Display omitted Prenatal phthalate exposure has been linked to reductions in fetal growth in animal and laboratory studies, but epidemiologic evidence is equivocal. Examine the association between prenatal phthalate metabolite mixtures and fetal growth and evaluate whether that association is modified by fetal sex or omega-3 intake during pregnancy. Analyses included 604 singleton pregnancies from TIDES, a prospective pregnancy cohort with spot urine samples and questionnaires collected in each trimester. Pregnancy-averaged phthalate exposure estimates were calculated as the geometric means of specific-gravity corrected phthalate metabolites. Fetal growth outcomes included birthweight and length, and ultrasound-derived size and velocity of estimated fetal weight, femur length, abdominal and head circumferences in the second and third trimesters. We used a novel application of quantile g-computation to estimate the joint association between pregnancy-averaged phthalate exposure and fetal growth, and to examine effect modification of that association by infant sex or omega-3 intake during pregnancy. There were few statistically significant differences in birth size and fetal growth by exposure. A one-quartile increase in the phthalate mixture was modestly associated with reduced birthweight(β 95% confidence interval): −54.6 −128.9, 19.7 grams; p = 0.15) and length (−0.2 −0.6, 0.2 centimeters; p = 0.40). A one-quartile increase in the phthalate mixture was associated with reduced birth length in males (−0.5 −1.0, 0.0 centimeters) but not for females (0.1 −0.2, 0.3 centimeters); interaction p = 0.05. The phthalate metabolite mixture was inversely associated with ultrasound-derived fetal growth among those with adequate omega-3 intake. For example, a one-quartile increase in the phthalate mixture was associated with reduced abdominal circumference in the third trimesters in those with adequate omega-3 intake (−3.3 −6.8, 0.1 millimeters) but not those with inadequate omega-3 intake (1.8 −0.8, 4.5 millimeters); interaction p = 0.01. Prenatal phthalate exposure was not significantly associated with fetal growth outcomes, with some exceptions for certain subgroups.