Prenatal screening can be done early in pregnancy. It is optional. There are different types of testing.
FIRST TRIMESTER SCREENING (FTS):
This involves having an ultrasound to measure nuchal translucency and doing bloodwork between 11+2 weeks and 13+3 weeks. The results will estimate the chances of the baby having Down’s Syndrome, trisomy 18 or trisomy 13. It is not a diagnostic test. If the screen is positive then a further diagnostic test will be offered to know for certain if the baby is affected or not.
MATERNAL SERUM SCREENING (MSS):
This involves having bloodwork done between 15-20 weeks of pregnancy to estimate the chance of the baby having Down’s Syndrome, trisomy 18, or an open neural tube defect. It is not a diagnostic test. If the screen is positive then further diagnostic tests will be offered to know for certain if baby is affected or not. This test is often used if a person is too late to have first trimester screening or integrated prenatal screening done.
NON-INVASIVE PRENATAL TESTING (NIPT):
This is a new test to screen for trisomy 21, 18, 13, as well as sex chromosome abnormalities. This test involves having bloodwork done. It will look at pieces of the baby’s genetic material which can be found in the mother’s bloodstream. This test is only funded by OHIP for women older than 40, women who have had one of the above screening tests come back positive, women with a nuchal translucency >3.5mm, women with a previous child affected by these conditions, or with an ultrasound suggestive of a chromosomal problem. Women who do not meet the criteria for OHIP coverage but who wish to pay for this test themselves can request it. Although very accurate, this test can still have false positive and false negative results, and further diagnostic tests will be offered to women with a positive test.
THINGS TO CONSIDER:
These tests are screening tests, not diagnostic tests. The result will come back “positive” if the estimated risk of your baby having one of the screened-for conditions is above a pre-determined level. For example, if the estimated risk of Down’s syndrome by FTS is greater than 1:200 then that screen is called positive. It does not necessarily mean that the baby has Down’s syndrome, just that the testing reveals a higher risk than usual. For women with a positive screen, genetic counseling and further screening or diagnostic tests will be offered. These tests can include chorionic villus sampling (CVS), amniocentesis, non-invasive prenatal testing (NIPT), or detailed ultrasound. These tests are also optional. CVS and amniocentesis carry a small risk of miscarriage.
False positive results are possible. This means that the screening test is positive, but the baby does not actually have the condition. This is why diagnostic tests are offered whenever a screening test is positive.
There are small numbers of false negative tests. These are screening tests which do not indicate a higher than usual risk of any of the conditions, when in fact the baby is affected by one of them. This is sometimes suspected or discovered by ultrasound, but sometimes may only be discovered after delivery.