This chart shows the long-term effects over the next three centuries of slightly different assumptions about birthrates.  The high scenario assumes a birthrate of 2.35 children per woman; the medium assumes 2.1 (replacement level); and the low assumes 1.85 children per woman.  From World Population to 2300, United Nations, 2004, p 27

In the book, Australian demographer John Caldwell notes that the “low” scenario relies on a continuation of the current downward trend in birthrates, but it reflects birthrates in European countries higher than they are currently.  Here’s what he wrote (pp 1113-114):

The medium

scenario from 2175 posits fertility at replacement

level with long-term stability around 9 billion, a

figure first approximately reached as early as

2045. The low scenario exhibits an eventual total

fertility of 1.85 children per woman and a global

population slowly sinking from 7.5 billion in 2075

to 2.3 billion in 2300 and then continuing to fall at

0.3 per cent per annum. This would mean a fur-

ther halving in the following quarter of a millen-

nium with a 2550 world population back to the

level of 1850. The high scenario is finally charac-

terized by a total fertility of 2.35 children per

woman and a 2300 population of over 36 billion

increasing by 0.54 per cent annually. …

One point should be noted. The relatively mod-

est populations projected for 2050, with low, me-

dium and high scenarios at 7.4 billion, 8.9 billion

and 10.6 billion, respectively, and the modest

peak populations for the low and medium scenar-

ios of 7.5 billion and 9.2 billion are heavily de-

pendent on continuing fertility falls across the de-

veloping world towards sub-replacement fertility

over the next half century. This, in turn, depends

on continuing economic and social globalization,

with all countries eventually becoming consumer

societies. …

Surprisingly, the low scenario is by no means

implausible. It posits a 2100 population of 5.5

billion people, only 27 per cent or two billion per-

sons below the peak, over two generations earlier,

and equal to global population in the early 1990s.

What would render such a population acceptable

is that some sensitive national populations are

projected to be higher in 2300 than is commonly

thought. Some low projections for 2300 of this

type are the following: Germany, 31 million (and

still 40 million in 2200), France, 22 million,

United Kingdom, 26 million and the United

States, 153 million. This would be achieved with a

final total fertility of 1.85 children per woman,

admittedly well above total fertility rates in much

of contemporary Europe…