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Blood Test 101

If it seems as if your doctor is drawing endless amounts of blood from you during your pregnancy, here's why:

Blood Type (first visit)

Doctors will need to know this in case you have complications during labor and need a transfusion.

Rh Status (first visit)

If you're an Rh-negative and carry an Rh-positive baby, there's a danger of damage to the fetus if your Rh antibodies cross the placenta. To prevent this, doctors inject you with an Rh immune globulin at 28 weeks.

Complete Blood Count (first visit)

White and red blood cells and platelets are measured to see if you're anemic. The test also determines if your blood clots properly when you bleed.

Rubella (first visit)

The doctor will check to see if you're still immune to rubella. If resistance has waned and you contract rubella during the first trimester, your baby is at risk for mental retardation and for heart problems.

Hepatitis B (first visit)

If you're a carrier of the disease, your newborn may contract it from contaminated blood during delivery. Doctors then will give her injections of hepatitis B immune globulin to prevent infection, and follow them with a vaccine for long-term protection.

Rapid Plasma Regent Syphilis Test (first visit)

If you have untreated syphilis, there's a 50 percent chance your baby will be infected, and he may develop other complications. Antibiotics usually prevent it from crossing the placenta.

Triple Screen Test (between 15 and 18 weeks)

This measures the levels of protein produced by your baby's liver, and of two important pregnancy hormones, estriol and hCG. It helps doctors screen for chromosomal abnormalities such as Down syndrome and spina bifida.

Glucose Tolerance Test (between 24 and 28 weeks)

After you've swallowed a soda-like substance that contains 50 grams of glucose, the levels of sugar in your blood are measured to check for gestational diabetes.

Complete Blood Count (second trimester)

Your doctor will repeat this test to make sure you haven't developed anemia since your first visit.