Low fertility is one of the most important demographic challenges of our country with economic, social and geopolitical implications. The purpose of this article is to identify the impellers and consequences of the transition to the third child by applying a qualitative approach and contextual theory. The participants in this research are 25 women under the age of 40 living in Tehran who have three children and the process of transitioning to their third child occurred after the announcement of the general population policies in 2013 until the time of the survey. The findings indicate that the intention of pure religious practice, extending kinship network, unintended pregnancy, and gender preference are included among the most important causes of transition to a third child. The influence of the mentioned factors has taken place through some background conditions (such as age of marriage, economy, and changes in demographic policies) as well as intervention (family integration). Moreover, the results indicated that the benefits of the extending family kinship network for the future of parents and children as well as its educational-emotional functions for the children were among the most important positive consequences affecting the transition to the third child. Nevertheless, the foresaid families with three children bear financial expenses, the cost of the opportunity to have child, social costs and health (physical and mental) costs. Therefore, applying and implementing policies to support families for increasing the benefits of childbearing and reducing the foresaid expenses can result in an increase in fertility in our country.