Why do pregnant women get anemic?

They shouldn't: If the prenatal course and supervision is adequate (including lab. Tests, recommendations, repeat blood tests, appropriate prenatal vitamins (possibly with iron and folic acid!) and a good diet anemia shouldn't be an issue, unless the patient is genetically predisposed to anemia!

Blood changes: First of all, most pregnant women are not really anemic. We use different definitions of anemia (low blood count) when pregnant. But their blood counts do lower. Why? Because they double the volume of fluid in the blood but only increase the amount of blood cells by about a third. So the proportion goes down. But overall, they increase the ability of the body to get oxygen to tissues and baby.