This is one of those articles where the introduction is very informative
In the last 50 years there has been a substantial reduction in the world's total fertility rate (TFR). The Latin American and the Caribbean (LAC) region is not an exception to this rule, and it has experienced a deep process of demographic transition. The reduction in the total fertility rates is a homogeneous phenomenon throughout the entire region. An illustration of the state of demographic transition in the LAC region can be observed from the behavior of the TFR during the first decade of this century. The region's TFR was 2.67 children per woman in 1999, and by the end of 2010 the TFR was 2.12. Surprisingly close to the widely accepted replacement rate of 2.1.The evolution of the fertility rate for young populations is especially important because of the negative consequences of teenage childbearing. This is a problem widely associated in the literature to low human development and poverty (Joshi and Schultz, 2007; Buvinic, 1998; Burt, 1998; Gage, 1995; Singh and Wulf, 1990; Hayes, 1987). During the last decade, there has been also a reduction in the Fertility Rate for women between 15 and 19 (FR15-19) in the LAC region. The FR15-19 decreased from 83.95 children per 1000 women in 1999 to 71.68 in 2010. Certainly, this is an important reduction in the FR15-19; nevertheless, in general terms it has been smaller that the reduction that the TFR has experienced during the same period. Between 1999 and 2010, the reduction in the TFR was 26% while the reduction in the FR15-19 was 17%. This is a very interesting phenomenon which, among other things, implies that adolescent fertility has become a more important component of total fertility in most of the LAC countries. This means that relative to adult fertility, adolescent fertility is becoming greater and greater in Latin America.
With some exception like Argentina and Peru, the contribution of adolescent fertility to the total fertility has increased continuously in almost all LAC Countries. As mentioned before, this is due to the faster reduction of the fertility rate for the adult population in comparison to the rate for teenagers' population (Florez and Soto, 2007B). A good illustration of this phenomenon can be observed from the evolution of the ratio adolescent fertility to total fertility (per 1000 women). In 1999, adolescent fertility was 15.72% of total fertility in developing LAC countries, and by 2010 this ratio had increased to 16.29%. There are some remarkable cases as Brazil and Ecuador where the ratio adolescent fertility to total fertility increased by more than 2 percentage points between 1999 and 2010. The result of this phenomenon is, on average, an earlier individual onset of childbearing.
The article is by Leonardo Fabio Morales who attributes to peer effects why the reduction on FR15-19 is lower than TFR in LA.HT: Clarence Nkengne Tsimpo