Lab Tests on Blood

At your first prenatal visit, you will probably donate a surprising quantity of blood for “blood tests”. Most are “routine”, and some will give your clinician a good overview of your general health. Other blood tests are very specific for common disorders which could impact your pregnancy. These blood tests (and others done later in pregnancy) may include the following:

  • Complete Blood Count
  • Blood Group and Rh typing
  • Antibody Screen
  • Sickle cell screen(if your ancestors were African)
  • Hepatitis B screen
  • Syphilis screen
  • German measles (aka, Rubella) antibody screen
  • HIV screen

The Complete Blood Count (CBC)

A CBC is a test which analyzes the cells in a sample of blood. The three different types of cells in the blood (red blood cells, white blood cells and platelets) are measured with regards to quantity and general cell characteristics.

The red blood cells (RBCs) are the chief oxygen-carrying components in the body. An important molecule, hemoglobin, is the end-user of most of the iron in the body. Without sufficient iron and hemoglobin, the oxygen-carrying capacity of the blood is reduced, a condition called “anemia”. The most common form of anemia is iron-deficiency anemia. Iron-deficiency anemia is usually treated with iron supplements and specific nutritional counseling.

White blood cells (WBCs) are primarily responsible for clean-up in the body. WBCs are sometimes referred to as leukocytes. They roam the blood as scavengers, ingesting debris and foreign invaders. Some are able to leave the blood stream and do their job between the cells of the body. There are specialized WBCs which clean up bacteria, viruses, allergens, and even cancer cells.

When large numbers of certain WBCs are produced (referred to as an “elevated white count”), it is often a sign of infection. However, normal pregnancy causes an elevation in the WBC count. But if the WBC count is too high, a doctor or midwife may suspect an infection somewhere in the body. With urinary tract infections, WBCs leak into the urine to fight the infection. Thus, large amounts of WBCs can be measured in the urine with a urinary tract infection.

The third cellular components of blood are the platelets. They are crucial factors in the blood’s ability to clot. When activated, the platelets become very sticky, and begin to cause the blood to clot. Clotting of the blood is usually a local phenomen. That is, when a body needs blood to clot it is usually only at the site of injury where clotting is desired. In some rare circumstances (and some diseases of pregnancy are included), the blood begins to clot throughout the blood stream. During this abnormal process, the small clots are pushed into the smallest arteries and veins where they become logjammed, and leave the remaining blood with no clotting factors. This is called “Disseminated Intravascular Coagulation” or DIC. DIC is sometimes a complication of severe forms of toxemia/pre-eclampsia or may be seen have the body loses large amounts of blood (hemorrhage).

In some pregnant women, platelets are “consumed” by the body for unknown reasons. This usually is not a problem until the “platelet count” drops below the 25,000-50,000 level. Most healthy women have platelets counts above 150,000.

Blood Types (and the Rh factor)

The RBCs are also the key to “typing” one’s blood. Certain proteins in and on the red blood cell determine if a person is type A, B, AB, or O. These “major blood types” are traits inherited from your parents. They are important in the event that a pregnant woman needs a blood transfusion. Also, “incompatibilities” between the mother and the newborn’s blood type are the causes of some forms of newborn jaundice.

Another blood grouping that is important in pregnancy is the Rh factor. The Rh factor, a protein residing on the red blood cell surface, is important because an incompatibility between the mother’s Rh type and the baby’s Rh type can, in rare instances, result in severe problems for the baby. One is either “positive” or “negative” for the Rh factor. Fortunately, since 1968, there has been a treatment which has all but eradicated the problems associated with “Rh sensitivity” (go to Rh Sensitization).

The Antibody Screen

Your blood will be screened for several antibodies which are possibly harmful to your baby. One is the antibody which Rh-negative women produce after “sensitization” to Rh-positive blood. It is the most clinically important one due to the number of Rh-negative women and the potential severity of the problem for some newborns. The other antibodies detected by an “antibody screen” are somewhat less important and not nearly as common. If you have a positive antibody screen, the antibody will be identified, and your doctor or midwife will know if the antibody is potentially harmful to your baby.

The Sickle Cell Screen

If you have family members/ancestors who are of African descent, you should have a sickle cell screen. That goes for the father of the baby, too. If you are both sickle cell “carriers”, you may want to talk with a genetic counselor about the possibilities of having a child affected by sickle cell disease.

If you are a “carrier”, you have the sickle gene in your chromosomes, but you do not have the disease. Sickle cell disease is inherited from both parents, each of whom is a carrier. In sickle cell disease, abnormal hemoglobin is produced. This causes red blood cells to become deformed and less able to carry oxygen.

In addition, for some unknown reason, women who carry the sickle cell trait have a greater chance of having urinary tract infections during pregnancy.

