In some countries women average five or six children, or even seven children in a few countries. Even if the next generation has fewer children than the previous one, the population will grow as long as couples average more than two children each. Because of high fertility in the past, most developing country populations are young. As young people reach childbearing age themselves, most will have several children of their own. Even after fertility falls to replacement level, it takes at least another generation or two for population size to stabilize.
Time is of the essence.
If replacement-level fertility were reached worldwide by the end of this decade which is virtually impossible world population would eventually level off at less than 9 billion. The actual date and thus the ultimate world population size is likely to be somewhere in between.