Boletín de la RSEHN. Sección Biológica

La población andaluza ha experimentado en la segunda mitad del siglo XX cambios importantes en sus modelos reproductores que se han reflejado en la estacionalidad de la natalidad. Entre 1941 y 1960 se ha observado una mayor frecuencia de nacimientos en los meses de invierno, cuyas concepciones serían primaverales; en las dos dŽcadas m‡s recientes el modelo muestra un único m‡ximo en septiembre, definido por concepciones en diciembre. La distribución mensual de los matrimonios tambiŽn ha variado desde un patrón caracterizado por m‡ximos en los últimos meses del año hacia otro, actual, donde las bodas se celebran preferentemente en agosto y septiembre. La mayor probabilidad de concebir en primavera se ha relacionado con un mecanismo hormonal modulado por el fotoperiodo, propio de poblaciones con fertilidad natural; el m‡ximo de septiembre se debe al incremento en la frecuencia de concepciones observado en torno a las fiestas de navidad y año nuevo; la nupcialidad no muestra un efecto importante sobre la distribución temporal de la natalidad. La estacionalidad de los nacimientos habría evolucionado desde un modelo regulado por el ambiente a otro determinado por condicionantes de tipo sociocultural en respuesta a los cambios recientes experimentados por la población andaluza en su dinámica demográfica.

Seasonality of birth is a commonly described phenomenon in human populations. The great variety of observed patterns can be explained by different factors related to the dynamics of human reproduction. Those factors include environmental ones (such as light, temperature or availability of food), sociocultural ones (such as seasonality of marriages, festivities, holidays or annual cycles of work), and others related to the seasonal variation in the probability of pregnancy loss. Human populations can modify the seasonal rhythms of births in response to changes in their reproductive models. In the second half of the twentieth century, Andalucía has experienced important changes in the dynamics of human reproduction. The transition in fecundity has been defined by a reduction of family sizes, an increase at the reproductive ages and a higher extramarital reproduction. Simultaneously, seasonality of births has shown important changes. In this paper, time trend in the seasonality of births is studied in Andalucía between 1941 and 2000. During this period, monthly distributions of births have shown important changes and three stages can be recognized (Fig. 1). In the first twenty years, the distribution of births has shown a peak in winter months; consequently, conceptions were in springs months. In the next twenty years, the differences between month frequencies have smoothed. In recent years, a new pattern characterised by a maximum in September has defined the distribution; so, the highest frequencies of conceptions were in December. Monthly distribution of marriages has been studied in order to assess the role that nupciality has played on the seasonality of births (Fig. 2). Time distribution of marriages has changed from a model defined by a maximun in the last months of the year to a pattern in which the highest frequencies of weddings were in the summer months, August and September. Except for the first decade analysed, November has shown low frequencies of marriages. There is no coincidence between the months with the highest number of marriages and those with the highest frequencies of estimated conceptions. Consequenty, seasonality of births is not strongly modulated by monthly distribution of marriages. High frequencies of conceptions in spring months is a widely observed phenomenon in pretransitional populations with natural fertility models from Spain and the rest of the world. Several authors have proposed that annual rhythms in human reproduction have a physiological basis in which photoperiodism and melatonin are essential factors. Long days in spring could favour conceptions by activating the reproductor axis in a metabolic pathway modulated by melatonin. Seasonality of the first studied years could be explained by this mechanism. High frequencies of births in September are explained by an increased number of conceptions around Christmas and, mainly, New Year festivities. Daily distribution of conceptions between 1976 and 2000, estimated from birth«s dates, shows only one peak around the last day of the year (Figure 3). High frequency of conceptions in December and January is a widely observed phenomenon in current populations; it has been related to an increase in sexual activity during Christmas and New Year festivities. This maximun can«t be explained by marriages. Changes in the seasonality of reproduction in Andalucia in the second half of the twentieth century can be explained by the transition between two different models. One of them has a biological basis and it is modulated by environmental factors, like photoperiod. For recent years there is a behavioral explanation; the period from Christmas to New Year is associated with an intensified social activity that provides an increase in sexual activity. This increased sexual activity during Christmas and New Year has been related to unwanted outcomes by differents authors. Several studies have described a higher frequency of induced abortions and sexually transmitted infections in relation to the maximum in the seasonal pattern of conceptions. In Andalucía that kind of data are not available, but the similarity in the seasonal distribution of conceptions suggests similar consequences. This new model may have biological implications in human populations: sexually transmitted diseases and induced abortions are not adaptative traits and they can reduce individual«s probability of survive. If this relation is confirmed, behavioral changes associated to the demographic transition may be establishing new relations between seasonality of reproduction and fitness in human populations.