Birth interval is one of the major determinants of the fertility rate. In this paper structural change approach is used to study the instabilities in the effects of different socio-economic factors on birth intervals of children with respect to couple's years of marriage factor in Ahvaz-Iran. A class of M-fluctuation tests for parameter instability is used and based on these tests different evidences of instability including the years of change and factors which changed are estimated and identified. The data analysed come from a sample of women referred to "Health and Medical Centres'' during October and November 2002.