Hepatitis B surface antigen

The Hepatitis B virus (HBV) causes an inflammation and destruction of the cells of the liver. In those individuals who become “chronic carriers” of the HBV, the lifetime risk of cirrhosis and liver cancer is greatly increased. Chronic carriers of HBV may continue to infect other individuals, including mothers who may infect their babies at the time of birth. Screening for the Hepatitis B surface antigen is an important prenatal blood test since newborns can be immunized against Hepatitis B immediately after birth if a mother is a carrier of the HBV. (More about Hepatitis B.)

Syphilis Screen

Screens for syphilis are known by many names (RPR, VDRL, STS, Wasserman). By any name, this is still an important test due to the grave consequences of syphilis on a baby infected during pregnancy.

Syphilis is an unusually silent, sexually transmitted disease. The painless lesions which occur shortly after initial infection may go unnoticed. The signs and symptoms of “secondary” syphilis mimic many other common health problems. And “tertiary” syphilis has been misdiagnosed for centuries.

Syphilis is usually easily cured with common anitbiotics. A syphilis screen at the first prenatal visit, followed by prompt treatment and later follow-up has prevented the tranmission of disease to many babies. (More about Syphilis.)

Rubella Screen

The Rubella virus is the cause of German measles, or 3-day measles, one of the five common, childhood viral diseases. German measles is usually a mild disease in most children and adults. Typically, a rash, swollen lymph nodes, a mild fever and mild flu-type symptoms are all that occur. However, for a small 7-week embryo inside a pregnant woman, the virus can attack the developing nervous system and cause a myriad of problems including miscarriage, stillbirth, blindness, deafness, and other defects associated with the brain/nervous system, heart and eyes. (More about Rubella.)

Tests for Gestational Diabetes

For some reason, the state of pregnancy may cause a diabetes-type disease in a small percentage of women (about 4-5%). This is called Gestational Diabetes, or GDM. Elevated blood sugars over prolonged periods of time cause disease in the human body. During pregnancy, if not adequately controlled, high blood sugars may contribute to higher rates of stillbirth, unusually small babies (if the disease is severe) or unusually large babies (if the disease is mild).

It is currently recommended that all pregnant women be screened for GDM between 24 and 28 weeks of pregnancy. Some clinicians will test some women earlier than this if certain risk factors are present. If early results are normal, the test should probably be repeated again in the 24-28-week range, since the condition tends to worsen as a woman approaches her due date.

The test commonly used to screen for GDM is called the Glucose Tolerance Test, or GTT. During a “one-hour” GTT, a women is given a measured amount of sugar (in the form of a sweet drink) and her blood sugar level is measured one hour later. It should be below 140. If the the level is above 140, a “three-hour” GTT should be done to get a better evaluation of the woman’s ability to process glucose. A 3-hour GTT should be done in the fasting state (nothing to eat in the previous 12 hours). At the beginning of the test, a Fasting Blood Sugar (FBS) is drawn. Then, the woman is given the glucose load, and blood is drawn 1, 2 and 3 hours later. If the FBS or any two measurements is elevated, the woman has GDM. (More about GDM.)

HIV Screen

The Human Immunodeficiency Virus (HIV) is the cause of AIDS (Acquired Immune Deficiency Syndrome). In the recent past, in all but a few cases, AIDS has been a fatal disease. New drug therapies have radically improved the short-term prognosis for those infected with HIV. HIV is spread through infected body fluids (blood, semen, cervical fluid) from one person to another. HIV can be spread by sexual intercourse, blood transfusion, and sharing of intravenous needles.

Recent studies have shown that mothers who carry the HIV virus can dramatically reduce the risk of infection to their babies if they take certain medications during pregnancy. Without treatment, the chance that the baby of a HIV-positive mother will become infected is almost 30%. If the mother receives the proper medications, the risk of transmission to the baby decreases to less than 10%.

In most parts of the country a blood screen for HIV antibodies is a routine part of the initial prenatal visit. The CDC (Center for Disease Control) has recommended that all pregnant women receive HIV screening. Anonymous HIV screening is available in most communities, often through the county health department or a community health clinic. Your medical chart, your health history, and the conversations you have with your doctor or midwife about your health are strictly confidential. Share any and all information you have about your risks of HIV with your doctor or midwife.

Alpha-fetoprotein (AFP or MSAFP)

Alpha-fetoprotein is a protein secreted by the developing fetus. It leaks into the mother’s blood (maternal serum AFP or MSAFP). AFP is secreted in large amounts by babies with neural tube defects (NTD). NTDs are birth defects involving the developing nervous system (spinal cord and brain). Thus, it is possible to draw a mother’s blood at a certain time in pregnancy (15-18 weeks) and measure the amount of AFP. This may help detect those babies with NTDs. But it’s not all that simple……read on.

Early in the history of the MSAFP test it was discovered that most babies with Down Syndrome (and some other chromosomal abnormalities) secreted abnormally low levels of AFP. This allowed the test to be used to screen for 2 birth defects, NTDs and Down Syndrome. Recently it has been discovered that by adding a couple more measurements to the test (for estriol and hCG), the test becomes more accurate.

