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
| Test | Test Type | Screens For: | Detection Rate | FPR | Weeks | |
|---|---|---|---|---|---|---|
| 1 | First Trimester Screen (FTS) | Blood plus Ultrasound | Trisomy 21 and 18, late pregnancy problems | Good | 5% | 11 - 13 |
| 2 | Second Trimester Screen | Blood | Trisomy 21 and 18, Spina Bifida, Abdominal Wall Defects, Late Pregnancy Problems | Good | 5% | 15 - 20 |
| 3 | Combined Screen (Combines 1 and 2) | Blood plus Ultrasound | Trisomy 21 and 18, Spina Bifida, Abdominal Wall Defects, Late Pregnancy Problems | Better | 5% | (1 and 2) |
| 4 | Cell-Free DNA | Blood | Trisomy 21, T18, T13, Sex Chromosomes, Deletions | Best | <5% | >10 |
| 5 | AFP | Blood | Spina Bifida, Abdominal Wall Defects | 5% | 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.