Determinants of birth interval in a rural Mediterranean population (La Alpujarra, Spain)

The fertility pattern, in terms of birth intervals, in a rural population n ot practicing contraception belonging to La Alta Alpujarra Oriental (southe ast Spain) is analyzed. During the first half of the 20th century, this pop ulation experienced a considerable degree of geographical and cultural isol ation. Because of this population's high variability in fertility and there fore in birth intervals, the analysis was limited to a homogenous subsample of 154 families, each with at least five pregnancies. This limitation allo wed us to analyze, among and within families, effects of a set of variables on the interbirth pattern, and to avoid possible problems of pseudoreplica tion. Information on birth date of the mother, age at marriage, children's birth date and death date, birth order. and frequency of miscarriages was c ollected. Our results indicate that interbirth intervals depend on an expon ential effect of maternal age, especially significant after the age of 35, This effect is probably related to the biological degenerative processes of female fertility with age. A linear increase of birth intervals with birth order within families was found as well as a reduction of intervals among families experiencing an infant death. Our sample size was insufficient to detect a possible replacement behavior in the case of infant death. High na tality and mortality rates, a secular decrease of natality rates, a log-nor mal birth interval, and family-size distributions suggest that La Alpujarra has been a natural fertility population following a demographic transition process.