The health of pregnant women and their fetuses are especially sensitive to socioeconomic conditions. This study analyzes the impact of maternal socioeconomic status (SES), evaluated by occupation and maternal education level, in preterm births (PTBs) and in small for gestational age (SGA) fetuses, considering the effect of the potential mediating factors on the SES and birth outcomes.A total of 2497 mother/newborn dyads from the INMA-Spain project were studied. We examined maternal occupation and education in relation to PTB and SGA along with covariate data, using logistic regression analysis. Adjusted models for each of the outcome variables in relation to SES indicators were estimated, considering potential mediating factors.About 4.7% of babies were PTB and 9.7% SGA. Full adjusted logistic regression models showed similar odds ratio (OR) for SGA in both SES indicators. Manual working women or without university studies had higher risk of SGA than their counterpart groups (OR = 1.39% CI = 1.03-1.88 and OR = 1.39% CI = 1.00-2.00, respectively). Likewise, mothers with a manual occupation were at more risk of PTB than those with a non-manual occupation (OR = 1.74 95% CI = 1.13-2.74), but there was no association between education and PTB. Smoking, pre-pregnancy BMI and underweight gain during pregnancy were significantly associated to SGA births. The mother's age, presence of complications and overweight gain during pregnancy were related to PTB.The mother's socioeconomic disadvantage was consistently associated with birth outcomes giving rise to intergenerational transmission of health inequalities. Reducing inequalities requires eliminating the upstream causes of poverty itself.