Pregnancy hormones decrease tear production which can lead to eye dryness, irritation and discomfort. Hormones also cause fluid buildup in your eyes, the same way they make your ankles and feet swell up.
But fret not! Your sight usually goes back to normal after you've given birth. More serious vision problems, however, can be a sign of gestational diabetes or high blood pressure, so be sure to talk to your doctor.
You can read more in this article, they even provide simple solutions for tired blurry eyes during pregnancy:
http://www.whattoexpect.com/pregnancy/symptoms-and-solutions/vision.aspx
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