Abnormal Screening Results

Pregnancy should be a joyful time, but for many it is a time of stress and worry. Pregnant women who are over the age of 35, or have other health or family issues that put their baby at risk of birth defects or genetic abnormalities, may wish to have prenatal screening done to ensure the health and well-being of their babies and themselves.

What Prenatal Screening Can Find
Different screening methods will be better suited to detecting certain abnormalities. Your doctor can tell you which defects you're most at risk for and which tests would be most appropriate for you. Abnormalities, diseases and defects that can be found using prenatal screening include:


  • Down syndrome, also called trisomy 21. Down syndrome is the most common chromosomal abnormality.
  • Neural tube defects, such as spina bifida and anencephaly
  • Genetic defects such as, Tay-Sachs, cystic fibrosis, muscular dystrophy, sickle cell anemia and fragile X syndrome
  • Congenital heart defects


Prenatal Screening Tests
Different types of prenatal screening that your doctor may wish to perform on you and your baby include:

Test: What it’s for: What it indicates:
Amniocentesis: Uses a long needle, inserted through the abdomen, to draw amniotic fluid from the sac surrounding the baby. Usually done between weeks 15 and 18 of the pregnancy. To look for abnormal chromosomes, genetic defects, to check levels of alpha fetoprotein (AFP) and take fetal tissue samples. Abnormal results can indicate: neural tube defects, such as spina bifida and hydrocephalus; genetic defects, such as cystic fibrosis and down syndrome.
Chorionic Villus Sampling: Small projections that form part of the placenta are removed and tested for chromosomal abnormalities. This procedure can be done earlier than an amniocentesis – between 10 and 12 weeks after conception. To look for abnormal chromosomes and genetic defects. Abnormal results can indicate: neural tube defects, such as spina bifida and hydrocephalus; genetic defects, such as cystic fibrosis; chromosomal defects, such as down syndrome.
Maternal Serum Screening: Blood samples may be taken from the pregnant mother’s blood. To check heck levels of AFP and look for chromosomal defects. Abnormal results can indicate; neural tube defects, such as spina bifida and hydrocephalus; physical birth defects, such as cleft palate.
Percutaneous umbilical cord blood sampling (PUBS): Involves removing a sample of fetal blood from the umbilical cord. The procedure can also be used to transfuse blood into the fetus. Performed between weeks 20 and 23 of the pregnancy, usually on mothers who’ve already had amniocentesis or Chorionic Villus sampling done, and abnormal results were found. Looks for blood disorders, metabolic disorders, infections, structural malformations, intrauterine growth retardation and some chromosomal abnormalities. Abnormal results can indicate: diseases, such as anemia; genetic defects, such as cystic fibrosis; chromosomal defects, such as down syndrome; congenital defects, such as heart defects and physical birth defects, such as cleft palate

Abnormal Results
If you have prenatal screening done and the tests find abnormal results, further tests will be ordered. When the result of the tests is positive, that only means that a change in a chromosome, protein, or gene was detected, and does not necessarily indicate a problem. False positives are also not uncommon.

If the results of further testing come back positive, you will be given the option of carrying the pregnancy to term or aborting the fetus. If you choose to continue the pregnancy, knowing what problems your baby will face ahead of time will give you and your medical team time to prepare for the birth and the special needs of the child after it is born. Counseling will also be made available to you, no matter which option you choose.

Accuracy of prenatal screening results vary, depending on which test has been performed, and how many tests have been done. A U.S. study looked at the results of 38,167 women who underwent genetic screening using a blood test and ultrasound during their first trimester and a blood test during their second trimester, and followed up after the births. The first trimester test, done at 11 weeks, accurately identified 87% of the Downs syndrome cases. The latter test caught 81%. When both tests are performed, 95% of cases were accurately identified. Still, this means that in five per cent of cases the test was resulting in a false positive.


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