Social inequalities, advanced maternal age, and birth weight. Evidence from a population-based study in Spain.

OBJECTIVE: To examine whether advanced maternal age (≥40 years) is linked to an increased likelihood of low or high birth weight among native and foreign-born mothers giving birth in Spain.

METHOD: A cross-sectional study was conducted using a novel database provided by the Spanish National Statistics Office which links the 2011 Census with information on individual births (2011-2015) from the Vital Statistics (Natural Movement of the Population). First, multinomial logistic regression models were used to estimate the potential association between maternal age and the likelihood of having a baby with low or high birth weight. Second, average adjusted predictions of giving birth to children with low, high, and adequate weight for the origin and the maternal age at birth were also calculated.

RESULTS: Findings indicate that women with advanced maternal age showed an increased probability of giving birth to low birth weight infants. Conversely, mothers aged below <30 years had an elevated risk for high birth weight infants. When considering maternal migratory status, the findings were mixed. On one hand, foreign-born mothers showed a higher likelihood of delivering infants with high birth weight; on the other, they displayed a lower risk of low birth weight among newborns in comparison to Spanish natives.

CONCLUSIONS: The study addresses two key aspects. First, it highlights the increased risk of low birth weight in mothers delivering at an advanced age. Second, it emphasizes the importance of accounting for maternal migratory status when investigating the association between age at birth and birth weight outcomes among immigrant mothers.