Prenatal Screening and Diagnostic Tests

Every pregnant woman wants desperately to believe that her baby will be “normal” with no genetic abnormalities or birth defects. Never before have we had so many screening options, but which ones are best and which ones should you use?

It is important to remember that a screening test cannot tell with 100% certainty whether or not a fetus will be affected by a given disorder. As Dr. Mark Evans, a geneticist and fetal medicine specialist, said on a recent Creating a Family radio show on Prenatal Genetic Testing, prenatal screening tests are for assessing the risk of problems, rather than giving a definitive diagnosis. If the results are abnormal, genetic counseling and diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis will be needed.

Pre-Pregnancy Screening Tests

Carrier Screening is a blood test on the mother, father, or both to determine risk of the couple passing on a genetic abnormality to their child. Most genetic diseases are “recessive disorders,” which means that each parent needs to pass along an affected gene to the baby in order for the child to be affected. If only one parent has the gene, the child will not inherit the condition. Even if both parents screen positive, there’s only a 25 percent chance your baby will have the disease.

First Trimester Prenatal Screening Tests

First trimester screening is a combination of maternal blood testing and ultrasound usually performed between weeks 11 and 14 of pregnancy. This testing assesses the risk of the fetus having certain birth defects and specific chromosomal abnormalities, including Down Syndrome Trisomy-21 and Trisomy-18.

There are three parts of first trimester screening:

  • Ultrasound test for fetal nuchal translucency (NT) examines the area at the back of the fetal neck for increased fluid or thickening.
  • Pregnancy-associated plasma protein screening (PAPP-A)-a maternal blood test to assess the level of a protein produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.
  • Human chorionic gonadotropin (hCG)—a maternal blood test to assess the level of a hormone produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.

Cell-free fetal DNA testing is a maternal blood test usually performed around week 10 of pregnancy that analyzes fetal DNA that has leaked into the mother’s bloodstream. The cellfree fetal DNA (cffDNA) test is a relatively new test introduced in late 2011 that can assess the risk of a fetus having a chromosome disorder, such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), or Patau syndrome (trisomy 13). This screening does not assess risk of fetal anomalies such as neural tube defects or ventral wall defects.

According to the American College of Obstetricians and Gynecologists:

“Given the performance of conventional screening methods, the limitations of cell-free DNA screening performance, and the limited data on cost-effectiveness in the low-risk obstetric population, conventional screening methods remain the most appropriate choice for first-line screening for most women in the general obstetric population.”

Second Trimester Screening Tests

Quad screen test is a maternal blood screening test usually performed between the 16th and 18th week of pregnancy analyzing the mother’s blood for four specific substances: AFP, hCG, Estriol, and Inhibin-A.

  • AFPalpha-fetoproteinis a protein that is produced by the fetus. Low levels of AFP and abnormal levels of hCG and estriol may indicate that the developing baby has Trisomy 21(Down syndrome), Trisomy 18 (Edwards Syndrome) or another type of chromosome abnormality. High levels of AFP may suggest that the developing baby has a neural tube defect such as spina bifida or anencephaly.
  • hCGhuman chorionic gonadotropinis a hormone produced within the placenta
  • Estriolestriolis an estrogen produced by both the fetus and the placenta
  • Inhibin-Ainhibin-Ais a protein produced by the placenta and ovaries

Prenatal Diagnostic Tests

Screening tests can only assess risk of the fetus having a problem. If results are abnormal the mother will be advised to seek genetic counseling and one or both of these diagnostic tests. According to Dr. Mark Evans, even if you have had preimplantation genetic screening of embryos with IVF, you should also consider having on of these diagnostic tests.

Chorionic Villus Sampling (CVS) is usually performed between weeks 10 to 12 of pregnancy, and involves taking a small Chorionic villus sampling graphicsample of cells from the placenta. Placental tissue contains the same genetic material as the fetus and can be analyzed for chromosomal abnormalities and other genetic disorders. CVS cannot identify neural tube defects, such as spina bifida, which can be detected by amniocentesis. 

Depending upon where the placenta is located and using ultrasound for guidance, a small tube is inserted through either the mother’s abdomen or her vagina and a small tissue sample is withdrawn from the placenta. CVS has a slightly higher risk of miscarriage than amniocentesis, although according to Dr. Mark Evans on a recent Creating a Family Radio Show, the risk is small and with an experienced doctor about the same as amniocentesis.

Amniocentesis is considered by many to be the gold standard for prenatal diagnostic testing, and is usuallyamniocentesis graphic performed between the 15th and 20th week of pregnancy. During amniocentesis a small amount of amniotic fluid is removed by inserting a long, thin needle through the mother’s abdomen into the amniotic sac surrounding the fetus. Amniotic fluid contains fetal cells and other substances, such as alpha-fetoprotein (AFP). It can test for genetic abnormalities as well as certain neural tube defects (diseases where the brain and spinal column don’t develop properly), such as spina bifida and anencephaly. With a skilled doctor, amniocentesis carries a low risk of miscarriage–about 1 in 400.


For a thorough discussion of genetic testing, both preimplantation with IVF and prenatal, I recommend this Creating a Family Radio Show on Preimplantation and Prenatal Genetic Testing with Dr. Mark Evans, geneticist and fetal medicine specialist. He is the Medical Director at Comprehensive Genetics, Professor of Obstetrics & Gynecology at Mt. Sinai School of Medicine, President of the Fetal Medicine Foundation of America, and President of the International Fetal Medicine and Surgery Society Foundation.




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Image credit: Stanford Children's Health (graphics)