Brazil has experienced a rapid demographic transition, going from an estimated TFR of 6.28 children per woman in 1960 to 1.90 in 2010 (IBGE 2012). In this dissertation, I explore the social and demographic determinants of low fertility in the country. The first study analyzes the association of human development, gender equality, and work-childrearing support environment with the likelihood of having a birth in 1991, 2000, and 2010. The results show a declining relative importance of the Human Development Index (HDIm) with time, while the gender equality and work-childrearing support environment increase its relative importance. The emergence of these associations in 2000 and 2010 is likely to be driven by a rise in the share of professional and managerial females in the labor market. In the second study, I explore the extent in which childbearing is a constraint on the employment of mothers with young children in Brazil. Specifically, I test whether an exogenous increase in public child care centers between 2007 and 2009 is associated with an increase in labor force participation and hours worked of mothers with the youngest child aged four or less. The results indicate that the increase in child care availability had a positive impact on female labor force participation and hours worked, specially for married mothers with lower levels of education attainment. The third study assess the plausibility of a demographic technique, the P/F Brass method, used to estimate Brazil’s official TFR. The method increased the TFRs observed in Census Surveys by approximately 12% in 1991, 10% in 2000, and 19% in 2010. Because of the dramatic fertility decline in Brazil in the age groups of 15-19 and 20-24, specially in 2010, and the violation of the stable population assumption, I conclude that the Brass method should no longer be used when estimating the official TFR in the country.