Fewer boys born in New York after 9/11 attacksBMJ 2006; 333 doi: https://doi.org/10.1136/bmj.333.7567.516-b (Published 07 September 2006) Cite this as: BMJ 2006;333:516
Anxiety and stress in New York city in the aftermath of the terrorist attack on the World Trade Center five years ago may have resulted in the birth of fewer boys months later, research shows.
Based on more than 700 000 births in New York city between January 1996 and June 2002, the study shows that the birth sex ratio for the city dropped to below one in the January after the attacks—its lowest level (Human Reproduction 2006 Aug 26, doi: 10.1093/humrep/del283).
One theory is that the stress of the attack, particularly in women in the second and early third trimesters of their pregnancy, resulted in a disproportionate loss of male fetuses, so lowering the odds of a male birth.
Previous research has suggested that the sex ratio falls in populations subjected to external stressors, with the odds of a male birth falling with earthquakes, political and social upheavals, and economic downturns. Another explanation is that stress may reduce the conception of boys.
Because 11 September 2001 was a Tuesday, the data for the new study were separated into 28 day periods, amounting to 91 in total, each beginning on a Tuesday.
According to the fetal death explanation, data on pregnancies resulting in live births during periods 77, 78, or 79—November 2001 to the end of January 2002—should show a lower than expected sex ratio. The reduced conception explanation suggests that the ratio would be lower during periods 82, 83, or 84 (end of March 2002 to mid June).
Out of all 91 test periods, the lowest ratio of 0.9995 was in the 79th period, 1-28 January 2002. The mean ratio in the 81 test periods unaffected by the events of 11 September was 1.0510, with a standard deviation of 0.0234.
“Our findings support the male fetal loss explanation,” write the authors from the school of public health at the University of California in Berkeley. They add, “The sex ratio in New York City in the period 1 January to 28 January 2002 fell significantly below (P<0.05; two-tailed test) the value expected from history.”
Because the sex ratios eight, nine, and 10 months after the attacks did not fall, the reduced conception theory seems unlikely.
It is not clear why cohorts in the 20th-24th weeks of pregnancy seemed to be the most vulnerable, say the researchers. “The answer may be related to the finding that the foetal response to maternal stressors appears strongest in the second half of gestation and that mothers may use that response as a test of male fetal robustness,” they say.