Evolución de ciertas características demográficas de las madres de niños sin defectos congénitos a lo largo de los últimos 26 años y por Comunidades Autónomas

Abstract

In this study we analyzed the evolution of maternal age along time (the last 26 years) and by Spanish Regions. We also estimated the sex ratio by each year of maternal age in the first pregnancy and in women with more than one previous pregnancy. The results showed that in all Spanish Regions, with some differences among them, there was an important increase in maternal age as the number of sibs increased. We also observed throughout the country an increase in the number of women working outside of the home. Additionally, the sex ration in the first pregnancy by each year of maternal age showed a high proportion of males in each maternal age from 15 years of, age to 36, but from 37 onwards, there were more females than males. This inversion was not observed in mothers with more than one previous pregnancy, who had more males than females in almost all ages. The progressive incorporation of women to the workforce is one of the factors used to explain the observed increasing age for the first pregnancy. This increasing maternal age could have different consequences, such as the alleged potential effect on the sex ratio at birth. Several authors have tried to explain these changes by applying the Trivers and Willard's model that hypothesizes that vertebrates adaptively vary the sex ratio of their offspring in response to the mother´s physical condition. We posit that this relationship is more complex than this. The notable advances in biomedical scientific knowledge and obstetric care have had a marked influence on human beings, not only as it relates to reproduction but also during all the stages of their lives. This influences all adaptive biological mechanisms that, together with the physiological differences with animals including primates, does not make it possible to apply the Trivers and Willard model to human beings.