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AFP (Alpha-fetoprotein) AFP is a protein produced mainly in the developing baby's liver and released into the amniotic fluid. A small amount crosses the placenta and can be measured in the expectant mother's blood towards the end of the first trimester. Levels rise steadily throughout the second trimester. |
In most babies affected with open spina bifida, anencephaly, or an abdominal wall defect, an increased amount of AFP enters the amniotic fluid and subsequently causes a higher than usual level of AFP in the maternal blood. In contrast, maternal blood AFP may be lower in a pregnancy if the baby is affected by Down syndrome, trisomy 18 or SLOS. hCG (Human chorionic gonadotropin) hCG is a hormone produced by the placenta. The levels rise rapidly in early pregnancy and then decline between the 10th and 20th weeks. Second trimester maternal blood levels of hCG may be higher in a pregnancy in which the baby has Down syndrome. The level of hCG may be lower than usual in a pregnancy if the baby is affected with trisomy 18 or SLOS. uE3 (Unconjugated estriol) Unconjugated estriol is a hormone produced by the placenta and the developing baby. Levels rise throughout normal pregnancy. Maternal blood levels of uE3 may be lower in a pregnancy if the baby is affected with Down syndrome or trisomy 18 or SLOS. Inhibin-A Inhibin-A is a hormone made by the placenta and the developing baby. Maternal blood levels remain relatively constant through the 15th and 20th weeks of pregnancy. Maternal blood levels of inhibin-A may be higher than usual in a pregnancy if the baby is affected with Down syndrome. What birth defects are found by the Program? Neural tube defects (NTD - spina bifida, anencephaly)
The neural tube is a structure that forms in the developing embryo very early in pregnancy. This tube eventually develops into both the baby's brain and spinal cord. The neural tube is completely formed by 5 weeks after conception. If the tube does not close completely, an opening may remain along a part of the baby's spine (back) or head. A neural tube defect occurs in 1 or 2 out of every 1000 births. The two most common types are spina bifida and anencephaly. In spina bifida there is an opening along the baby's spine. This defect often causes paralysis or weakness of the legs. There may also be problems with bowel or bladder control, as well as other medical problems such as hydrocephalus (water on the brain). Children with spina bifida require surgery and may also need ongoing special medical care. Anencephaly occurs when the brain and back of the skull are incomplete. These babies are often stillborn or die shortly after birth.
Babies with these defects have abnormal openings along the abdomen (stomach) so that intestines and other organs grow outside the baby's body. An opening where the umbilical cord is attached is called an omphalocele . This occurs in about 1 out of 4,000 births.
An opening beside the umbilical cord
is called gastroschisis. This occurs in about 1 out of 10,000
births. These openings may be present along with other birth
defects. Surgery after birth can often repair an abdominal
wall defect. Down syndrome is a common cause of mental retardation. It is caused by an extra chromosome #21 in all of the cells in the body. Normally, in each human cell there are 46 chromosomes. Individuals with Down syndrome have a total of 47 chromosomes in their cells. This condition is also called trisomy 21. People with Down syndrome have a distinct physical appearance and varying degrees of mental retardation. They are also at increased risk for certain birth defects, such as congenital heart disease. Overall, approximately 1 in about 800 babies is born with Down syndrome. Older women have a greater risk than younger women for having a baby with Down syndrome or other chromosome abnormality. Although this risk gradually increases with age, a woman of any age can have a baby with Down syndrome. Trisomy 18 is caused by an extra chromosome #18. Babies with trisomy 18 have severe mental retardation and usually die before birth or in early infancy. Older women have a greater risk than younger women for having a baby with trisomy 18 or other chromosome abnormalities. Although this risk gradually increases with age, a woman of any age can have a baby with trisomy 18. Smith-Lemli-Opitz Syndrome or SLOS is a very rare autosomal recessive genetic disorder that causes mental retardation, poor growth and birth defects. (To learn more about autosomal recessive conditons, please see Genetics 101, Inheritance Patterns ) SLOS is caused by a genetic defect in the body's ability to make cholesterol. All babies need cholesterol even before birth to help their brain and body develop. Babies with SLOS cannot make enough cholesterol because two important genes that make cholesterol do not work correctly. Children born with this condition may die in infancy or be mentally retarded and may have serious birth defects. Some are severely affected, a few are mildly affected, but most are somewhere in the middle. No one can predict how long someone with SLOS will live. It depends on the severity of their medical problems. There is no way to replace the “non-working” genes at this time, so each problem caused by SLOS needs to be treated separately. Adding extra cholesterol to the baby’s diet or giving it directly as medication has been shown to improve some of the development and growth problems in some patients. However, this does not reverse the mental retardation. SLOS occurs in approximately 1 in 100,000 births.
