Over the last 50 years, least developed countries have experienced dramatic population growth due to high fertility rates and poor economic conditions within these countries. In order to tackle this issue, many least developed countries have made remarkable strides to lower their fertility rates. However, while for some countries fertility rates have been on the decline, for other countries such as those in the Middle East and North Africa (MENA) and sub-Saharan Africa (SSA), their fertility rates are still among the highest in the world. Such variations in fertility rates have led to many studies on this issue. However, the possible role of populations with typical high fertility rates, such as those in slums, have been given much less attention. This paper investigates the role of growing slums as a moderating factor that could possibly contribute to explaining the reasons behind the failure of some regions, such as MENA and SSA, to bring down fertility rates to a satisfactory level. Our panel fixed effects and two-stage least square results of 72 developing countries during the period 1990-2014 support the positive effect of slums on fertility rate after controlling for endogeneity, country and time fixed effects, as well other drivers of fertility.