Low birth weight and preterm babies have been increasing in Spain since 1980, coinciding with important changes in the social and demographic structure of childbearing populations—including the contribution of a 25% of foreign mothers—and with increasing medical intervention in births. This study, based on 5,990,613 births, compares the temporary trends in reproductive patterns and birth outcomes in Spanish and foreign mothers during the period 1996–2009 and evaluates for the years 2007 to 2009 the relative contribution of mother's origin and Caesarean section to birth weight variability. Foreign mothers maintain their own reproductive pattern, whereas negative birth outcomes increase in all groups. Results from logistic regression analysis show that besides late maternity and primiparity also Caesarean section increases the risk for low birth weight. The reduction in Caesarean section rates between 2007 and 2009 might explain the reduction of low birth weight detected. A change of tendency simultaneously appears in most maternal and newborn characteristics, and in the mode of delivery in all ethnic groups since 2008. Coincidence in the timing of the change of trends points to a common factor. We suggest that the current world financial crisis could be this common cause, a hypothesis to be contrasted in future research. 1. Introduction Low birth weight and preterm babies, which are widely used for assessing the health of populations, have been steadily increasing in Spain since 1980 [1] as in other Western countries. Population variability in birth outcomes and their temporal trends are closely related with the social and demographic changes in the structure of childbearing populations. European countries, including Spain, have experienced maximum decrease in fertility and a huge increase in the age of maternity [2], characterizing the so called “Second Demographic Transition” [3]. In Spain the reduction of fertility started later (1977) but occurred at a faster rate, reaching the lowest value in 1996 [4]. Nevertheless, whereas the decrease of fertility in Europe slows down and stabilises from the mideighties onwards, fertility in Spain inverts the trend and increases from 1997 to 2008, coinciding with a rapid increase of economic migrant women and men. Spain—which was a country of emigrants until the second half of the Twentieth Century—has become over the past twenty years a host nation for immigrants, most of which coming from South America, the Maghreb, and East Europe, whose populations are undergoing their specific demographic transitions [5]. The
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