Read J. B. DeLong's Notes: In the Shadow of Malthus:
We know what a pre-industrial pre-birth-control population that is moderately well-off (an annual per-capita GDP of, say, $1100 in Maddison's 1990 international dollars) does: like the North American colonists, it doubles in a generation with a population growth rate of more than 2% per year. We know that the pre-1800 human race as a whole never achieved this.
What held back population growth? What keeps the numbers of the human race from growing at more than 0.2% even under the most favorable pre-industrial conditions? The conclusion seems inescapable: desperate poverty. For the overwhelming bulk of recorded history, population growth rates have been kept low by poverty so dire that women's fat reserves are so low that ovulation is a hit-or-miss affair, and by poverty so dire that nutritional deficits are so great as to seriously compromise immune systems' abilities to deal with the endemic disease pool.
Some commenters have different assessments, though they're hardly more encouraging: the likely culprits are disease and cyclical famine (with ensuing social upheaval), rather than infertility due to constant malnutrition. Anyway, the thread's worth reading.