Prenatal Genetic Testing Explained

One of the choic­es you will make as a prospec­tive par­ent involves whether or not to test for genet­ic con­di­tions in your devel­op­ing baby. There are many choic­es, as well as many mis­con­cep­tions about genet­ic testing. 

There are two types of con­di­tions that your doc­tor may dis­cuss with you: Tri­somies (when the baby has an abnor­mal num­ber of chro­mo­somes) and car­ri­er muta­tions (when a par­ent car­ries a less-com­mon ver­sion of a gene, which may be passed to the baby).

Chro­mo­some or Tri­somy Testing

Most peo­ple have a total of 46 chro­mo­somes, half from their moth­er and half from their father. Chro­mo­somes car­ry the genet­ic mate­r­i­al, or genes, from gen­er­a­tion to gen­er­a­tion. Tri­somies occur when the sperm or egg car­ries an abnor­mal num­ber of chro­mo­somes, result­ing in a baby hav­ing 47 chro­mo­somes. Down syn­drome is a well-known tri­somy con­di­tion. Peo­ple with Down syn­drome have an extra chro­mo­some num­ber 21, so this con­di­tion is called Tri­somy 21. Tri­somies may affect peo­ple dif­fer­ent­ly, with vary­ing degrees of pos­si­ble abnor­mal­i­ties. There are genet­ic tests offered to help detect the risk or pres­ence of cer­tain trisomies.

You may be aware that a moth­er’s age affects her risk of con­ceiv­ing a child with a tri­somy. For exam­ple, the over­all rate of Down Syn­drome is approx­i­mate­ly 1 in 700 births; how­ev­er, as a moth­er’s age increas­es, so do her chances of deliv­er­ing a baby with Down Syndrome.

For chro­mo­some con­di­tions, you can choose to do no test, a screen­ing test, or a diag­nos­tic test.

No Test­ing

Many prospec­tive par­ents choose to decline any type of test­ing for chro­mo­some con­di­tions. They may feel that the results of the test would not alter their approach to their preg­nan­cy, or they may feel that find­ing out infor­ma­tion about tri­somies may increase their wor­ry dur­ing their pregnancy.

If you decline any genet­ic test­ing, remem­ber that an ultra­sound is rou­tine­ly done mid­way through the preg­nan­cy. Ultra­sounds may find clues about poten­tial genet­ic con­di­tions, but they can­not diag­nose trisomies.

Screen­ing Tests

A screen­ing test cal­cu­lates the chance that your baby has a cer­tain tri­somy. It can­not say whether or not your baby has or does not have the con­di­tion. For exam­ple: Jane Doe has a screen­ing test that shows her baby has a 1:100 chance of hav­ing Tri­somy 21. In this sit­u­a­tion, her baby has a 1% chance of hav­ing the con­di­tion and a 99% chance of not hav­ing it. 

Tra­di­tion­al screen­ing tests are offered at two dif­fer­ent times dur­ing the pregnancy:

  • First Trimester Screen is a test that uses an ultra­sound and a small amount of the moth­er’s blood to test for the odds of Tri­somy 21 (Down Syn­drome) and Tri­somy 18 (Edwards Syn­drome). This test is often called a Nuchal Translu­cen­cy, or NT test, and is per­formed at 12 — 13 weeks of pregnancy.
  • AFP-tetra, or quad screen, is a blood test done around 16 weeks. Just like the first trimester screen, this test cal­cu­lates the chance that your baby can have tri­somy 21 and tri­somy 18. In addi­tion, this test also assess­es the chance that your baby can have an open neur­al tube defect, which is a type of mal­for­ma­tion of your baby’s spine or brain that is not usu­al­ly genetic.

If you choose a screen­ing test, you would choose one or the oth­er; both would not be done. If some­one does a first trimester screen, they can do a sep­a­rate test, called an MSAFP test, to assess the risk of neur­al tube defects in the sec­ond trimester.

If your screen­ing test comes back with a risk for tri­somy high­er than you are com­fort­able with, you do have the choice to then pro­ceed with a cell-free fetal DNA test or a diag­nos­tic test. 

Cell-Free Fetal DNA Testing

Cell-free fetal DNA test­ing is gen­er­al­ly offered to high­er-risk cou­ples: preg­nan­cies involv­ing old­er moth­ers, cou­ples with pre­vi­ous chil­dren with tri­somy, or those with abnor­mal screen­ing test results. This test is non­in­va­sive and only requires a blood sam­ple from the moth­er. This test assess­es risk by actu­al­ly detect­ing chro­mo­some mate­r­i­al for tri­somy 21, tri­somy 18, and tri­somy 13 (Patau Syn­drome). The test is con­sid­ered high­ly accu­rate, and can­not result in mis­car­riage. It is also pos­si­ble to test for the baby’s gen­der with these tests. Two com­mer­cial cell-free DNA tests are the MaterniT21 test and the Har­mo­ny test. 

Diag­nos­tic Testing

Diag­nos­tic tests have been avail­able for many years. Diag­nos­tic tests include chori­on­ic vil­lus sam­pling (CVS) and amnio­cen­te­sis. Both of these tests involve obtain­ing fetal cells to ana­lyze chro­mo­somes. CVS is done at the end of the first trimester, and involves a small biop­sy of the pla­cen­ta. Amnio­cen­te­sis is done ear­ly in the sec­ond trimester, and involves using an ultra­sound-guid­ed nee­dle to obtain some amni­ot­ic flu­id through the moth­er’s abdom­i­nal wall. Advan­tages of inva­sive test­ing include very high accu­ra­cy, the abil­i­ty to test for many more chro­mo­some con­di­tions, and very low risks of com­pli­ca­tions. There is a small risk of mis­car­riage with either pro­ce­dure, less than 1%.

Car­ri­er Screening

Car­ri­er screen­ing refers to tests that deter­mine whether you or your part­ner car­ry an unusu­al form of a gene that can lead to a dis­or­der or dis­ease in your child. Car­ri­er screen­ing is avail­able before or after you have dis­cov­ered you are expect­ing. Unlike test­ing for chro­mo­some con­di­tions in your baby, car­ri­er screen­ing tests for your poten­tial to pass a pos­si­bly harm­ful gene to your child. 

Most car­ri­er tests screen for con­di­tions that require both par­ents to car­ry the unusu­al gene in order for the child to have sig­nif­i­cant health issues. Hav­ing a gene from one par­ent makes you a car­ri­er, mean­ing you can pass the gene to your child but not be affect­ed by the con­di­tion your­self. Your child would only be affect­ed if he or she has unusu­al genes from both par­ents. Car­ri­er screen­ing can look for com­mon con­di­tions such as: cys­tic fibro­sis, tha­lassemia, spinal mus­cu­lar atro­phy, Tay-Sachs, sick­le cell ane­mia, and Frag­ile X syndrome. 

Are these tests cov­ered by my insur­ance plan?

Some insur­ance plans cov­er many genet­ic tests and car­ri­er screens. Some do not. Should you con­sid­er test­ing, it is always wise to con­tact your health plan to find out your cov­er­age. If you can­not find out eas­i­ly, ask your physi­cian’s office if they can help you find out. 

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