For many women, pregnancy can be as tiring as it is exciting. While some fatigue is normal for moms-to-be, excessive tiredness may be a sign of anemia, a deficiency of red blood cells (which carry oxygen in the blood) that affects up to half of pregnant women.

Why is this condition so common? During a normal pregnancy, the volume of fluid in a woman's blood increases by as much as 50 percent, diluting the concentrations of red blood cells and hemoglobin, a critical protein in these cells. If levels drop too low, the body's ability to carry oxygen to the lungs, tissues, and the baby can be compromised.

Untreated, a severe case of anemia can cause poor fetal growth, preterm birth, and low birth weight. But since women are routinely tested for the condition at their first prenatal visit and then again during their second trimester, doctors usually catch the condition before it becomes serious. "In the majority of cases, when the condition is diagnosed and treated, it's not going to harm you or your baby," says Laura Riley, M.D., a maternal-fetal medicine specialist at Massachusetts General Hospital, in Boston. Here's what you need to know:

• Causes: The most common cause is just pregnancy itself, says Dr. Riley. When anemia is the result of increasing blood fluid volume (the most common cause), it's called "physiologic" anemia of pregnancy. The second most common type, iron-deficiency anemia, comes from decreased iron stores in the body. (Iron is needed in pregnancy to make hemoglobin and red blood cells for both mom and her baby-to-be.) Less commonly, anemia in pregnancy can also be caused by a folic acid or vitamin B12 deficiency. A more serious and rare form is caused by an inherited predisposition to anemia called thalassemia, which is mainly seen among people from the Mediterranean and the Middle East. Even more rare, anemia in pregnancy can be the result of sickle-cell anemia, which is most commonly seen in African- Americans.

• Symptoms: Most women won't have any symptoms. If they do occur, symptoms can be identical to those of pregnancy: fatigue, a drop in exercise tolerance, and shortness of breath. In more acute cases, a woman may also notice pale skin, lips, and nails; dizziness; and a rapid heartbeat.

• Diagnosis: Most cases of anemia are detected during the complete blood count (CBC) done on every woman when she comes in for her first prenatal visit and again at 28 weeks, says Dr. Riley. If anemia is found, your doctor may order more tests to target the cause.

• Treatment: There's nothing you can do about physiologic anemia, says Dr. Riley, except wait it out. Moms-to-be with iron-deficiency anemia will be counseled to eat iron-rich foods and to take an iron supplement (in addition to a prenatal vitamin) together with vitamin C for better absorption. (Caffeine, on the other hand, can inhibit the absorption.) Sources of iron-rich foods include red meat, beans, peas, dark-green leafy vegetables, iron-fortified cereals, dried fruits, nuts, and whole-wheat breads.

Though taking an iron supplement can lead to constipation and nausea, it reduces symptoms and restores red-blood-cell count to normal levels, says Dr. Riley. Most iron-deficiency anemia cases will clear up in a few weeks. Exercise, as well as extra fluids and fiber, can help reduce the constipation associated with iron supplements. Those with folic acid- or B12-deficiency anemia will be instructed to take supplements of those vitamins.

• Prevention: Women who are pregnant or thinking about trying to become pregnant should remember to take a daily multivitamin that includes iron and folic acid, in addition to eating a variety of iron-rich foods.

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