The first trimester screen is used to determine a woman’s risk of carrying a baby with Down syndrome or trisomy 18.  First trimester screening is performed by drawing a sample of maternal blood in addition to an ultrasound. The first trimester screen can only be performed between 11 weeks 0 days and 13 weeks 6 days from the first day of a woman’s last menstrual period (LMP).  The ultrasound portion is to measure the fluid that collects behind a baby’s neck.  This is known as a nuchal translucency measurement.  Babies with excess fluid, or an increased nuchal translucency measurement, have an increased chance to have a chromosome abnormality, such as Down syndrome or trisomy 18.  The second part of the first trimester screen involves taking a sample of the mother’s blood to measure two proteins, known as free b-hCG and PAPP-A.

The first trimester screen is a screening tool; it is not a diagnostic test.  Screening tools can only provide statistical information and not a yes or no answer.  The screening may tell you if your baby has an increased or decreased chance of having Down syndrome or trisomy 18, but it cannot tell you definitely if there is a problem or not.  The screen appears to detect about 90% of babies with Down syndrome and Trisomy 18.  It is important to remember that a positive or abnormal screen result does NOT mean that the baby definitely has a problem.  Many women with a positive or abnormal screen result have healthy babies.