Prenatal and infant daily exposures to pyrethroid pesticides (PYRs), used in the elimination of harmful organisms in the family environment and agricultural activities, may have an impact on children's language development.To determine the impacts of prenatal and infant PYRs exposure on 2-year-old toddlers' language development.From January 2016 to December 2018, women in the third trimester of pregnancy, in Yunnan rural area, China, were recruited, and the development of their newborns was observed from birth till the age of two. We examined three PYRs metabolites: 3-phenoxybenzoic acid (3PBA), 4-fluoro-3-phenoxybenzoic acid (4F3PBA), and cis-2,2dibromovinyl-2,2-dimethylcyclopropane-1-carboxylic acid (DBCA) in urine samples collected from women in the third trimester of pregnancy and their infants of 6-8 months after birth, and assessed language development of 2-year-old toddlers by the Bayley Scales of Infant and Toddler Development-Third Edition (BSID-III). Generalized linear models were used to analyze the impacts of exposure to PYRs on 2-year-old toddlers' language development.The median concentrations of 3PBA, 4F3PBA and DBCA creatinine-adjusted were 0.21, 0.19, and 0.15 μg/g in pregnancy, and 0.25, 0.72, and <LOD μg/g in infancy, respectively. The average scores of Language Composite (LC), Receptive Communication (RC), and Expressive Communication (EC) in 2-year-old toddlers were 100.0 ± 16.5, 10.0 ± 3.1, and 10.0 ± 3.1, respectively. Multiple linear regression analyses manifested that the concentration of 4F3PBA in infancy was negatively correlated with toddlers' RC scores [β = -0.43 (95 % CI: -0.85, -0.01), p = 0.046]; the total PYRs metabolites in infancy were negatively correlated with boys' LC scores and RC scores [β = -1.14 (95 % CI: -2.25, -0.04, p = 0.042 and β = -0.23 (95 % CI: -0.44, -0.01), p = 0.039, respectively]. Generalized linear models showed that exposure versus non-exposure to PYRs with DBCA in infancy increased 4.58 times risk of language development delay (OR = 5.58, 95 % CI: 1.76-17.68, p = 0.004) and a 21 % risk of not passing expressive communication items (OR = 1.21, 95 % CI: 1.01-1.45, p = 0.040). The risk of not passing RC items increased 9 % with 4F3PBA increasing per 1 μg/g in infancy (OR = 1.09, 95 % CI: 1.02-1.15, p = 0.007). The probability of not passing RC items ascended with infant 4F3PBA (rs = 0.57, 95%CI: 0.48-0.67, p < 0.001).Women in the third trimester of pregnancy and their infants were widely exposed to low-dose PYRs. Infant daily exposure to PYRs may negatively impact toddlers' language development, with the 6-8 months age bracket being a sensitive window. The probability of toddlers' language development delay may be predicted by PYRs metabolites of infants aged 6-8 months.Copyright © 2022. Published by Elsevier B.V.