Objectives To identify factors predicting maternal sex steroid hormone concentrations in early pregnancy. Methods The Infant Development and the Environment Study recruited healthy pregnant women from academic medical centers in four US cities. Gold standard liquid chromatography-tandem mass spectrometry was used to measure maternal sex steroids concentrations (total testosterone [TT], free testosterone [FT], estrone [E1], estradiol [E2], and estriol [E3] concentrations) in serum samples from 548 women carrying singletons (median = 11.7 weeks gestation). Women completed questionnaires on demographic and lifestyle characteristics. Results In multivariable linear regression analyses, hormone concentrations varied in relation to maternal age, body mass index (BMI), race, and parity. Older mothers had significantly lower levels of most hormones; for every year increase in maternal age, there was a 1-2% decrease in E1, E2, TT, and FT. By contrast, each unit increase in maternal BMI was associated 1-2% lower estrogen (E1, E2, E3) levels, but 1-2% higher androgen (TT, FT) concentrations. Hormone concentrations were 4-18% lower among parous women, and for each year elapsed since last birth, TT and FT were 1-2% higher (no difference in estrogens). Androgen concentrations were 18-30% higher among Black women compared to women of other races. Fetal sex, maternal stress, and lifestyle factors (including alcohol and tobacco use) were not related to maternal steroid concentrations. Conclusions for Practice Maternal demographic factors predict sex steroid hormone concentrations during pregnancy, which is important given increasing evidence that the prenatal endocrine environment shapes future risk of chronic disease for both mother and offspring.