9 Things Your Doctor Looks for With a Prenatal Blood Test

It's standard for obstetricians to perform a blood test on expectant or possible mothers early in pregnancy.  Blood tests aren’t only used to confirm pregnancy, but also to check for maternal infections or signs that further fetal testing may be necessary. Prenatal testing can also relieve some of the anxiety most expectant mothers face, along with providing insight into the pregnancy. Here’s a look at what your doctor is looking for with a prenatal blood test. 

1. Blood Type 

The antibodies on the surface of red blood cells determine the mother's blood type (A, B, AB, and O). Knowing maternal blood type helps the treating physician prepare for possible complications that may later arise, such as a required blood transfusion or hemolytic disease in the newborn.

2. Rh Factor

The presence or absence of the Rh antigen in the mother's blood determines whether she’s Rh positive or negative. This test is crucial because an Rh negative mother carrying an Rh positive baby can result in hemolytic disease. Fortunately, treatment of the mother prenatally and immediately after birth can manage the disease during the current pregnancy as well as prevent it from occurring in any subsequent ones.

3. Glucose Levels

Testing the glucose levels in the mother's blood measures her body's ability to metabolize sugar. This serves as a screening test for gestational diabetes. If the blood glucose level of the mother is between 130 to 140 milligrams per deciliter, the treating physician will order additional testing in the form of a glucose tolerance test. Regardless of these results, most physicians perform an addition glucose test around the sixth month.

4. Iron and Hemoglobin

An iron deficiency in the mother's blood indicates she’s at risk for anemia. If a mother does have low iron levels, her physician may prescribe iron supplements to prevent her from developing anemia. Hemoglobin is an iron-containing protein found in the red blood cells. It’s responsible for transporting oxygen throughout the body. Normal hemoglobin levels are between 12 and 14 grams. If the expectant mother's hemoglobin level is below 10 grams, her physician will prescribe treatment for anemia.

5. Sexually Transmitted Diseases

Blood testing can diagnose sexually transmitted diseases such as Hepatitis B, Syphilis, and HIV. Treatment can greatly reduce the risk of transmission of these diseases to, or otherwise harming, the baby during pregnancy and birth.

6. Rubella 

Blood testing determines whether the mother carries immunity to the Rubella virus, which can cause severe birth defects in an unborn baby. Precautions to avoid contracting the disease will be advised to expectant mothers without sufficient antibodies to the disease.

7. Toxoplasmosis

Toxoplasmosis is a parasitic infection that’s harmless to an expectant mother, but may harm the baby if it crosses the placenta. Treatment with antibodies greatly reduces the risk of harm to the baby during pregnancy.

8. Genetic Diseases

Some genetic diseases can also be diagnosed by prenatal testing of the mother's blood or may indicate a need for further testing. These diseases include cystic fibrosis, familial hypercholesterolemia, sickle-cell anemia, Tay-Sachs disease, and thalassemia.

9. Risks and Side Effects of Prenatal Blood Testing

A blood test is a routine diagnostic tool and normal during prenatal care. The test has neither risks nor side effects for the baby or mother, other than the mild discomfort of drawing blood.


Source: American Pregnancy Association,

Last Updated: November 03, 2017