Prenatal Genetic Screening Tests
Prenatal genetic screening tests are performed before your baby is born to help you know if your child has a genetic disorder. Prenatal genetic testing differs from genetic carrier screening, which helps you better understand your own hereditary risk factors that could contribute to having a child with a genetic disorder.
First trimester prenatal genetic screening tests
First trimester screening tests are used to screen for Down syndrome, Trisomy 13 and Trisomy 18. This test may also show if a baby is at increased risk for heart defects. The test is conducted between 11 and 14 weeks. It is completed by ultrasound and primarily measures the thickness at the back of the baby’s neck, called nuchal translucency. Maternal blood screening tests may also be offered after completion of ultrasound and counseling.
Second trimester prenatal genetic screening tests
Second trimester screening tests are completed between 15 and 22 weeks. Maternal blood screening tests are used to detect neural tube defects and chromosomal birth defects (e.g., Trisomy 18 and abdominal wall defects).
Two different tests may be offered:
- Alpha-fetoprotein (AFP) is used for women who have undergone first trimester screening as it screens only for neural tube defects. If abnormalities are found on your screening tests, a detailed ultrasound evaluation will be performed, and you may be offered diagnostic testing such as amniocentesis.
- Quad screen is used for women who have not undergone first trimester screening, as it tests for Down syndrome, Trisomy 18 and neural tube defects.
*Additional testing and screening may be offered for high-risk patients.
Disorders detected by prenatal genetic screening tests
Abdominal wall defects called gastroschisis and omphalocele affect about 1 in 5,000 births in the U.S. Affected babies have intestines that protrude outside the body through an opening in the abdominal wall or through a hernia in the umbilical area. Surgery can help correct these defects.
Cystic fibrosis (CF) is a genetic disease. About 30,000 children and adults in the United States have the disorder. Cystic fibrosis severely affects breathing and digestion. It is caused by an abnormal gene that makes the body produce thick mucus in the lungs. CF is an autosomal recessive disorder, meaning that both parents must be carriers for a child to be at risk. If you are found to be a carrier, then your partner will also be offered carrier testing.
About 1 in 800 babies in the U.S. is born with Down syndrome, which is caused by an extra copy of chromosome 21. Affected children have varying degrees of mental intellectual disabilities, characteristic facial features and, oftentimes, have heart defects in addition to other physical problems.
Fragile X syndrome is the most common inherited cause of autism and intellectual disability. Symptoms are variable from mild to severe. Females usually have milder symptoms than males. Approximately 1 in every 3,600 boys and 1 in every 6,000 girls is born with Fragile X. Fragile X is an X-linked disorder; the gene is on the X chromosome.
Open neural tube defects, often called spina bifida, affects the backbone and sometimes the spinal cord. Children with spina bifida may have varying degrees of leg paralysis and bladder and bowel control problems. Anencephaly is a severe form of neural tube defect and is a fatal condition in which a baby is born with a severely underdeveloped brain and skull.
Spinal muscular atrophy or SMA is the most common inherited cause of infant death. It affects a child’s ability to control their muscles, including those involved in breathing, eating, crawling and walking. One in every 6,000 to 10,000 babies born in the U.S. has SMA. Spinal muscular atrophy is an autosomal recessive disorder, meaning that both parents must be carriers for a child to be at risk. If you are found to be a carrier, then your partner will also be offered carrier testing.
About 1 in 8,000 babies in the U.S. is born with Trisomy 13, which is caused by an extra copy of chromosome 13. Affected babies have severe mental intellectual disabilities, heart development delays and other birth defects. Most babies with Trisomy 13 will die within the first year of life, if not before delivery.
About 1 in 6,000 babies in the U.S. is born with Trisomy 18, which is caused by an extra copy of chromosome 18. Affected babies have severe mental intellectual disabilities, heart defects and other birth defects. Most babies with Trisomy 18 will die within the first year of life, if not before delivery.