A "screen negative" result means that the chance for certain birth defects in this pregnancy, such as Down syndrome, neural tube defects, abdominal wall defects, trisomy 18 or SLOS is low enough that follow-up testing may not be necessary. The risks are calculated using measurements of the amounts of AFP, hCG, uE3 and inhibin-A in the pregnant woman's blood, in combination with other factors such as race and weight. (Inhibin-A is only used in the calculation for the risk for Down syndrome). Maternal age is also part of the calculation for the risk of Down syndrome and trisomy 18. Up to 95% of women will have a "screen negative" result. It is important to remember that a screening test will not tell for certain whether the baby actually has a birth defect, but instead tells the chance of a specific problem occurring. Some women whose babies actually have these birth defects will not be picked up through the XAFP screening test. A "screen positive" result means that there is an increased chance for certain birth defects in this pregnancy, such as Down syndrome, neural tube defects, abdominal wall defects, trisomy 18 or SLOS. The risks are calculated using measurements of the amounts of AFP, hCG, uE3 and inhibin-A found in the pregnant woman's blood, in combination with other factors such as race and weight. (Inhibin-A is only used in the calculation for the risk for Down syndrome). Maternal age is also part of the calculation for the risk of Down syndrome and trisomy 18. Most of the time, however, the reason for the "screen positive" result is NOT a birth defect. The most common reasons for this type of result include:
Women with "screen positive" results are eligible for genetic counseling and follow-up diagnostic services at a Northern California Kaiser Permanente Prenatal Diagnosis Center (PDC), or any one of the other State-approved Prenatal Diagnosis Centers. My blood test is screen positive for Down syndrome Your test result was "screen positive", indicating an increased risk for Down syndrome. This risk is based on your age and the amounts of AFP, hCG, uE3 and inhibin-A in your blood. Most of the time, the reason for this result is NOT a birth defect. The most common reasons for a "screen positive for Down syndrome" result are:
To help determine why your result was "screen positive", you are eligible for genetic counseling and follow-up diagnostic services at the Northern California Kaiser Permanente Prenatal Diagnosis Center (PDC) closest to you, or any one of the other State-approved Prenatal Diagnosis Centers. You may want to review the following California State XAFP Program publication: Screen positive for Down syndrome. This booklet provides a brief explanation of the meaning of an Expanded AFP test result that is "Screen positive for Down syndrome". The booklet is intended to supplement individual genetic counseling. My blood test is screen positive for trisomy 18 Your test result was "screen positive" based on lower than expected amounts of AFP, hCG, and uE3 in your blood. This indicates an increased chance that the fetus may have trisomy 18. (Inhibin-A is not used as a factor in calculating the risk for trisomy 18). Most of the time, the reason for this result is NOT a birth defect. Sometimes low levels of AFP, hCG, uE3 occur without any known pregnancy problem (normal variation). To help determine why your result was "screen positive", you are eligible for genetic counseling and follow-up diagnostic services at the Northern California Kaiser Permanente Prenatal Diagnosis Center (PDC) closest to you, or any one of the other State-approved Prenatal Diagnosis Centers. You may want to review the following California State XAFP Program publication: Screen positive for trisomy 18. This booklet provides a brief explanation of the meaning of an Expanded AFP test result that is "Screen positive for trisomy 18". The booklet is intended to supplement individual genetic counseling. Your test result was "screen positive" based on a higher than expected amount of AFP in your blood. This indicates an increased chance that the baby may have a neural tube defect (such as spina bifida) or an abdominal wall defect. Most of the time, the reason for this result is NOT a birth defect. The most common reasons for a high AFP level are:
To help determine why your result was "screen positive", you are eligible for genetic counseling and follow-up diagnostic services at the Northern California Kaiser Permanente Prenatal Diagnosis Center (PDC) closest to you, or any one of the other State-approved Prenatal Diagnosis Centers. You may want to review the following California State XAFP Program publication: Screen positive for Neural Tube Defects and Abdominal Wall Defects: This booklet provides a brief explanation of the meaning of an Expanded AFP test result that is "Screen positive for neural tube defects". The booklet is intended to supplement individual genetic counseling My blood test is screen positive for SCD Follow-up for Screen Positive Results If you have a screen positive XAFP test result indicating an increased risk for any of the above birth defects, you are eligible for follow-up services at a Northern California Kaiser Permanente Prenatal Diagnosis Center or any other state-approved Prenatal Diagnostic Center (PDC) at no extra cost. At the PDC, you will meet with a genetic