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July 01, 2006 - It was two days before Thanksgiving when the call came from my obstetrician's office.
"Your test results came back positive for Down syndrome," the nurse said. "I wanted to see if you're going to be home tomorrow so we can try to get an appointment with a specialist."
I could not believe this was happening to me. I knew the triple screen blood test I'd taken had a five percent false positive rate, but when I decided to take the test, I never thought it would affect me.
When I was pregnant with my son, my test results showed that I had a one in 2676 chance of having a child with Down syndrome. I opted to take the test during my second pregnancy because I wanted similar reassurance. One in 42 was not reassuring.
As I waited for the nurse to call back, I called my husband at work and filled him in.
"That still means that out of 100 babies only two will have Down syndrome," he said, trying to calm me down. I was not comforted.
When the nurse called back, she told me that the first available appointment with the genetic counselor was Monday morning. The plan was for me to visit my doctor's office first and do a regular ultrasound to be sure that the dates were right on my pregnancy. If they were, my husband and I would go to the specialist for an afternoon appointment. There we would be offered a more detailed ultrasound, genetic counseling and amniocentesis. If the dates were incorrect, my doctor would call the lab and have the test results recalculated based on my new due date.
I pleaded with the nurse not to make me wait all of Thanksgiving weekend to learn if the dates were wrong and she agreed to squeeze me in for the regular ultrasound the next morning.
After a sleepless night, I had decided that if the dates were right, I was having amniocentesis. I had to know for sure.
Fortunately, the ultrasound showed the baby's development was one week behind where it should have been based on my last menstrual period. That meant the test was done at 14 weeks, one week too soon. We redid the test the following Monday and the new results showed a one in 648 chance that the baby had Down syndrome. That was helpful, but still not what I had hoped for.
I've asked myself again and again if I had known ahead of time what the outcome was going to be, would I still have taken the test? I would. Had my daughter had Down syndrome, I would have wanted to know ahead of time so I could prepare.
However, I should have been better educated about the prenatal screening options available and what the test results really mean.
There are two types of prenatal testing available—screening and diagnostic. Screening tests assess your risk of having a child with a birth defect such as Down syndrome, a neural tube defect or cystic fibrosis. Diagnostic tests determine whether the baby you are carrying actually has the defect.
FIRST TRIMESTER
First trimester screen
A new test called the First Trimester Screen assesses a woman's risk of having a child with Down syndrome or Trisomy 18.
An extra chromosome, #21, causes Down syndrome, which results in mental and physical abnormalities, including mental retardation. Approximately one in 800 babies is born with Down syndrome. A woman's risk of having a baby with Down syndrome increases as she ages.
Trisomy 18 results when the baby has an extra chromosome, #18. Trisomy 18 causes serious mental retardation and physical deformities including heart defects. Only one out of 10 babies affected by Trisomy 18 will live past one year.
The First Trimester Screen is done between the 11th and 13th week of pregnancy and includes drawing a blood sample from the mother and a special ultrasound. At the lab, technicians measure the levels of two proteins produced by the baby and the placenta, PAPP-A and human chorionic gondotropin (hCG). The ultrasound, known as a nuchal translucency, measures a fluid-filled space in the back of the baby's neck. Extra fluid can indicate problems.
The screen detects roughly 83 percent of Down syndrome cases and 80 percent of Trisomy 18 cases.
The advantage of the First Trimester Screen is that the mother often learns whether the baby is at an increased risk of having a birth defect before anyone else knows she is pregnant, says Dr. Mary Pell Abernathy, a maternal fetal medicine specialist at the Indiana University School of Medicine.
"Any pregnant woman could potentially benefit from this," she says.
Abernathy said the screening is popular on the East and West coasts, but has been slower to catch on in the Midwest.
Jennifer Dickerson, a certified genetic counselor at St. Vincent Maternal Fetal Medicine, says most women opting for this screening are over 35, but the test is available to any pregnant woman. However, insurance companies may be reluctant to pay for the test for women who are not considered high risk.
CVS
Chorionic Villi Sampling, CVS, is also performed during the first trimester between 10 and 12 weeks of pregnancy. During CVS, a doctor withdraws a small amount of tissue from the placenta with a catheter inserted through the patient's cervix or abdomen. The tissue is then sent to a laboratory where the baby's chromosomes are analyzed.
