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Mosquitoes also prefer pregnant women, a fitting prey since only female mosquitoes bite at all out of a need to develop fertile eggs. Pregnant women on average have higher metabolic rates than nonpregnant women. One study found pregnant women exhale 21% more CO2 than their nonpregnant counterparts. Pregnant women are also at another disadvantage: their body temperatures tend to be higher, another mosquito attractor.

https://www.scientificamerican.com/article/do-mosquitoes-bite-some-people-more-than-others/