Prenatal screening has become an issue of importance over recent years and even though most aspects of standard prenatal care are straightforward, the issue of screening continues to have some emotional charge to it. Screening and diagnostic testing for chromosomal abnormalities continues to be confusing and carries uncertain risks.
One of the most commonly used genetic tests is amniocentesis. However, the rate of fetal loss with this test averages about one in 200. As a result, doctors are relying more on the non-invasive procedure of serum analyte testing which helps to identify the risk of neural tube defects, trisomy 21 and trisomy 18. Neural tube defects appear in 1.2 of 1,000 live births - the most common defect being spina bifida.
Trisomy 21, responsible for Down syndrome, presents with mental retardation, malformation of the heart, gastrointestinal tract, eyes, and ears, and is associated with early onset Alzheimer's disease. Risk for this defect is one in 1,000 live births. The risk in the second trimester in women 35 to 40 years of age is one in 270 live births, and in women over the age of 40, the risk is one in 100. Even though the risk does increase with the age of the mother, 75 percent of babies affected with trisomy 21 are born to mothers under the age of 35 years. As a result, it is suggested that all pregnant women be screened.
Trisomy 18, also known as Edwards' syndrome, occurs in one in 6,000 live births. Babies born with this syndrome have low birth weight, mental retardation and malformations of the head and heart as well as renal malformations. These babies frequently die within the first year of life.
A triple screen is a blood test that measures alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol. The test only indicates that there may be a problem and serves to help the doctor see if the baby is at risk for certain birth defects. However, an abnormal test does not necessarily indicate a birth defect in the baby. Most often, the results are abnormal because the age of the baby is either younger or older than the doctor thought. To be most effective, the tests should be run between the 16th and 18th weeks of gestation.
If the screening is abnormal, it is likely the doctor will want to run other tests or will refer the woman to a specialist. An ultrasound exam is first on the list of additional testing. The ultrasound can better check the age of the baby as well as look at the brain, spinal cord, kidneys, and heart. The doctor may request an amniocentesis, which checks the fluid around the baby in the sac. This test may also help identify any problems.
It is important to understand that many of these tests are not conclusive and only indicate a possible problem.