After conception, there are three options available for detecting genetic issues in the fetus: chorionic villus sampling (CVS), amniocentesis, and preimplantation genetic diagnosis (PGD).
Amniocentesis
Amniocentesis is a prenatal test used to detect genetic or chromosomal conditions in a developing baby, such as Down syndrome, Edwards syndrome, or Patau syndrome. The procedure involves extracting a sample of amniotic fluid, which surrounds the baby in the womb.
Amniocentesis is typically performed between 12 to 16 weeks of pregnancy and is guided by an ultrasound. A needle is carefully inserted through the mother's abdominal wall — or, in some cases, through the vagina and uterus — to reach the amniotic sac. A small amount of amniotic fluid is then collected for testing.
Amniocentesis is a relatively quick procedure that lasts around 10 minutes. While it may cause discomfort similar to menstrual cramps, it is generally not very painful.
CVS
Chorionic villus sampling (CVS) is a prenatal test that can be conducted as early as 10 weeks into pregnancy. It involves sampling chorionic villi, which are small, finger-like projections of tissue from the placenta that contains the same genetic material as the baby. The purpose of the CVS test is to detect chromosomal abnormalities in the fetus.
CVS is considered an alternative to amniocentesis because it can be performed earlier in pregnancy, providing intended parents with more time for counseling and decision-making. However, unlike amniocentesis, CVS does not provide information on neural tube defects such as anencephaly or spina bifida. It is recommended that all pregnant women, especially those at higher risk for chromosomal abnormalities (such as older women, those with a history of chromosomal disorders in previous pregnancies, or those with abnormal screening results), consider this test.
During the procedure, depending on the location of the placenta, chorionic villi are sampled either through a thin plastic tube inserted through the cervix or a small needle inserted through the abdomen. Under ultrasound guidance, a small amount of tissue is collected into a syringe. The entire procedure typically lasts about five minutes and is associated with minimal discomfort, similar to that of a Pap smear. A follow-up ultrasound is often performed two to four days later to ensure no complications have arisen.
While complications from CVS are rare, they can include infection, bleeding, and in very rare cases, miscarriage. These risks are carefully weighed against the potential benefits of obtaining valuable genetic information about the fetus early in pregnancy.
Accuracy
Neither CVS nor amniocentesis is 100 percent accurate. A test result may fail to detect an existing abnormality (a false negative) because some embryos consist of both normal and abnormal cells, and by chance, only the normal cells may be sampled. However, if the results of a CVS or amniocentesis indicate that a fetus likely has a chromosomal abnormality or genetic disorder, the parents must decide whether they want to terminate the pregnancy.
PGD
Preimplantation genetic diagnosis (PGD) involves testing embryos typically on day 3 of their development when they consist of four to nine cells. During the procedure, one or two cells are removed from the embryo after the outer layer (zona pellucida) is opened using mechanical, chemical, or laser techniques. A sufficient number of eggs are retrieved initially as not all will fertilize and progress to a stage suitable for biopsy. While most embryos (96%) survive the biopsy, pregnancy rates may be slightly lower compared to non-biopsied embryos.
PGD testing can identify a range of disorders including cystic fibrosis, beta-thalassemia, sickle cell disease, spinal muscular atrophy, myotonic dystrophy, Huntington’s disease, Charcot-Marie-Tooth disease, fragile X syndrome, hemophilia A, and Duchenne muscular dystrophy.
There are two primary groups that benefit from PGD testing. The first group comprises couples at risk of transmitting a single-gene defect, an X-linked disorder, or other chromosomal abnormalities. The second group includes women with a history of recurrent miscarriages or repeated unsuccessful implantations, indicating an increased risk of chromosomal defects. Test results help determine which embryos are suitable for transfer during IVF procedures.
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