From the Economist (November 15, 2007)
A Boy or a girl? That is usually the first question asked when a woman gives birth. Remarkably, the answer varies with where the mother lives. In rich countries the chances of its being a boy are about 5% higher than in poor ones.
Strange as it might seem, the terrorist attacks of September 11th 2001 shed light on the enigma. Studies noting the sex of babies conceived in New York during the week of the attacks found a drop in the ratio of males to females. That is consistent with earlier studies, which revealed a similar shift in women who became pregnant during floods and earthquakes and in time of war.
Dr Obel suspects the immediate cause is that male pregnancies are more likely to miscarry in response to stress than female pregnancies are, especially during the first three months.
While there are many factors that could potentially influence the gender of a child, the most accurate tool may after all be the ancient Chinese birth chart.