AFP Tetra Test
AFP Tetra screening can identify pregnant women who are at an increased risk for having a baby with certain birth defects. When a woman has reached a certain point in her pregnancy (between 15 and 21 weeks), a simple blood test can estimate her risk of having a baby with open neural tube defects, Down syndrome, or Trisomy 18, compared to other women her age.
AFP Tetra is a screening test that measures four proteins in a pregnant woman’s blood: AFP (alpha-fetoprotein), hCG (human chorionic gonadotropin), uE3 (unconjugated estriol), and dimeric inhibin A (DIA). The levels of these proteins, combined with clinical information about the pregnant woman, such as weight, race, and whether she takes insulin, can help identify a woman who has a higher chance of having a baby with certain birth defects.
AFP Tetra screens for the following birth defects:
Open Neural Tube Defects: Open neural tube defects, such as spina bifida, occur when the baby’s neural tube, or spine, does not close completely during development, leaving the spinal cord exposed. Approximately 1 in 1000 babies has an open spina bifida. The effects of open spina bifida range from bladder control problems to paralysis and mental retardation. The AFP Tetra test can identify greater than 80% of pregnancies that may have open neural tube defects.
Down Syndrome: Down syndrome, which occurs in about 1 in 800 births, is a disorder in which the baby has an extra chromosome. Chromosomes are the “units of inheritance” or “packaging” for our genetic material, or DNA. In a normal pregnancy, each baby inherits a total of 46 chromosomes, 23 from the mother, and 23 from the father. Sometimes an error in division will cause a baby to inherit too many or too few chromosomes. In Down Syndrome, instead of inheriting one chromosome 21 from the mother and one chromosome 21 from the father, the baby inherits a total of three of chromosome 21. Therefore, Down Syndrome is often called Trisomy 21. Babies with Down Syndrome have some degree of mental retardation and often have physical abnormalities, such as heart defects. AFP Tetra can detect 75% to 80% of Down Syndrome pregnancies.
Trisomy 18: Trisomy 18 is also called Edwards Syndrome. It is rare and only occurs in 1 out of every 8000 births. Like Down Syndrome, Trisomy 18 is a chromosome abnormality, but is caused by the baby having three of chromosome 18. Babies with Trisomy 18 have significant birth defects and severe mental retardation. Few live beyond the age of one year. AFP Tetra can detect more than 60% of Trisomy 18 pregnancies.
What does it mean if my AFP Tetra test is negative?
A negative test result significantly reduces the likelihood that your baby has an open neural tube defect, Down Syndrome, or Trisomy 18; however AFP Tetra is not diagnostic and cannot completely rule out the possibility of these problems. AFP Tetra does not screen for any other defects.
Does a positive AFP Tetra test result mean that my baby has a birth defect?
No. AFP Tetra is a screening test; it cannot diagnose problems with your baby or pregnancy. The test can only identify those women who are more likely to have a baby with an open neural tube defect, Down Syndrome, or Trisomy 18. Normally a positive AFP Tetra result will only mean that more diagnostic testing may be offered.
What other tests are offered if my AFP Tetra test is positive?
Follow up options are determined privately between the couple and the physician/nurse practitioner. In general, most women with a positive test result are referred for genetic counseling, ultrasound, and amniocentesis.
Genetic Counseling is a communication process between patient and a trained specialist, such as a certified genetic counselor, perinatologist (high risk pregnancy physician), the patient’s obstetrician. Genetic counseling is designed to help you understand your test results and follow-up options and may include a discussion of your family and pregnancy history.
Ultrasound is the use of high frequency sound waves and a computer to create images of the developing baby. Ultrasound is an important tool to help determine gestational age, or how far into the pregnancy you are. The levels of the proteins measured in the AFP Tetra test vary with each week of pregnancy, so unless the exact gestational age used during the AFP Tetra analysis was accurate, your test results may not be accurate. If ultrasound dating changes your gestational age by 10 days or more, your physician/nurse practitioner will ask the lab to recalculate your test results. Your result may change after the gestational age adjustment.
Ultrasound may also reveal the presence of twins, which will affect the AFP Tetra result. Certain birth defects, such as open spinal bifida, may also be seen on ultrasound. Babies with Down Syndrome and Trisomy 18 may have certain features that can be seen on ultrasound, but in general, neither can be diagnosed by ultrasound alone.
Amniocentesis a test wherein a sampling of your baby’s amniotic fluid is tested for definitive chromosome analysis. In amniocentesis, a very thin needle is inserted into the uterus through the abdomen and a small amount of amniotic fluid is removed. You will feel very little pain during the procedure, and you’ll probably be advised to take it easy for a few hours afterward. An amniocentesis can give an accurate diagnosis of chromosomal abnormalities, as a count of all chromosomes is performed through DNA analysis.
Before you decide to have this procedure, think about whether you want to know the results and what you would do with the knowledge. Some women, for instance, want to know everything about their fetus, including any genetic abnormalities, in order to be prepared. Some women want to know the results because they might choose to terminate the pregnancy. Some women do not want to know the test results because they feel they would maintain the pregnancy no matter what the results were, and a bad result would only be upsetting to them during their pregnancy. No one can make these decisions but you, so it is important to consider all the options before you decide to have amniocentesis or any prenatal screening for chromosomal defects.