Genetic Testing

We offer prenatal genetic screening to all patients and will discuss this at your first prenatal visit. Initiation of prenatal care is very important as some of these tests are valid only if performed early in the pregnancy.

Screening for Birth Defects

Birth defects occur in up to 3-5% of pregnancies with a wide range of severity and outcomes.

Different types of birth defects:
  • Chromosomal – Chromosomes can be abnormal number, missing or extra pieces of chromosomes.
    • Trisomy – extra chromosome (Trisomy 21 is commonly called Down syndrome).
    • Monosomy (missing chromosome), deletions and duplications of parts of chromosomes).
  • Inherited genetic disorders- muscular dystrophy, cystic fibrosis, or sickle cell disease, others
  • Structural birth defects- cleft lip, club foot, neural tube defects, cardiac defects, other

Diagnostic Vs. Screening Tests

Diagnostic Tests: Amniocentesis, Chorionic Villus Sampling
  • Usually used for patients with risk factors (age, prior pregnancy, family history, abnormal screening test)
  • Definitive results (yes or no)
  • Invasive – Risk of leakage of amniotic fluid, infection, bleeding and miscarriage.
Screening Tests:
  • Usually for low risk pregnancies, but may also be used with advanced maternal age if patient desires screening only
  • Result not definitive (Not yes or no result, but a calculation of risk, which if increased may lead to further testing.)
  • Non-invasive- no risk
Carrier Screening: 
  • Blood from parents can be tested to see if they carry a genetic condition that could be passed on to the baby. This can be done anytime during or before pregnancy.
  • Cystic fibrosis- All patients regardless of ethnic or family history should consider screening.
  • Other genetic disorders- Testing should be considered based upon personal or family history or ethnic background

Comparison of Screening Tests

TestTest TypeScreens For: Detection RateFPR Weeks
1First Trimester Screen (FTS)Blood plus UltrasoundTrisomy 21 and 18, late pregnancy problemsGood5%11 - 13
2Second Trimester ScreenBloodTrisomy 21 and 18, Spina Bifida, Abdominal Wall Defects, Late Pregnancy ProblemsGood5%15 - 20
3Combined Screen (Combines 1 and 2)Blood plus UltrasoundTrisomy 21 and 18, Spina Bifida, Abdominal Wall Defects, Late Pregnancy ProblemsBetter5%(1 and 2)
4Cell-Free DNABloodTrisomy 21, T18, T13, Sex Chromosomes, DeletionsBest<5%>10
5AFPBloodSpina Bifida, Abdominal Wall Defects5%15 20

Cell-free DNA Screen v. First trimester Screen/ Combined Screen

Cell-free DNA screen
  • More sensitive for trisomy, sex chromosome abnormalities
  • Lower false positives than other screening tests
  • Sex determination available
  • Screening for chromosome deletions available but sensitivity is lower than for trisomy.
  • Will not pick up other rare chromosome abnormalities
  • Not recommended for twins
First trimester screen and combined screen
  • Less sensitive for trisomy and sex chromosome abnormalities
  • Higher false positives
  • Cannot determine sex
  • May detect other rare chromosome abnormalities that cell-free DNA would miss.
  • Ultrasound (FTS) allows early screening for anatomical birth defects
  • Blood levels individually can screen for late pregnancy problems such as preeclampsia, low birth weight and stillbirth.
  • Can be used for twins.

Deciding to screen for birth defects

Who should screen for birth defects:

  • Patients at increased risk or who want definitive results should consider referral for genetic counseling and diagnostic testing
  • Any positive or abnormal result should be followed up with genetic counseling and consideration of further testing.
  • May be helpful to know in advance the health of the baby for planning and possibly improved care before and after birth
  • Decision should be made by the parents based upon information about the available tests.