Although women are having fewer children than ever before in some countries the population is expected to continue growinf for another 20 years, how is this possible?

I believe the answer is: Population momentum

Even if women generally want to have fewer children, the number of population in childbearing age is still around, and would most likely grow up having the old traditional principles. Because of this, the population could only be expected to decrease in the next child-bearing generation (about 20-30 years later)