Prenatal Genetic Screening Options
This information is intended to help you decide which, if any, prenatal screening tests you would like to have performed during your pregnancy. Most babies are born healthy; however, the general risk of having a child with a birth defect is about three to four percent. The risk of having a baby with a chromosomal abnormality, like Down syndrome, increases with the age of the mother. The approximate risks of having a baby with a chromosomal disorder related to age of the mother are listed below:
Down Syndrome or Trisomy 21: This is a chromosomal abnormality that can cause mental retardation and birth defects that may involve the heart, face or digestive system. Individuals affected with Down syndrome can have varying degrees of mental retardation and physical defects ranging from mild to severe.
Trisomy 18 and 13: These are rare chromosomal disorders that cause profound mental retardation and multiple birth defects. Many of these children do not live longer than one year.
Neural Tube Defects: This is an abnormality in the formation of the neural tube. It causes abnormalities of the brain or spinal cord (e.g., spina bifida). They occur in about in 1-2 of every 1000 births.
Chorionic Villus Sampling or CVS: This invasive test is typically performed at about 10-11 weeks and involves obtaining a sample of the placenta for genetic testing by either a vaginal or abdominal approach. There is a small risk of pregnancy loss after this procedure. About 1 in 10 women are unable to have this procedure performed because a sample cannot be safely obtained. In these cases an amniocentesis is usually recommended. CVS testing is more than 99 percent accurate for detecting chromosomal abnormalities.
Amniocentesis: This invasive test is typically performed at 15-18 weeks. In an amniocentesis, a small sample of amniotic fluid is obtained by inserting a thin needle through the mother’s abdomen into the amniotic sac. There is a small risk of pregnancy loss after this procedure. It is more than 99 percent accurate for ruling out chromosomal abnormalities and 95 percent accurate for detecting open neural tube defects.
Ultrascreen/Nuchal Translucency/First Trimester Screening: This noninvasive test is done between 11 and 13 weeks. It predicts risks based on the patient’s age, a blood sample from the mother (obtained by a finger stick), and an ultrasound that looks at the thickness of the area behind the baby’s neck or nuchal translucency. This test detects 82-87 percent of Down syndrome pregnancies and 95 percent of trisomy 18 pregnancies.
Alpha-fetoprotein or AFP: This test screens for neural tube defects in your baby and can be tested in the mother’s blood or done with an amniocentesis. This test is usually done in the second trimester in combination with first trimester testing or as part of the second trimester blood tests.
Tetra Screen or Quad Screen: This is a noninvasive second trimester blood test done around 16-18 weeks that screens for Down syndrome, trisomy 18, and neural tube defects. This test measures the level of four of the following substances in the mother’s blood: alpha-fetoprotein, estriol, human chorionic gonadotropin and inhibin A. This test detects Down syndrome in 81 percent of the cases, trisomy 18 in more than 60 percent of the cases and neural tube defects in 80 percent of the cases.
Ultrasound: This is a noninvasive evaluation of your baby’s anatomy that can detect some birth defects, including neural tube or spinal defects.