Prenatal Diagnostic Testing
last authored: July 2011, David LaPierre
last reviewed: Nov 2011, Carrie Blake
Prenatal diagnostic testing, which can include ultrasound, blood analysis, and sampling,
- can be done to identify babies who are at increased risk of defects
- is carried out for mothers who want information about potential conditions the fetus may have, and
- is generally offered dependent on risk factors
Women who are 35 years and older and/or have a family history of inherited disease or congenital abnormalities are considered at higher risk for having a baby with birth defects. However, it should be noted that the majority of babies born with congenital defects, such as Down Syndrome, Trisomy 18, Cystic Fibrosis, and Duschenne Muscular Dystrophy, are born to mothers who are not technically at a higher risk.
Women or their partners who are African, Asian, Hispanic, Mediterranean, or Middle Eastern should be considered for screening for hemoglobinopathies such as thalassemia and sickle cell disease.
- blood biochemistry
- completed between 11-14 weeks gestation
- offered to mothers at increased risk of birth defects
- can confirm viability, gestational age, assess number of fetuses, and allow for nuchal translucency measurement and nasal bone scan
- an increased nuchal translucency and lack of nasal bone indicate that the fetus is at higher risk of defects such as Down Syndrome or other Trisomy disorders
Fetal Anatomy Ultrasound
- completed around 20 weeks gestation
- used to evaluate fetal anatomy and organ systems, including evaluation of the brain and heart anatomy and fetal face
- physical birth defects, such as neural tube defects (spina bifida), heart defects, cleft lip/palette, polycystic kidneys, and club feet can be diagnosed by ultrasound
- chromosomal abnormalities can only be suspected via ultrasound and must be definitively diagnosed by sampling (see Sampling section)
First Trimester Screening (FTS)
- completed between 10-14 weeks gestation
- consists of blood tests for beta-hCG and pregnancy-associated plasma protein A (PAPP-A)
- is used in conjunction with ultrasound Provides information about the risk for Down Syndrome in single gestations.
Maternal Serum Screening (MSS)
- completed between 15-21 weeks gestation
- includes blood tests for maternal serum alpha-fetoprotein (AFP), beta-hCG, and unconjugated estrogen
- provides information about the possible presence of Down Syndrome, Trisomy 18, and open neural tube defects
- in order to verify the presence of any of these disorders, sampling must be completed (see next section)
Inherited Genetic Condition Screening
- completed at various points in pregnancy, dependent on condition being screened
- includes testing for Cystic Fibrosis, Thalassemia, and Tay-Sachs disease
Amniocentesis, typically carried out at 15-20 weeks gestation, is an ultrasound-guided sampling of amniotic fluid. Amniocentesis can be done as early as 13 weeks gestation. It is used to assess risk of neural tube defects, known inherited genetic disorders, and fetal lung maturity. Amniocentesis is more accurate and less risky than chorionic villus sampling (see below), but still carries a 1:200 risk of spontaneous abortion and can result in injury to the fetus.
Chorionic Villus Sampling (CVS)
Chorionic Villus Sampling (CVS), carried out at 10-12 weeks gestation, allows for diagnosis of congenital defects including Down Syndrome, Cystic Fibrosis, and Duschenne Muscular Dystrophy.
CVS is an ultrasound-guided needle biopsy of the fetal-derived chorion. Small fragments of the chorionic villi are drawn up and examined and direct analysis is performed on trophoblastic cells. Culture analysis examines the karyotype of cells from the mesenchymal core of the chorionic villi. It should be noted that CVS analysis of chromosome structure is not as detailed as is amniocentesis analysis.
Risks of CVS include:
- spontaneous abortion due to fetal damage
- development of a positive Indirect Coombs antibody titre due to mixing of maternal Rh- and fetal Rh+ blood
- birth defects due to placental damage
- negative results due to maternal tissue mistakenly being sampled instead of fetal tissue.
- Carried out at 10-12 weeks gestation
- Ultrasound-guided needle biopsy of the fetal-derived chorion
- Allows for diagnosis of congenital defects including Down Syndrome, Cystic Fibrosis, and Duschenne Muscular Dystrophy
return to top
Resources and References
return to top