counselor to discuss further testing options such as detailed ultrasound and amniocentesis. These tests may help determine the reason for your "screen positive" test result, including normal variation, a change in your due date, more than one baby, or the presence of a pregnancy problem or birth defect. A genetic counselor is a healthcare professional trained in providing information regarding inherited conditions or birth defects to individuals and families. The genetic counselor will discuss your specific test result and the follow-up tests that are available to you. In addition, she/he will review you and your partner's family health history to see if there are other risk factors and/or tests that you should consider. All follow-up tests are completely optional. Talking about your choices with a genetic counselor can help you make the best decisions for you and your pregnancy. Everyone with a screen positive XAFP test result is offered a detailed ultrasound by a State-approved ultrasonographer. This ultrasound will tell you if the pregnancy dating used to interpret your XAFP result was correct and if there is more than one baby present. If either of these conditions are present, your blood test may have to be re-interpreted or drawn again for an accurate reading. In addition, the ultrasound will take a very close look at your baby to see if there is any kind of birth defect that may have caused your test result to be screen positive. Although ultrasound alone cannot detect all birth defects, it may identify some physical problems like spina bifida and the abdominal wall defects. Down syndrome cannot be determined by prenatal ultrasound alone. Amniocentesis is a diagnostic test generally used to identify problems with the baby's chromosomes, or genetic material. It can also help identify neural tube defects (like spina bifida) and abdominal wall defects when the ultrasound picture is not conclusive. Amniocentesis involves removing a small amount of amniotic fluid (the water surrounding the developing baby) with a thin needle inserted through the mother's abdomen. Using the ultrasound, the doctor is able to locate and remove this fluid without touching the baby. The fluid contains some specific substances produced by the baby (like AFP) which can help identify neural tube and abdominal wall defects, as well as cells which contain the baby's chromosomes. Since Down syndrome and trisomy 18 are caused by an extra chromosome, these conditions, as well as other chromosome abnormalities, can be detected by amniocentesis. The results of the amniocentesis are usually available in about two weeks. Although amniocentesis is considered a safe procedure when performed by medical experts at a State approved PDC, there is a small risk of miscarriage with this procedure (less than 1 in 300). A genetic counselor will discuss with you the risks, benefits and limitations of amniocentesis. If the ultrasound or amniocentesis shows that your baby has a birth defect or chromosome abnormality, you will be given more information about the specific problem and how it may affect the baby. All available treatments and options for continuing or ending the pregnancy will be discussed at that time. Remember, most women with a screen positive XAFP result will have normal, healthy babies. Frequently Asked Questions (FAQs) Do I need to have the Expanded AFP test? No. This is a voluntary test and it is your choice whether to have the test or not. You have a number of options for testing: 1) You may choose to have No Testing for birth defects during your pregnancy. 2) You may choose to have First Trimester Screening (FTS) to screen your pregnancy for Down syndrome, trisomy 18 and trisomy 13. 3) You may choose to have Expanded AFP blood screening to screen your pregnancy for Down syndrome, trisomy 18, neural tube defects, abdominal wall defects and Smith-Lemli Opitz syndrome. 4) You may choose to go directly to a diagnostic procedure such as amniocentesis or CVS. Your doctor, nurse practitioner, nurse midwife and/or genetic counselor can help you decide whether testing is appropriate for you. You may also want to visit Prenatal Testing for Birth Defects: A Guide to Helping You Decide for more information. Does my age affect the results of the blood test? Yes. As your age increases, the chance that your blood test will fall into the screen positive range also increases. Table of maternal age risks for chromosome abnormalities.
Does what I eat affect the results of the blood test? No. The XAFP screening test measures the amounts of four substances in a pregnant woman's blood. They are: AFP (alpha-fetoprotein), hCG (human chorionic gonadotropin), uE3 (unconjugated estriol) and inhibin-A. What you eat does not affect these substances. Does smoking affect the results of the test? Cigarette smoking does affect the levels of some of the substances being measured in the blood test. If you have smoked one or more cigarettes during the 7 days before you have had your blood drawn, please tell your prenatal provider. This should be noted on your lab request form. Where do I go to have the test done? XAFP blood tests are done at Kaiser
Permanente Laboratories between 15 and 20 weeks of pregnancy.
It is important that you have the correct lab form so that results
will be mailed to your doctor/nurse practitioner/nurse midwife.
