That's called infertility, and it's a separate issue. Fertility rate refers to children per woman, and this is decreasing globally to unsustainable levels. There is a phenomenon known as the "demographic transition" that refers to the demographic shift of pre-industrial high births and high mortality, to high births and low mortality due to modern science, to finally low births and low mortality. The only successful policy to reverse baked-in population decline so far has been to accept large numbers of immigrants. Obviously this can't be universally applied because those immigrants have to come from somewhere.