Background: Proper time intervals among pregnancies has a major role in family provision. Investigating the dynamism of the birth spacing, identify factors and their effects on the time interval among pregnancies and determination of optimal spacing is the utmost importance for health planners and families. The purpose was to study the abortion and stillbirth due to short inter-birth spacing, factors affecting inter-births and optimal birth spacing.

Material and Methods: In 2005, a cross-sectional study was conducted in a population of 1064 women of childbearing age in Fars province by multistage random sampling. Information on reproductive history was collected by resident health workers. Variables affecting inter-births were checked by the frailty recurrent events model using S-plus 2000 and SPSS 13 software.

Results: The rate of abortion or stillbirth was 1.7%, which fell with increasing interval between births and reached their lowest (1.9%) at 36-48 months. Mother's level of education, duration of breastfeeding of previous infant, maternal age at birth and father's job showed positive correlation with inter-birth. Mother's age at marriage, having a job other than household, socioeconomic status was negatively correlated with the the time interval to the next delivery. With increasing birth order, inter-birth of each delivery with regard to previous delivery increased on average by one to two months.

Conclusion: The risk of abortion and stillbirth were observed more in subsequent delivery with very low or very long gap interval.