Maternal urine collected at approximately 17 wk gestation was shipped overnight, unrefrigerated, to the central biorepository in Oslo, Norway for immediate processing. Urine was transported in a commercially available urine transport tube with a preservative to prevent bacterial growth (chlorhexidine plus ethyl paraben and sodium propionate) (UAP Vacutainers; Becton-Dickinson) (Rønningen et al. 2006 (link)). In a previous quality control (QC) study in MoBa, no impact was found on the measurement of phthalates from this preservative (Ye et al. 2009 (link)). Analysis of urine for phthalate metabolites was conducted at the Norwegian Institute of Public Health. Methods have been previously described (Sabaredzovic et al. 2015 (link)). Briefly, on-line column switching liquid chromatography coupled with tandem mass spectrometry was used to measure 12 phthalate metabolites: monoethyl phthalate (MEP), a metabolite of diethyl phthalate; mono-iso-butyl phthalate (MiBP), a metabolite of di-iso-butyl phthalate; mono-n-butyl phthalate (MnBP), a metabolite of di-n-butyl phthalate; monobenzyl phthalate (MBzP), a metabolite of BBzP; mono-2-ethylhexyl phthalate (MEHP), mono-2-ethyl-5-hydroxyhexyl phthalate (MEHHP), mono-2-ethyl-5-oxoyhexyl phthalate (MEOHP), mono-2-ethyl-5-carboxypentyl phthalate (MECPP), and mono-2-methylcarboxyhexyl phthalate (MMCHP), metabolites of DEHP; and mono-4-methyl-7-hydroxyoctyl phthalate (OH-MiNP), mono-4-methyl-7-oxooctyl phthalate (oxo-MiNP), and mono-4-methyl-7-carboxyheptyl phthalate (cx-MiNP), metabolites of di-iso-nonyl phthalate (DiNP). A QC sample of pooled urine was created to assess batch-to-batch variability and assay precision. In each analytic batch, procedural blank samples, two in-house control urine samples and 4–6 QC pooled urine aliquots were included. External reference samples from the National Institute of Standards and Technology [NIST; Standard Reference Material (SRM) 3673] were also analyzed in every fourth analytical batch. Cases and controls were randomly allocated across analytic batches. The analyst was blinded to QC, case, and control samples. To account for urinary dilution, specific gravity was measured using a pocket refractometer (PAL-10S) from Atago. In brief, 180μL of the urine sample was placed onto the prism surface, and the specific gravity was measured with the refractometer. The coefficient of variation (CV) was <0.1% for the in-house control urine samples. In laboratory-blinded QC samples, average batch CVs were <5% .