2.2. Urinary phthalate measurement

Urine samples were obtained from the mothers at the time of the two pregnancy interviews (mean (SD): 14.0 (4.8) and 26.9 (2.5) weeks gestation) and were stored at −80°C until shipment to the Centers for Disease Control and Prevention in Atlanta, GA for analysis. Concentrations of 11 metabolites of 8 parent phthalate compounds were measured using solid phase extraction coupled with isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (Silva et al., 2007). Phthalate metabolites quantified were mono-ethyl phthalate (MEP), mono-n-butyl phthalate (MnBP), mono-isobutyl phthalate (MiBP), mono-benzyl phthalate (MBzP), mono(carboxyoctyl) phthalate (MCOP), mono(3-carboxypropyl) phthalate (MCPP) and four metabolites of diethyl-hexyl phthalate (DEHP): mono-2-ethylhexyl phthalate (MEHP), mono-(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP), mono-(2-ethyl-5-oxohexyl) phthalate (MEOHP), and mono-(2-ethyl-5-carboxypentyl) phthalate (MECPP). Limits of detection (LOD) ranged from 0.2–0.6 ng/mL. Concentrations below the limit of detection (LOD) were assigned an imputed value less than LOD randomly selected from the lognormal distribution using maximum likelihood estimation (Lubin et al., 2004). Urinary specific gravity was measured using a hand-held refractometer (National Instrument Company Inc., Baltimore, MD) and metabolite concentrations were corrected for urinary dilution using the formula: metabolite concentration * (mean specific gravity - 1)/(sample specific gravity – 1).