Now there's evidence of it.
A team at Ohio State University's Wexner Medical Center found that a baby's gender is linked to a mother's immune response.
The immune cells of women carrying girls released more inflammatory proteins than those carrying boys.
"Too much inflammation can really be unhelpful to our bodies' functioning. It can create or contribute to symptoms like fatigue or achiness," says postdoctoral researcher Amanda Mitchell, who led the study.
It could also explain why some women with allergies, asthma, and other medical conditions have stronger symptoms during pregnancy.
Pregnant women who contracted the H1N1 flu during the 2009-2010 outbreak often fared much worse than any other group.
At the time, there were numerous stories of women who needed intubation, ECMO, and other breathing assistance to survive.
The Ohio State team now plans to explore HOW a baby's gender could alter inflammation