Prenatal Blood Tests

Early in your pregnancy, usually at your first prenatal visit, your practitioner will do certain standard blood tests to learn basic information about your body, check for specific conditions, or spot any potential health problems. Here’s what your blood test may reveal:

Blood type and antibody screen

First of all, a blood test will disclose your blood type if you don’t know it already. Each of the major blood types — A, B, AB, or O — comes in two different varieties: negative and positive. People with a negative blood type lack a certain protein called an Rh antigen. People with a positive blood type have this antigen. This information is crucial because complications can arise if your baby is Rh positive and you aren’t, or if you have antibodies that will react with a protein on your baby’s red blood cells. This test needs to be done in the first trimester so you can start treatment if necessary.

Complete blood count

A complete blood count (CBC) is a panel of tests that offers clues about your general health by analyzing the components of blood: red blood cells, white blood cells, and platelets. If any of these cells is out of balance, you may have a condition that needs to be treated.

For example, one CBC test estimates the percentage of red blood cells in a certain volume of blood. This measurement is called the hematocrit. If your hematocrit is low, you may be anemic — a common condition during pregnancy. Your health care provider may recommend you boost your iron stores by taking an iron supplement and eating more iron-rich foods such as liver, lean red meat, dried fruits and nuts, leafy green vegetables, and iron-fortified breads and cereals.

Other tests look at your levels of hemoglobin to measure the amount of oxygen-carrying protein in the blood, while others count the number of white blood cells and examine the five different types. An elevated or low white blood cell count may indicate infection or inflammation because these cells are the body’s infection fighters. Still another test counts the number of platelets in a given volume of blood. Since platelets are very small cellular components of blood that help the clotting process, an increase or decrease in them could signal problems clotting or excess bleeding.

German measles (rubella)

Another blood test will confirm whether your blood has antibodies to the rubella virus. Antibodies are special proteins produced by your body’s immune system as a response to a foreign substance, such as a virus. If you previously had rubella, you develop “natural immunity,” or protection against the illness and you’re unlikely to get it again.

If you don’t have any immune defenses against rubella, your doctor will advise you to steer clear of anyone who might have the disease. This is because the disease can cause serious complications during pregnancy, especially during your first trimester. Potential complications include miscarriage, stillbirth, or significant birth defects such as deafness, stunted growth, heart irregularities and mental retardation. Your doctor may give you antibodies to help fight off infection.

If you aren’t immune to rubella, pregnancy is not the time to get vaccinated because the virus in the injection could be passed on to your fetus. Doctors advise getting vaccinated after your baby is born if you’re planning to have more children.

Hepatitis B

A blood test is the only surefire way to tell whether you’ve been infected with hepatitis B, a virus that attacks the liver. Hepatitis B usually spreads through sexual contact, shared needles, or bodily fluids. Although many people with this disease are entirely symptom-free, you can pass on the infection to your baby during childbirth.

Hepatitis B doesn’t usually cause problems during pregnancy for either you or your unborn child, though your practitioner will want to monitor your health closely. If you haven’t been vaccinated for hepatitis B, and you’ve come into contact with the virus during your pregnancy, your doctor may give you a drug called hepatitis B immune globulin to help protect you from developing the disease. During labor and delivery, large amounts of blood and other fluids are exchanged between mother and child, putting your infant at greater risk of exposure to the virus. To lower the risk of spreading the infection, your baby will also be given hepatitis B immune globulin, as well as a vaccine immediately after birth.


A blood test is also the best way to diagnose syphilis, a sexually transmitted disease that can easily go unnoticed in women. This uncommon but serious infection can also be transmitted to your developing child during pregnancy and delivery.

Syphilis can cause miscarriage, stillbirth, or premature rupture of the amniotic sac or membranes. An infant born with the disease may have brain, liver, spleen, skin, bone, ear, or eye problems. If you are infected, it’s important to get treated with penicillin — particularly during the first few months of pregnancy — to greatly lower the risk of long-term damage to your developing baby. Treating a newborn immediately will prevent further harm in many cases.


Your practitioner will ask you if you want your blood tested for human immunodeficiency virus (HIV), the virus that causes AIDS. If you have HIV, there are drugs you can take during pregnancy that dramatically reduce the risk of infecting your baby with this life-threatening virus. Without treatment, one out of every four babies born to women with the virus will also have HIV.

New treatments can reduce the chances of passing the virus from mother to child by 98 percent. That’s why the American Congress of Obstetricians and Gynecologists recommends that all pregnant women — even those not at high risk of contracting the disease — should get tested for HIV. The disease can be passed on to your baby during pregnancy, labor, and delivery, as well as through breast milk.

If you test positive for HIV, your health-care provider will discuss treatment options with you. These include drug therapies and birth by cesarean section (also known as a C-section) to further reduce the chances of your child contracting the virus during delivery. You’re at higher risk for the infection if you have unprotected sex, if you have sex with more than one partner, or if you or your partner use injectable drugs. If so, it’s wise to get tested for the virus again before you go into labor. Even if you are HIV-positive, with proper treatment your child may not be.

Sickle Cell Anemia (Sickle Cell Disease)

Depending on your ethnic group, you may also be screened for sickle cell anemia. This condition mostly affects people of African ancestry. Other ethnic groups, including people of Mediterranean or Middle Eastern descent, and those from Latin America are also at higher risk of inheriting the illness.

Abnormally shaped red blood cells are the hallmark of this disease. Instead of being flexible and round like a doughnut, the cells are rigid and curved like a crescent moon or a sickle. These oddly shaped red blood cells don’t move through the bloodstream easily, which clogs blood vessels and deprives tissues and organs of oxygen. These more fragile cells also break down more easily and more often, which can lead to anemia. Although there is no cure for the illness, there are treatments to relieve symptoms such as pain and fatigue.

Doctors can also identify sickle cell anemia before birth through a sample of amniotic fluid or tissue taken from the placenta. This method, which is not without risk, can be done as early as the first few months of pregnancy. These few samples of blood provide a lot of information about your health. Make sure your doctor clearly explains all test results, and if you have any questions, don’t hesitate to ask.

Blood Glucose Testing

This test will look at your levels of blood glucose, or blood sugar, which can suggest whether you have diabetes or gestational diabetes. If you do, your doctor will let you know what to do to keep yourself and your developing baby healthy during your pregnancy.

Chromosomal Disorders

Instead of diagnostic tests like amniocentesis, which are invasive and can slightly increase the risk of miscarriage, the American Congress of Obstetricians and Gynecologists now recommends that all pregnant women — regardless of age — be offered non-invasive blood tests and ultrasound to assess their risk of passing on Down syndrome and other chromosomal defects. If the screening revealed a potential risk, the women could then opt for diagnostic testing.


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