Why do women with screen negative results occasionally have babies with Down syndrome or an Open Neural Tube Defect?
It is unusual for women to have a baby with either of these abnormalities, and it is even more unusual for a woman with a screen negative result, but it does sometimes happen. This is because the screening test cannot completely distinguish affected from unaffected pregnancies. However small the risk is, we cannot rule out the possibility of the baby having Down syndrome or an open neural tube defect.
What does a screen positive result for Down syndrome mean?
A screen positive result means that you are in a high risk group for having a baby with Down syndrome. If your result is in this group, you will be offered genetic counseling and the option of a diagnostic test such as CVS or amniocentesis.
The result is called screen positive if the risk of Down syndrome in your pregnancy is 1 in 200 or greater in the first trimester or 1 in 270 or greater in the second trimester. Most women with screen positive results do not have a pregnancy with Down syndrome. For example, of 20 women with screen positive results for Down syndrome, only one would actually have a pregnancy with Down syndrome.
What does a screen positive result for Open Neural Tube defects mean?
A screen positive result means that you are in a group with an increased risk of having a baby with an open neural tube defect. If your result is in this group, you will be offered genetic counseling, an ultrasound scan examination at 18 to 20 weeks of pregnancy, and the option of an amniocentesis.
This is organized by your doctor or hospital.
What is Chorionic Villus Sampling (CVS)?
If your result is screen positive in the first trimester then CVS may be an option. CVS is a diagnostic procedure that extracts a small amount of placental material which is usually an excellent source of genetic material from the fetus. This test is performed either transcervically or transabdominally between 10 to 12 weeks of gestation, but only transabdominally after 12 weeks gestation. There is a small risk associated with CVS. Less than one percent of women may have a miscarriage as a result of the procedure.
Results are usually available in five to seven days, and the detection rate is greater than 99 percent for chromosomal abnormalities, such as Down syndrome. Follow-up evaluation for open neural tube defects is necessary.
What is Amniocentesis?
Amniocentesis is usually performed between 15 and 20 weeks, and is a procedure in which the doctor obtains a sample of amniotic fluid that surrounds the developing fetus. The sample is then sent to the laboratory for testing. This fl uid sample can be used to diagnose both chromosomal problems such as Down syndrome and Trisomy 18, as well as open neural tube defects such as spina bifida. Amniocentesis is an invasive procedure, which means that there is a small risk of miscarriage (less than 1 in 200) associated with it. Results of the test for Down syndrome and Trisomy 18 will take about 7-14 days. Results of the test for open spina bifida will take about 2-5 days.
No test can guarantee that your baby will be free of all birth defects, but if the result of the amniocentesis is negative, it will almost certainly rule out Down syndrome and/or other chromosome abnormalities.
What are the advantages of risk assessment?
The test may give you and your healthcare provider important information about your pregnancy and your developing baby. If your baby is found to have a serious birth defect, you can receive professional counseling about how your child’s physical and mental development may be effected. The individual capabilities and potential of children with birth defects are considerations which you may wish to discuss with your genetic counselor or other healthcare provider. Other options, such as adoption and termination of pregnancy may be discussed with you by your healthcare provider. Further information and support are available through groups such as your local Down Syndrome Society and Spina Bifida Association.
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