We all know that stress is bad for you — particularly when you're pregnant. But researchers recently tested how harmful stress can be to pregnant women by looking at birthing data following one of the most stressful recent events to occur in the U.S., the attacks on 9/11.  

The history books claim that almost 3,000 people died as a direct result of those attacks. It turns out that death toll may be even higher than previously thought. According to a study published in the journal BMC Public Health, pregnant women across the nation were so distressed by the attacks that many suffered a miscarriage.

Researchers for the study discovered that the male fetal death rate increased significantly in September 2001, so much so that it affected the ratio of boys to girls born in a later month. Why did the male miscarriage rate rise so drastically?

According to researchers, male fetuses are believed to be more sensitive than females to stress hormones. Previous studies have suggested that the percentage of male births drops after natural disasters, economic decline or catastrophes. For instance, a 1998 study from Japan found a decline in male births after the Kobe earthquake in January 1995.

The 9/11 attacks were so incredibly tragic, it's hard to imagine the statistics getting any worse. But this study shows that the attacks were even worse than we thought, affecting pregnant women and their families all across the nation.

The opinions expressed by MNN Bloggers and those providing comments are theirs alone, and do not reflect the opinions of MNN.com. While we have reviewed their content to make sure it complies with our Terms and Conditions, MNN is not responsible for the accuracy of any of their information.