Abstract In human reproduction process, analysing the birth interval is a crucial and important aspect. Normally, the distribution of fetal development stage interval of any exact order may be the same as the distribution of Post-partum Amenorrhoea interval of the same order but it cannot be same. Here the interval between the distribution of a specific order of fetal development stage interval and Post-partum Amenorrhoea interval of the same order is analysed. In this context, it is demonstrated the distinction between the Post-partum Amenorrhoea interval and fetal development stage interval empirically by allowing for a actual data set. Further, the distinction between these distributions is demonstrated theoretically, by taking certain proposed values of fertility factors involved in the stochastic model proposed for the purpose.