Call the Advice Line at your OB/GYN clinic for information
about the XAFP form and laboratory location. If the ultrasound or amniocentesis shows that your baby has a birth defect or chromosome abnormality, you will be given more information about the specific problem and how it may affect the baby. This may include discussion with your primary OB provider, a genetic counselor, geneticist, perinatologist or other pediatric specialist. All available treatments, and options for continuing or ending the pregnancy will be discussed when the problem is identified. Can I have the XAFP blood test repeated? No. The California Department of Health Services Genetic Disease Screening Program has a "no repeat" recommendation. The reasons for not recommending a redraw are based on standard statistical theory and not enough data has been collected on repeat specimens to develop a mathematical formula for the repeat population. Is this a California State Program or a Kaiser Program? The XAFP Screening Program was established by the California Department of Health Services to provide prenatal screening for all pregnant women in California. Kaiser Permanente has a Northern California Regional Prenatal Screening Office in Oakland that is set up to coordinate all aspects of the program for our members. Our region covers the 43 Northern California Kaiser facilities, including clinics, laboratories and Prenatal Diagnosis Centers. The Prenatal Screening Office staff work to ensure complete, accurate results and that appropriate follow up is initiated when necessary. You may call the Prenatal Screening Office at (510) 752-6190 with any questions you might have about the Expanded AFP Program. Expanded AFP Program information. This is the official website of the California State Department of Health Services, Genetic Disease Branch. Links to valuable patient oriented information about Expanded AFP blood screening tests. You may talk with your doctor, nurse practitioner, or nurse midwife about any questions or concerns you may have. You may also call the Northern California Kaiser Permanente Prenatal Diagnosis Center assigned to your clinic for additional information.
Prueba de Escrutinio de la Alfa-Fetoproteína (AFP) Extendida La prueba de escrutinio de la AFP Extendida es un análisis de sangre voluntario que puede ayudar a identificar a mujeres que pueden correr un riesgo mayor que el riesgo normal al tener un bebé con algunos defectos de nacimientos. La prueba de AFP Extendida no puede detectar todos los tipos de defectos de nacimientos. Puede ayudar a identificar a muchos bebés con Defectos del Tubo Neural (aperturas a lo largo de la espina dorsal y/o del cerebro del bebé), Defectos de la Pared Abdominal (aperturas a lo largo del abdomen o del estomago del bebé), el Síndrome de Down (una anomalía cromosómica que causa retraso mental y algunos problemas físicos), Trisomía 18 (otra anomalía cromosómica que causa severos problemas mentales y físicos) y el Síndrome de Smith-Lemli-Opitz (un condicion que causa retraso mental y problemos físicos). Se hace la prueba de escrutinio de la AFP Extendida entre las semanas 15 y 20 del embarazo porque es el único período cuando la prueba es fiable y precisa. De la pequeña cantidad de sangre que le saca del brazo a la madre, el laboratorio mide la cantidad de 4 substancias diferentes: AFP(Alfa-fetoproteína), hCG (gonadotropina coriónica humana), uE3(estriol no conjugada), y inhibin-A. Estas 4 substancias son fabricadas en la placenta de la madre y en el bebé que se está desarrollando (feto), y se encuentran normalmente en la sangre de cada mujer embarazada. Cada semana del embarazo, esperamos ver cantidades diferentes de cada una de esas substancias. Las cantidades medidas en su análisis de sangre son comparadas con las cantidades que normalmente se encuentran en otros embarazos al mismo tiempo. La mayoría de las mujeres que hace la prueba de sangre de la AFP Extendida tendrá un resultado "negativo". Esto significa que la cantidad de las 4 substancias encontradas en su sangre está comprendida en el promedio que corresponde a cuán avanzada está en su embarazo. Aunque un resultado negativo no quiera decir que no hay riesgo de tener un bebé con defectos de nacimientos, sí nos asegura de que el riesgo es bastante bajo. Una cantidad pequeña de mujeres tendrá un resultado "positivo" a la prueba de la AFP Extendida. Esto significa que la cantidad medida de las substancias no estaba comprendida en el promedio para esa etapa del embarazo. Esto no significa que el bebé tenga un defecto de nacimiento, sino que tiene sentido tener más cuidado con el embarazo haciendo más pruebas. Como todas los bebés son diferentes, algunos producen más y otros menos de la cantidad "media" de cada substancia. La mayoría de las veces, un resultado positivo es debido a esta "variación normal". Algunas veces se obtiene un resultado positivo porque la mujer no está tan avanzada, o está más avanzada en su embarazo de lo que pensábamos. Sin embargo, algunas veces un resultado positivo se debe a un defecto de nacimiento en el bebé o puede indicar una complicación en el embarazo. Si Usted tiene un resultado positivo, un consejero genético se pondrá en contacto con Usted y le explicará su resultado particular y contestará a las preguntas que Usted pueda tener. Además, el/ella puede hacer los arreglos necesarios para que Usted haga otras pruebas como un ultrasonido detallado y una amniocentesis, lo cual puede ayudar a determinar la razón del resultado positivo de la prueba. |
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Created by: XAFP Screening |