This paper examines the nexus between age at first birth and fertility differentials among
women in Osun State, Nigeria. The rationale for this central objective was based on the fact
that a lucid explanation on the relationships that exist between various indicators of socioeconomic
development and fertility level after controlling for the age at first birth is an
important phenomenon in understanding developmental process at both individual and
societal levels. Thus, basic knowledge about the intricacies and interrelationship between
these variables is of direct relevance to planners and policymakers attempting to integrate
population variables into development planning. To achieve the central objective of the
paper, empirical quantitative data from Osun State were used. Findings of the study show that
irrespective of socio-demographic characteristics of women (such as place of residence,
current age, occupational status, educational level, marital status and form of marriage),
women who had their first birth below age 20 years exhibit significant higher number of
children ever born in comparison with women who had their first birth by age 20 years and
above at P<0.01 most especially when the later were at similar categories to the former.
Thus, age at first birth is a strong determinant, among others, responsible for relatively high
fertility level in the State. Consequently, it is imperative for policymakers and development
partners in the State to develop appropriate policies and programmes that will address and
inhibit early age at first birth in order to control the high natural population growth rate of
Osun State, Nigeria