Perturbations in fetal growth may have adverse consequences for childhood and later life health. Organophosphate pesticide (OP) exposure has been associated with reduced birth weight at delivery but results are not consistent. We investigated this question by utilizing ultrasound measures of size in utero in combination with measures from delivery. Within Generation R, a population-based prospective cohort conducted between 2002 and 2006 in Rotterdam, Netherlands, we measured dialkyl phosphates (DAPs), OP metabolites, in urine samples from early, middle, and late pregnancy and created a subject-specific average to estimate OP exposure ([Formula: see text]). Ultrasound measures of head circumference, femur length, and estimated fetal weight from middle and late pregnancy and delivery measures were converted to standard deviation scores (SDS). Associations with DAP average were examined in linear mixed effects models that included an interaction term between gestational age at measurement and DAP average to investigate whether the relationship differed over time. Windows of vulnerability to exposure were assessed by modeling urinary DAPs from each visit in relation to growth measurements. A 10-fold increase in average DAPs was associated with a [Formula: see text] SDS decrease in fetal length (95% [Formula: see text], [Formula: see text]) and a [Formula: see text] SDS decrease in estimated fetal weight (95% [Formula: see text], [Formula: see text]) at 20 weeks of gestation. These differences corresponded to 5% and 6% decreases relative to the mean. Effect estimates were greatest in magnitude for DAP concentrations measured early in pregnancy. Associations between average DAPs and growth measures at delivery were positive but not significant for head circumference and length and were null for weight. Maternal urinary DAPs were associated with decreased fetal weight and length measured during mid-pregnancy, but not at delivery. https://doi.org/10.1289/EHP4858.