While there is a 1 to 2 percent chance the mother's tissue may contaminate the sample, CVS is generally considered a diagnostic test. The baby's sex and any chromosomal abnormalities can be firmly established by the test. CVS carries a 1 to 3 percent risk of miscarriage.
SECOND TRIMESTER
Integrated Screen
The Integrated Screen is also relatively new. It combines the results of the First Trimester Screen with a second blood test done between 15 and 22 weeks of pregnancy. The results are not available until after the second blood draw.
The Integrated Screen offers a significant increase in accuracy and will also screen for neural tube defects like spina bifida.
The neural tube develops into the baby's brain and spinal cord. If the tube does not close completely, an opening remains along the baby's spine or head. Neural tube defects affect one to two babies out of every 1000.
The Integrated Screen detects approximately 92 percent of Down Syndrome cases, 90 percent of Trisomy 18 cases and 80 percent of open neural tube defects.
Triple or Quad Screen
The Triple Screen or Quad Screen, also known as maternal serum screening, is the test most women can expect to take in their doctor's office. The screening test is done between the 15th and 22nd week of pregnancy and requires a sample of the mother's blood. The optimal time for the test is between weeks 16 and 18.
Once the blood sample is drawn, it is sent to a laboratory where technicians measure the level of three or four pregnancy hormones. The Triple Screen looks at alpha-fetoprotein, hCG and estriol. The Quad Screen also looks at the level of Inhibin A.
The Triple Screen catches about 70 percent of Down syndrome cases, while the Quad Screen increases your chances of detection to about 81 percent. Both tests detect about 80 percent of Trisomy 18 cases and open neural tube defects.
The screening is considered positive for Down syndrome if it shows a one in 270 chance that the baby will have the defect. At age 35, a mother's risk of having a child with Down syndrome is 1 in 270. Basically, a positive Triple or Quad Screen entitles a younger woman to the same right to have additional tests that a pregnant woman over 35 is given as part of her standard care.
"It's important, I think, that people realize it's not a diagnosis," Dickerson says.
The screening is considered positive for a neural tube defect if the alpha-fetoprotein level is above 2.5 and a 1 in 100 chance is considered a positive for Trisomy 18.
Approximately 5 percent of all positive Triple or Quad screens are false positives.
"You want to try to pick up as many as you can," Abernathy says. "You don't want to be in that 8 percent you miss that may have been falsely reassured."
Detailed ultrasound
If a screening test comes back positive, the next step is usually a detailed ultrasound where highly trained technicians look for soft markers that indicate a problem with the baby. Detailed ultrasounds are routinely offered to women over 35 or couples with a family history of birth defects.
During the detailed ultrasound, the technician looks for problems such as heart defects or abnormal growth which indicate the baby may have a birth defect. An ultrasound can detect at least 50 percent of babies who have Trisomy 18, Down syndrome and neural tube defects.
"It's a more detailed ultrasound than what you would have in your OB's office," Dickerson says.
Amniocentesis
The best way to diagnose a genetic defect like Down syndrome is amniocentesis, which is 99.8 percent accurate. The test is generally done between 15 and 22 weeks, although it can be done at any point during pregnancy. During amniocentesis, a doctor uses an ultrasound to visually guide a thin needle through the abdomen and into the uterus to withdraw a sample of amniotic fluid.
"We try to be where the baby's not, where the placenta's not and where there's a good pocket of fluid," Abernathy says.
She adds that amniocentesis hurts about as much as having your blood drawn.
The fluid is sent to a lab where skin cells from the baby are grown in a Petri dish and examined to determine the baby's chromosome composition. The test, which generally takes between one and three weeks, determines whether there are any chromosome defects and shows the baby's sex.
In a relatively new development, the laboratory can put special markers on certain chromosomes and have preliminary results within 1 to 3 days. The test is known as fluorescent in-situ hybridization or FISH.
"It's nice to be able to give that Mom a little more info," Abernathy says.
The downside of amniocentesis is that there is a small chance it will cause a miscarriage.
Other tests
If you have a family history of genetic disorders such as cystic fibrosis or sickle cell anemia, you may be offered additional tests to determine whether you and your husband carry the genes for an inherited disorder. Sometimes this screening can occur even before you become pregnant.
However, there is still no guarantee you will have a healthy baby. Every couple faces a three to five percent risk that their child will have a developmental delay or birth defect, Dickerson says.
"The one drawback is that of all the things we can test for, we can't detect everything," she adds.
I was fortunate. My daughter was born May 12 and shows no signs of Down syndrome. That's reassuring.
