How is diagnostic testing different from screening?
Diagnostic testing tells you for sure whether or not your baby has one of the conditions being screened for. Screening tells you the chance that your baby has one of the conditions to help you decide whether or not to have diagnostic testing
What diagnostic tests are available?
A very thin needle is put into the woman’s belly into the womb. A few teaspoons of the fluid surrounding the baby (amniotic fluid) is taken out through the needle. The needle is guided by ultrasound, so that it does not touch the baby.
The amniotic fluid contains skin cells from the baby. Each cell has a complete set of the baby’s chromosomes. A picture is taken of the chromosomes so they can be counted and looked at closely.
Amniocentesis can tell you for sure whether or not the baby has Down syndrome or trisomy 18. However, amniocentesis is associated with a 1 in 200 risk of miscarriage. In other words, if 200 women have an amniocentesis, one would lose the pregnancy as a result of the procedure.
Amniocentesis is performed after 15 weeks of pregnancy and the result is ready in 2 – 3 weeks.
Chorionic villus sampling (CVS)
Chorionic villus sampling is a diagnostic test in which a needle is put either into the woman’s belly or the vagina. The needle is used to collect a small piece of the placenta, known as the chorionic villus. The needle is guided by ultrasound so that it does not touch the baby.
The sample contains cells from the baby. Each cell has a complete set of the baby’s chromosomes. A picture is taken of the chromosomes so they can be counted and looked at closely.
CVS can tell you for sure whether or not the baby has Down syndrome or trisomy 18. However, there is a 1 in 100 chance of miscarriage.
CVS is done between 11 – 13 weeks of pregnancy. Because it must be done early on in a pregnancy, it will not be offered as a diagnostic test after screening. Rather, it may be offered to women who are eligible for diagnostic testing without prior screening.
The result of CVS is ready in 2 – 3 weeks.
In a detailed ultrasound examination, a specialist looks carefully at every part of the baby to ensure the baby is growing and developing as expected. The ultrasound can tell whether the baby has an open neural tube defect . It can also confirm how far along you are in the pregnancy, and whether twins are present.
Unlike amniocentesis or CVS, detailed ultrasound has no risk of miscarriage. It is usually done between 18 and 20 weeks of pregnancy.