Atheism and fertility: Using World Values Survey and World Factbook data, I calculated the correlation for 59 countries between the percent who don't believe in God and: 1) the total fertility rate (TFR), and 2) the closeness of the TFR to 2. I computed the first for those concerned about Western countries with below-replacement fertility and the second for those concerned with population stability or over-population.
The Pearson correlation coefficient for the first pair is -.40; for the second pair, it's .25. So, atheism is moderately correlated with subreplacement, but only weakly correlated with population stability.
(If I get time later, I'll control for socioeconomic level since doing so might erase these correlations.)