It can also check for defects in the fetal sex chromosomes X or Y. Cell-free fetal DNA screening does not find structural birth defects, such as spina bifida or defects in the abdominal wall.
If first trimester screening is abnormal, you may need more testing for diagnosis. This may include chorionic villus sampling, amniocentesis, or another ultrasound. First trimester screening can tell if the baby might have a birth defect.
The screening test is usually offered to all pregnant women toward the end of the first trimester. Cell-free fetal DNA testing may be offered to women at increased risk, such as women over age These screening tests may not be accurate in women with a multiple pregnancy twins or more. First trimester screening involves an ultrasound and blood tests.
These are low-risk tests. In an unscreened population, about one in every babies is born with Down syndrome.
Usually, it is not inherited and so a baby can be affected even if there is no history of Down syndrome in the family. Although Down syndrome occurs more frequently as mothers get older, about 70 percent of babies with Down syndrome are born to women who are younger than 35 years old. Down syndrome is always associated with moderate to severe developmental disability and is often associated with physical problems such as heart defects and difficulties with sight and hearing. It is not possible to assess the degree of handicap before the baby is born.
About nine out of 10 babies with Down syndrome will survive their first year, and nearly half of these will reach 60 years of age. A sample of your blood is taken between 11 and 13 weeks of pregnancy. At the same time, an ultrasound scan is performed. The specific ultrasound marker of Down syndrome is: iii nuchal translucency NT thickness. The values of these markers are used together with your age to estimate the risk of having a pregnancy affected with Down syndrome.
A risk is the chance of an event occurring. For example, a risk of Down syndrome of one in means that if women have this risk, we expect that one of these women will have a baby with Down syndrome and that 99 will not.
This is the same as saying that the baby has a 1 percent chance of having Down syndrome and a 99 percent chance that it does not. The results of the test are usually ready within three working days of the blood sample being taken.
Results are sent to your health care provider. Your screening result is either screen positive or screen negative. A screen positive result means that you are in a high risk group for having a baby with Down syndrome. If you are in this group, you will be offered a diagnostic test. The result is screen positive if the risk of Down syndrome in the first trimester is one in or greater.
About one in every 20 women screened will be in this group. Most women with screen positive results do not have a pregnancy with Down syndrome. For example, of about 50 women with screen positive results for Down syndrome, only one would have an affected pregnancy. If the risk of Down syndrome based on the first trimester test is lower than one in , then the result is called screen negative and a diagnostic test is usually not offered.
Although a screen negative result means that your risk of having a baby with Down syndrome is not high, a screen negative result cannot rule out the possibility of a pregnancy with Down syndrome. Your use of this information means that you agree to the Terms of Use. Learn how we develop our content. To learn more about Healthwise, visit Healthwise.
Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated. It looks like your browser does not have JavaScript enabled. Please turn on JavaScript and try again. Important Phone Numbers. Top of the page. First-Trimester Screening for Birth Defects.
Topic Overview What is the first-trimester screening for birth defects? Nuchal translucency test. This test uses ultrasound to measure the thickness of the area at the back of the baby's neck. An increase in the thickness can be an early sign of Down syndrome. The test is not available everywhere, because a doctor must have special training to do it. First-trimester blood tests.
These tests measure the amounts of two substances in your blood: beta human chorionic gonadotropin beta-hCG and pregnancy-associated plasma protein A PAPP-A. Beta-hCG is a hormone made by the placenta. High or low levels may be related to certain birth defects.
PAPP-A is a protein in the blood. Low levels may be related to certain birth defects. How are the tests done? The health professional taking a sample of your blood will: Wrap an elastic band around your upper arm. This makes the veins below the band larger so it is easier to put a needle into the vein. Many women have their screening test as early as possible, in their first trimester, to find out if there could be a problem.
However if you are too late for the first trimester test or it is not offered in your area you may find a second trimester test valuable. Your results should be available within a week. They will be given to your doctor who will also tell you if a physical abnormality has been found during the ultrasound examination.
One in every 25 women tested will be told they are at increased risk. This does not mean there is definitely something wrong with your baby, but you may wish to think about having further diagnostic tests. There may be costs for these screening tests.
You may be able to claim part of this cost from Medicare external site. Please ask when you book your appointment about the costs and any rebates available. This publication is provided for education and information purposes only.
It is not a substitute for professional medical care.
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