However, the good news is that structural defects do not increase with age. Examples of a structural defect are when the skull bone doesn't form over the brain, or the spinal bone only partially forms over the spinal chord, or the intestines form outside the abdomen, or one kidney is missing. At between 15 and 20 weeks of pregnancy, non-invasive tests like ultrasound and the alpha fetal protein blood test can identify 85 to 90 percent of these structural defects. With amniocentesis, the detection rate rises to 95 percent with a one in 200 risk of miscarriage.
Dr. Patricia Baird, professor of medical genetics at the University of British Columbia, studied 26,859 children with birth defects. She excluded children with chromosomal abnormalities. She found there was no association between other types of birth defects and the age of the mother.
Get All Your Questions Answered Before Testing
Proper genetic counselling is essential if you are considering prenatal testing like amniocentesis or chorionic villi sampling. As Dr. R. D Wilson, Professor and Chairperson of the Genetics Committee at University of British Columbia, says, "All patients considering prenatal genetic diagnosis should have access to professionals who are knowledgeable in the field and skilled in the procedures."
If you are over 35, you should consider amniocentesis if you feel you could not face raising a child with Down's Syndrome or other serious genetic defect, and are prepared to undergo a late abortion at 20 weeks. A newer procedure known as chorionic villi sampling can be done at ten to 12 weeks of pregnancy, with results being available within a week. This newer test does not screen for structural abnormalities, so an alpha fetal protein test will have to be done between 16 and 18 weeks.
Another way to rule out structural defects of the baby and certain heart problems as well, is to consider having an ultrasound examination done. Ultrasound has no known risks to the unborn infant, but the final answer on its long-term safety for infants is not in yet. If the pregnancy is going well, and you are not having an amniocentesis, you might want to wait for a clear-cut reason such as unusual bleeding or suspected twins before having an ultrasound done.
Alpha Fetal Protein Testing
A blood test can measure the amount of a special protein known as alpha fetal protein (AFP) that is present in the mother's blood. This protein is made by the fetus as it grows, and normally is confined to the baby's circulation. With structural defects, there is a leakage of this protein into the mother's blood, and large amounts of this protein will be found in the amniotic fluid and in the mother's blood. The blood level of alpha fetal protein is best measured between 16 and 18 weeks of pregnancy. At this time in pregnancy, 85 to 90 percent of open spinal tube defects can be detected and 30 to 40 percent of Down's Syndrome.
A positive test does not mean your baby has a defect. It only means that further testing is necessary. Similarly, a normal blood test means you have a higher probability of a normal pregnancy, but does not guarantee a "normal" baby. The AFP test is only a screening test, aimed at only a few of the problems that can occur in pregnancy. False positive results are common, especially if you have miscalculated the dates of your pregnancy. The levels will also be high if you have twins or even due to normal variations in blood levels. Before proceeding to other testing, a repeat test should be done. If the level is still abnormally high, you should consider having a detailed ultrasound. If the ultrasound does not account for the high second reading, you may have to have an amniocentesis done as well.
Of 1,000 women having this test, an estimated 30 will have an abnormally high second reading. Of these 30, eighteen will end up having amniocentesis, and there will be only one to two actual spinal tube defects confirmed at birth. The remaining 16 women will receive high risk care and there is some evidence that these 16 women may be at higher risk from miscarriage, low birth weight, premature delivery and other complications.
Drawbacks of this blood test include the fact that the test will miss about 20 percent of structural defects and has a high rate of false positives, which can cause considerable and unnecessary anxiety.
A low level of alpha fetal protein may suggest but not prove a genetic abnormality such as Down's Syndrome.
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