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What Prenatal Screening Tests Should You Expect?

Monday, October 14th, 2013

Prenatal tests are diagnostic procedures used to uncover potential chromosomal or structural fetal disorders. All expectant mothers, but particularly mothers over 35, should have thorough prenatal testing because of the increased risk of birth defects and other abnormalities. These are a few of the tests your doctor may want you to have; talk to him or her about others that may be necessary or helpful.

Maternal Alpha-Fetoprotein

Alpha fetoprotein (AFP) is a protein normally produced by the liver and yolk sac of a developing baby during pregnancy. AFP levels decrease soon after birth. AFP probably has no normal function in adults. The amount of AFP in the blood of a pregnant woman can detect abnormalities in the fetus, be they chromosomal or structural, such as Down syndrome or spina bifida, respectively.

This test is routinely administered to every pregnant woman, except those over 35 who will be having amniocentesis. If screening reveals worrisome levels (too high or too low) of AFP in the mother’s circulation, additional tests like amniocentesis are administered to further investigate the problem.

Cell free fetal DNA testing

Noninvasive prenatal testing that uses cell free fetal DNA from the plasma of pregnant women is now being offered as a screening tool for fetal aneuploidy (Down syndrome and  other trisomies). The only problem with cell free fetal DNA testing is that it requires informed patient consent after pretest counseling and should not be part of routine prenatal laboratory assessment. Cell free fetal DNA testing should not be offered to low-risk women or women with multiple gestations because it has not been sufficiently evaluated in these groups. A negative cell free fetal DNA test result does not ensure an unaffected pregnancy.  So why have this test?  Because many patients do not wish to be stuck with a needle or have any invasive testing done.  To me, it just delays the inevitable, but it is important that my readers know it exists.  A patient with a positive test result should be referred for genetic counseling and should be offered invasive prenatal diagnosis, i.e., chorionic villus sampling or amniocentesis, for confirmation of test results.


Ultrasound, or sonogram, is a test commonly administered during pregnancy that uses sound waves (not X-rays) to render an image of your baby inside your womb. Ultrasound is safe to use throughout pregnancy, and can be helpful in determining  an accurate gestational age  if the Mom is uncertain and can detect gross abnormalities, such as anencephaly or open neural tube defects.  Ultrasound has also been used to attempt to screen for Down syndrome by using specific markers, such as thickness of the fold behind the neck, or abnormal kidneys or absence of or abnormal development of the nasal bone.

Ultrasound is typically used at 16-20 weeks, when fetal structures have grown large enough to be seen somewhat clearly, and many abnormalities can be detected. The ultrasound technician checks to make sure the skull is present, the spinal column has closed, and various organs are present and developing normally, including the heart. Ultrasound does have its limitations; for example, it is not a perfect way to detect fetal heart malformations, as this study shows. That is because the fetal heart is so tiny and complex. Nonetheless, ultrasound remains a safe and effective tool for detecting many potential fetal problems.  However, there is a tendency to overuse and abuse ultrasound in an attempt to “have a picture of the baby in utero.”   Any imaging study needs to be performed because of a medical indication and not for entertainment.


Amniotic fluid contains the baby’s metabolic products and desquamated fetal skin cells, and therefore can be tested for genetic disorders that may be present. During amniocentesis, ultrasound is used to detect pockets of amniotic fluid, and a small amount of it is drawn out through a needle inserted into the amniotic sac. This fluid will then be used for chromosomal analysis as well as checked for AFP.

Amniocentesis can detect conditions characteristic of chromosomal and developmental disorders. It is used in conjunction with ultrasound.  Amniocentesis has the advantage of actually examining the chromosomes, not just the structure and appearance.

Chorionic Villus Sampling

Chorionic villus sampling (CVS) is the best-kept secret in obstetrics, which is a pity because it provides the earliest opportunity for prenatal diagnosis. In contrast to ultrasound and amniocentesis at 16 to 20 weeks, CVS is done in the tenth to the twelfth week of pregnancy, with the tenth week calculated as starting at nine weeks and one day.

Chorionic villi are the beginnings of the placenta. Rapidly growing, fingerlike projections in the sidewalls of the uterus, they look like sea kelp. By means of a soft catheter introduced vaginally and guided by ultrasound, a sample of these villi can be aspirated and sent to the laboratory for culturing. The cells are dividing very rapidly at this early stage and they grow quickly in the culture, giving results in seven days, in contrast to amniocentesis, which takes two weeks for results. As well as providing material for a chromosomal analysis, CVS allows us to rule out Tay-Sachs, sickle-cell anemia, and any inborn error of metabolism or enzymatic problem. The only thing this test does not do is establish the AFP level, but because every woman not having amniocentesis is screened with a blood test for AFP, this is not a problem.

Chorionic villus sampling yields the same information as an amniocentesis done five, six, or seven weeks later. The presence of anomalies can be confirmed at ten weeks—before the pregnancy is showing, before the woman is feeling fetal movement, and before there is the degree of bonding with the baby that is likely to have taken place by 20 weeks. Thus, if the findings give rise to a decision not to continue with the pregnancy, it can be interrupted with a D and C with far less morbidity and psychological stress than occasioned by a second trimester termination.

Even parents who know that they would never terminate a pregnancy regardless of the circumstance should undergo prenatal testing because education and preparedness can make such a difference in the quality of life for parents and children. You can find more detailed information on these and other common prenatal tests, as well as advice on deciding how to proceed in the event that a test reveals something troublesome, in my book, Inside Information for Women.

– Yvonne S. Thornton, M. D., M. P. H.