Deciding whether or not to do this test may not be very easy for some prospective parents. You might want to ask yourself a difficult question. “If I do the test, what will I do with this information? Would I consider aborting the pregnancy if I knew my baby had a spinal cord defect or Down Syndrome?”

The incidence of NTDs is about 1 or 2 per 1, 000 live births in the U.S. In Great Britain, it is considerably higher, averaging about 5 per 1,000 births. The incidence of Down Syndrome (and other chromosomal abnormalities) increases with the age of the mother. At age 26, the risk of Down Syndrome is about 1 per 1,000 births. At age 35, the risk increases to 1 per 300 births; at age 40, 1 per 100.

If you would consider having an abortion for a baby with a spinal cord defect or a serious chromosomal abnormality, you probably will want to have this test. If you would NOT chose to consider an abortion in these cases, why would you want the test? Some women in this situation say they want the test “for reassurance”. That would be a good reason if the test was highly accurate. It is not. It IS extremely sensitive at detecting babies with NTDs, but not very specific……….what that means is that if your baby has a NTD, the test will detect it, but if your baby does not have a NTD, the test may still come back saying that it does (that’s called a “false-positive test”). The percentage of false-positive MSAFP tests is very high (1 out of every 20 tests). That means that 1 out of 20 women are told that the test is positive when the baby probably does NOT have a spinal cord defect. In these cases, an ultrasound examination is quite good at detecting large defects. Remember, however, that the test is extremely good at detecting all the babies who do have a NTD.

In a different way the accuracy of the AFP test to detect chromosomal abnormalities is also somewhat limited. The test can detect only about 70% of the babies with Down Syndrome. That means that we occasionally tell a woman who has a baby with a chromosomal problem that the test was negative (this is called a “false-negative test”). Unfortunately, the only way to positively detect a chromosomal abnormality is with amniocentesis. Amniocentesis is a more “invasive” test, with a miscarriage rate of about 1 in 250.

Some women who are at a higher than average risk for having babies with chromosomal abnormalities (for example, women over the age of 35) will use the AFP test to help decide whether or not to have an amniocentesis. If the AFP does not show an elevated risk for an abnormality, they will not have the amniocentesis. If the test indicates an elevated risk for a chromosomal abnormality, they will do the amniocentesis. However, remember that one of the big problems with the AFP test is the false-negative rate when trying to predict chromosomal abnormalities such as Down Syndrome. As many as 30% of those who have a baby with a chromosome abnormality will not be predicted by the AFP test.

One more way to think about this is to picture 4 women who all have a fetus with Down Syndrome. If all of them decide to do the AFP in order to help them decide to do an amniocentesis, one of them will be told that they do not have a fetus with a chromosomal abnormality (at least, based on the AFP test). That’s a falsely negative result. That person might elect not to have the amniocentesis which (with a very high degree of accuracy) would correctly predict that she DOES have a baby with Down Syndrome.

It’s hard stuff to think about, isn’t it? Welcome to the mental anguish of parenthood. Talk to yourself, your partner, and your midwife or doctor. Decide how you feel about all this. If you decide to do the test, rest assured that the odds are in your favor. If you are notified of a positive test, chances are good it is falsely positive.

You should also know that the MSAFP test is often falsely positive or falsely negative for other reasons. It is important that the gestational age of your baby is accurate when the test is done since the normal ranges are age-specific and figured only for singleton pregnancies (one baby). Thus, abnormal results are sometimes reflected when: 1) your dates are wrong, 2) you have more than one baby (twins) or, 3) you have had a miscarriage and don’t know it yet.

Testing for Cystic Fibrosis

Cystic fibrois (CF) is a disorder caused by a genetic defect. In people with CF, a defective gene causes problems in the lungs, pancreas and sweat glands. The average life expectancy for a person with CF is about 30 years.

CF is recessive gene disorder. Thus, a person must inherit the defective gene from both parents. The parents are considered “carriers” of the gene. Genes come in pairs (one from each parent). The parents of a child with CF do not have CF themselves because they are expressing a dominant (rather than a recessive) gene inherited from their parents. Since the carrier-parents carry both a dominant and a recessive gene, the chance that any one offspring would inherit both recessive genes is 25% (1/4).

Caucasians are carriers more often than people of any other race. 1 out of every 25 Caucasians is a carrier. Thus the disease is seen in about 1 out of every 2500 births among Caucasians (1/25 x 1/25 x 1/4). The CF gene is also common among Ashkenazi Jews (about 1/29). Fortunately, the gene is much less rare about other racial/ethnic groups.

It is now possible to test a person to see if they carry the CF gene. There are probably more than 40 mutations of the CF gene. The tests that are currently available test for less than 40 of mutations. Thus, the test may miss a small percentage of people who carry one of the mutations not covered by the test.