reported in previous studies from sites without an important source of pollution.The mean concentrations of most metals except As,Ni,Tl,and Pb showed statistically significant differences between both trimesters.The concentrations of Mo,Se,Cd,Cs,and Sb were higher in the first than in the third trimester,whereas the opposite was found for Co,Cu,and Zn.The concentrations of all metals in both sampling periods showed statistically significant correlations(p<0.01 for Mo and Cu,p<0.001 for the others).The significant correlations of metal urine concentrations in the first and third trimesters of pregnancy suggest that the observed differences between both periods are related to physiological changes occurring along it.As a conclusion,although physiological changes occurring during pregnancy may change excretion of trace metals along it,the measured urine concentrations during either the first or third trimesters can be used as estimates of exposure during the whole period and can serve as markers for prenatal intake of these metals in the studied cohort.