At current growth rates, forecasters expect to see a total of 9.1 billion humans on Earth by 2050, although small shifts in the birth rate might add or subtract one billion people. The rise could be slowed without taking any direct measures to control population. The most effective way to reduce fertility rates is to educate women: worldwide, each additional year of female education lowers the average birth rate further. Greater education correlates with more abstinence, birth control and female employment, each of which lowers birth rates.
Source: United Nations Population Division