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How Prenatal Testing Works


Blood Sampling Tests

In the first trimester of pregnancy, you may have blood drawn to determine the following:

  • hCG levels (discussed above)
  • Blood iron content - important for hemoglobin and delivering sufficient oxygen to the growing baby
  • Blood type and Rh compatibility - assess problems between mother and baby's blood types (see Rh Blood Testing for details)
  • Presence of antibodies to viruses (e.g., HIV, hepatitis B) or presence of bacteria (e.g., syphilis).

The hCG, viral antibody, syphilis and blood type tests look for and/or quantify the amount of antibodies to these substance in the blood. The iron test directly measures the iron by burning a sample in a high temperature flame and quantifying the amount of light given off at the wavelength of light specific for iron, which is related to the amount of iron present.

Glucose Tolerance Test

Between week 25 to 28 gestation, you will have a glucose screening test for gestational diabetes. You will drink a soda containing high amounts of glucose, or sugar, and will have your blood drawn one hour later. Blood glucose will be measured by a glucose oxidase reaction described in the previous section. If the glucose level is high, you may be asked to take an additional glucose-tolerance test. In this test, you drink a high-glucose solution on an empty stomach and blood samples will be taken at regular intervals (usually every hour) for three hours. Blood glucose levels will be measured again. The timecourse information from a glucose tolerance test is a better indication of your body's response to a glucose load to diagnose gestational diabetes.

Triple Screen Test

This test is done in the second trimester and measures three parameters:

  • alpha-fetoprotein (AFP)
  • hCG
  • estriol

AFP is produced by the baby and makes its way into the mother's blood. Typically, AFP levels are low. However, high levels of AFP indicate that the baby's neural tube has failed to close (i.e., a neural tube defect). An ultrasound examination may then be done to confirm this finding.

At this time in development, hCG levels in combination with AFP levels can tell us whether the baby has any abnormalities in the number of chromosomes. A high level of hCG in combination with a low level of AFP suggests a chromosomal abnormality. The most common disorder of this type is Down syndrome (extra chromosome #21 -- see Human Chromosomal Abnormalities for more details). If the level of hCG is high and no fetal heartbeat is detected, then there may be a molar pregnancy. A molar pregnancy is when a piece of tissue, usually leftover from a previous pregnancy, grows rapidly, destroys the baby and forms a benign tumor. An ultrasound examination may then be done to confirm this finding.

Estriol is a hormone made by the baby's adrenal glands. The levels of estriol in the mother's blood indicate the health of the baby. If the estriol levels drop, then the baby may be threatened and need to be delivered. Low levels of estriol may also indicate Down syndrome or neural tube defect.


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