Birth weight for gestational age (BW/GA) has been associated with a risk of adverse health outcomes. Biological indices of pregnancy complications, maternal mid-pregnancy serum biomarkers and placental pathology may shed light on these associations, but at present, they are most often examined as single entities and offer little insight about overlap. In addition, these indices are typically assessed in relation to the extremes of the BW/GA distribution, leaving open the question of how they relate to the entire BW/GA distribution. Addressing issues such as these may help elucidate why postnatal health outcomes vary across the BW/GA continuum. In this study, we focused on a subset of women who participated in the Pregnancy Outcomes and Community Health Study (n = 1371). We examined BW/GA (i.e. gestational age and sex-referenced z-scores) in relation to obstetric complications, second trimester maternal serum screening results and histologic evidence of placental pathology along with maternal demographics, anthropometrics and substance use. In adjusted models, mean reductions in BW/GA z-scores were associated with preeclampsia (β = −0.70, 95% CI −1.04, −0.36), high maternal serum alpha fetoprotein (β = −0.28, 95% CI −0.43, −0.13), unconjugated estriol (β = −0.31/0.5 multiples of the median decrease, 95% CI −0.41, −0.21) and high levels of maternal obstructive vascular pathology in the placenta (β = −0.46, 95% CI −0.67, −0.25). The findings were similar when preterm infants, small-for-gestational age or large-for-gestational age infants were excluded. More research is needed to examine how the factors studied here might directly mediate or mark risk when evaluating the associations between BW/GA and postnatal health outcomes.