Introduction: Pesticides are toxic substances that can also cause unintended adverse effects, e.g., endocrine disruption, in exposed non-target populations such as humans. Some endocrine disrupting compounds (EDCs) (e.g., the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT)) are identified as persistent organic pollutants while other suspected EDCs (e.g., atrazine) are less persistent in the body. A trend towards early puberty in girls has been observed in the United States and Europe. This decline in the age of onset of puberty has been associated with several factors including exposures to EDCs. The endocrine disrupting effects due to population exposures to pesticides are still not well understood. Objectives: 1) To characterize the in utero exposure of girls in the Avon Longitudinal Study on Parents and Children (ALSPAC) in Bristol, United Kingdom (UK) to organochlorine pesticides and examine the association with early menarche 2) To characterize the in utero exposure of girls in ALSPAC to atrazine pesticide and examine the association with early menarche.Methods: Exposure to 9 organochlorine compounds and 7 atrazine analytes measured in maternal serum or urine collected during pregnancy was characterized using non-parametric survival analysis methods. Logistic regression methods were used to examine the association between in utero exposure to these pesticides and early menarche. Results: Hexacholorbenzene (HCB), -hexacholorocyclohexane (-HCH), 2,2-Bis(4-chlorophenyl)-1,1,1-trichloroethane (p,p-DDT) and 2,2-Bis(4-chlorophenyl)-1,1-dichloroethene (p,p-DDE) were detected in more than 50% of the study participants. Diaminochlorotriazine (DACT) was the only atrazine analyte detected in more than 50% of the study participants. Overall, there was no association between in utero exposure to organochlorines and early menarche. In utero exposure to higher levels of the atrazine metabolite DACT in the ALSPAC cohort was associated with an increased odds of early menarche. Conclusions: More emphasis needs to be placed on measuring exposure in countries where organochlorine pesticides are still in use and evaluating their associations with adverse health effects. Our results suggest an association between in utero exposure to the atrazine metabolite DACT and early menarche in the ALSPAC cohort. To accurately assess atrazine exposure in populations, several metabolites including DACT need to be measured.