First trimester screening involves blood a blood test and an ultrasound. The ultrasound measures the thickness of the baby’s neck called nuchal translucency. First trimester screening looks for an increased risk of down’s syndrome, trisomy 18, other genetic abnormalities, as well as some cardiac or other structural abnormalities. Unlike second trimester screening it does not test for neural tube defects. First trimester screening is done between 11-13 weeks.First trimester screening has a high false positive and false negative rate.
First trimester screening tests will not be able to tell for certain that the baby has a problem, abnormal test results suggest an increase risk of the problem tested for. The cut-off for “abnormal” results depends on the individual lab but is generally between 1/250 – 1/300. A risk of 1/300 means that if there are 300 abnormal test results, only one of the 300 children tested will actually have the problem. So in most cases the overwhelming number of positive tests are false positives. There can be false negatives as well. The detection rate for first trimester screening is generally about 85%. This means that 15% of babies with the disease being tested for will test as normal. So a negative first trimester screen is not a guarantee that the baby will not be effected by the problems tested for. Because firs trimester screening does not test for neural tube defects and has a relatively high false negative rate, second trimester screening is also done at 16 weeks. None of the abnormalities tested for are treatable. If the baby has one of these abnormalities the question then becomes whether a pregnancy termination is done or the baby is carried to term.
If a first trimester screen is abnormal, invasive testing is the next step in determining if the baby does indeed have the problem. This can be done in the form of a CVS (chorionic villous sampling) or an amniocentesis. CVS can be done right away when the test results come back, amniocentesis is not done until 16 weeks, about a month after the first trimester screen comes back. The advantage to CVS is that it can be done much earlier that amniocentesis. The disadvantage to CVS is a higher risk of causing a miscarriage, higher false positive and false negative results, the possibility that after the procedure is done the results cannot be ascertained, and it does not test for neural tube results. CVS is not available in all areas as fewer specialist have the skill required to do CVS safely than amniocentesis. The disadvantage of amniocentesis is there will be about a month wait from the time the abnormal test results are obtained to the time the amniocentesis results are obtained.
First trimester screening is not for everyone. First trimester screening may be valuable for patients that want to aggressively pursue potential genetic abnormalities as early as possible. For women who live in an area in which CVS is available there is the potential to find a genetic abnormality at the end of the first trimester or the early second trimester. For women that choose not to do CVS or CVS is not available, first trimester screening can cause a lot of anxiety while waiting a month to see if the baby is effected or not. My recommendation is to consider what you would do if the results come back abnormal, is CVS available, would you terminate the pregnancy if the baby is abnormal, would you do invasive testing that carries some small risk of miscarriage? These hypothetical questions are important to consider before having the testing done. If the results of the test are not going to change what you will do, the test may not be worthwhile. If you would act on the results of the test they me be more valuable.