Never before have so many people shared the planet earth. For 99.9 percent of humankind's existence, our world population was less than 10 million people. By the year 2010, there will be twenty-one megacities each with a population greater than 10 million. From a historical perspective, this growth has occurred almost instantaneously. Global Fertility Trends and the Role of Population Momentum Fertility rate is the average number of children a woman will have in her lifetime given current birth rates.
Fertility rates vary widely from country to country and are dependent on a large number of factors.
Globally, average fertility rates have fallen since the early 1950s from about 5 births per woman to the current rate of 2.9 births. This decrease is due in large measure to increased educational and economic opportunities for women and the provision of voluntary family planning. In absolute numbers, we add approximately 77 million people to the planet every year. Even though each individual woman is having fewer children, there are simply so many women in their childbearing years that rapid population growth is still occurring. Childbearing years have been defined as the ages between 15-45.