The impact of delayed maternity on foetal growth in Spain: An assessment by population attributable fraction

Background Delayed childbearing is considered a risk factor for maternal–foetal health. As in other higher-income countries, in Spain age at maternity has steadily increased during the last two decades. Aim To quantify the impact of the delay in the age at maternity on small for gestational age (SGA) categories of <3rd, 3rd–5th and 5th–10th percentiles. Methods 2,672,350 singleton live births born to Spanish mothers in 2007–2015 were analysed. Adjusted relative risk was calculated to estimate the adjusted partial population attributable fractions (PAFp) for mothers aged 35–39 and ≥40 years for each category of SGA considering the interaction between age at maternity and parity. Findings Primipara 35–39 years old mothers have the highest PAFp in the three categories of SGA, with the maximum value for SGA <3rd percentile (2.57%, 95% CI 2.25, 2.88). PAFp for both primipara and multipara ≥40 years old mothers were less than 1%. PAFp for primipara older mothers increased significantly in 2007–2015 for the three categories of SGA, more clearly among those aged 35–39 years. The contribution of multipara mothers of both age groups did not increase significantly during the period. Conclusion Delayed maternity is a significant adjusted risk factor for SGA, contributing to the increase of its prevalence. However, results also suggest a limited clinical impact of delayed maternity on foetal growth. Positive changes in maternal profile associated with the shift in maternal age might contribute to explain the limited impact of mothers aged 35 years and older on negative birth outcome in Spain.