Objectives: The objective of this study was to evaluate the impact of phthalate exposure on pregnancy duration and birth outcomes based on the Polish Mother and Child Cohort (REPRO_PL). Material and Methods: Phthalate exposure was determined by measuring 11 phthalate metabolites (mono-ethyl phthalate (MEP), mono-iso-butyl phthalate (MiBP), mono-n-butyl phthalate (MnBP), 3OH-mono-n-butyl phthalate (OH-MnBP), mono-benzyl phthalate (MBzP), mono‑ (2-ethylhexyl) phthalate (MEHP), mono-(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP), mono-(2-ethyl-5-oxohexyl) phthalate (MEOHP), mono-hydroxy-iso-nonyl phthalate (MHiNP), mono-oxo-iso-nonyl phthalate (MOiNP), and mono-n-octyl phthalate (MOP)) in the urine collected from 165 mothers during the third trimester of pregnancy by high performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS). The following measures at birth were considered: gestational age, birth weight, length as well as head and chest circumference. Results: Pregnancy duration was inversely associated with natural log concentrations (μg/g creatinine) of MEP (standardized regression coefficient (β) = –0.2, p = 0.04) after adjustment for a variety of confounders. Significant impact of MOiNP on head circumference (β = –0.1, p = 0.05) was also observed. Conclusions: The study findings add further support to the hypothesis that phthalate exposure may be associated with shorter pregnancy duration and a decreased head circumference, and underscore importance of public health interventions to reduce that exposure.