First trimester screening uses a blood test and a specialized ultrasound to identify pregnancies at an increased risk for Down syndrome and trisomy 13/18.  This screening is performed between 11 weeks 1 day and 13 weeks 6 days of pregnancy.  The blood test can be performed as early as 9 weeks 0 days of pregnancy, however timing for the ultrasound is when fetus has a crown rump length of 45-84mm. The blood test measures 2 different proteins produced by the pregnancy (PAPP-A and free beta hCG). The specialized ultrasound measures the thickness of the fluid at the back of the fetal neck (the nuchal translucency).  Fetuses with Down syndrome, trisomy 18, trisomy 13 and some congenital heart defects often have an increased nuchal translucency thickness and/or abnormal protein levels.  The protein levels from the blood test and the measurement of the nuchal translucency are then combined with the mother’s age-related risk using a statistical analysis to estimate the risk for Down syndrome and trisomy 13/18 in the pregnancy.    

We now offer first trimester screening with instant risk assessment (IRA).  You may come to our office for a fingerstick as early as 9 weeks one day of pregnancy if you have had an early ultrasound placing you at least at 9 weeks 1 day.   If you have any genetic issue or will be over 32 years old at delivery, you will be scheduled for genetic counseling at this time.  Your ultrasound appointment for nuchal translucency measurement (fluid behind the baby’s neck) will be scheduled between 12 weeks and 13 weeks 6 days of pregnancy. 

The IRA program allows us to provide you with the earliest possible individualized risk assessment for the three most common aneuploidies (trisomies 21, 13, 18), in-person on the day of your ultrasound.  In the case of a high risk result, genetic counseling and CVS will be offered to you that day.  

First trimester screening detects over 90% of fetuses with Down syndrome and about 97% of fetuses with trisomy 13/18, two other chromosome abnormalities.  The false positive rate associated with this test is approximately 5%.  Unlike the quad screen, first trimester screening will also help detect some other birth defects -- an increased nuchal thickness with normal chromosomes may be associated with other fetal disorders such as congenital heart defects.  For this reason, a fetal echocardiogram (specialized sonogram of the fetal heart) may be recommended in cases of increased nuchal translucency with normal chromosomes.   First trimester screening cannot detect spina bifida or other neural tube defects.  Therefore, anyone having first trimester screening should follow up with maternal serum AFP screening between 15-18 weeks of pregnancy. 

As with the quad screen, both false positive and false negative results may occur.  A “screen negative” test provides reassurance that the risk for Down syndrome is low (below 1 in 500), but cannot entirely exclude the presence of Down syndrome in the pregnancy.  Women over the age of 32 or those with a family history indicating increased risk should still consider prenatal diagnostic testing through CVS or amniocentesis.  Some women may choose to use first trimester screening to decide between these diagnostic options, or as an additional method of assessing their risk if they decline invasive testing.   A positive first trimester screen result does not mean that a fetus is affected with Down syndrome or trisomy 13/18, but rather indicates that the risk for these conditions is increased over the general population (greater than 1 in 500 risk), and that either CVS or amniocentesis should be considered. 

Genetic counseling is available for anyone considering first trimester screening.