Diagnostic tests are tests that involve taking a sample of either the placenta, the amniotic fluid, or fetal blood. The samples are then sent away and examined in a laboratory for chromosomal or genetic abnormalities. The two main diagnostic tests are amniocentesis and chorionic villus sampling, or CVS. Since both of these tests carry a small risk of miscarriage, you will need to carefully consider the advantages and disadvantages of the tests before going ahead with either of them.
Chorionic villus sampling (CVS)
Chorionic villi are fragments of placental tissue. Because the placenta originates from the fertilized egg, the chromosomes in the cells that make up the placenta are representative of your baby's chromosomes. During CVS, a small amount of tissue from your placenta is removed and tested in a laboratory to reveal if your baby has a chromosomal disorder such as Down syndrome or another trisomy disorder, such as trisomy 18. CVS is done between 10 and 12 1/2 weeks' gestation and the results are usually available within 7 to 10 days. The procedure can also definitely identify your baby's sex, if you want to know. If you don't want to have this information, make your wishes clear before the test. In rare cases, the doctor will be unable to perform CVS due to the placenta's position. In this case, you may be asked to return for an amniocentesis at 15 weeks.
This is the most common diagnostic test, done at around 15 to 19 weeks of pregnancy. The amniotic fluid around your baby mainly consists of your baby's urine and contains cells from your baby's skin and urinary tract. During amniocentesis, a number of cells are collected from the amniotic fluid. They are then sent to a laboratory and grown in a cell culture, until there is a sufficient number of cells to examine your baby's chromosomes and identify whether your baby has a chromosomal abnormality such as Down syndrome. Amniotic fluid can also be tested for high levels of the substance alpha-fetoprotein (AFP), which could mean that your baby has a neural tube defect such as spina bifida or anencephaly. Other DNA testing can also be done during amniocentesis to check for congenital conditions that may run in either parent's family (see Premature Birth).