The American College of Obstetricians and Gynecologists (ACOG) has recommended that all pregnant women should be offered first trimester screening. First trimester screening uses information from a blood test (performed between 9 and 13 weeks of pregnancy) and a specialized ultrasound (performed at 11 and 13 weeks of pregnancy) to find pregnancies that have an increased risk for Down syndrome as well as two more rare chromosome conditions called Trisomy 18 and Trisomy 13.
The specialized ultrasound is typically external (over the abdomen) and takes approximately 30 minutes. During the ultrasound, the sonographer (ultrasound technologist) will measure the amount of fluid that is on the back of a baby’s neck (the nuchal translucency). Pregnancies affected with chromosome conditions including Down syndrome, often have an increased nuchal translucency thickness.
The blood sample (done by a finger stick) is sent to a lab called NTD labs to measure the amounts of two chemicals in a pregnant woman’s blood called free beta-hCG and PAPP-A. When a pregnancy is affected with Down syndrome or Trisomy 18/Trisomy 13, these chemicals tend to follow certain patterns.
The levels from the blood test and the measurement of the nuchal translucency are combined with the women’s age to estimate the risks for Down syndrome and Trisomy 18 and Trisomy 13 in the pregnancy. It is estimated that 85-90% of pregnancies affected with Down syndrome and 90% of pregnancies affected with trisomy 18 and 13 will be detected with first trimester screening.
There are two options in regards to first trimester screening, traditional first trimester screening and instant risk assessment.
With a traditional first trimester screening test, the patient has her ultrasound and finger stick done at our office. The blood sample and ultrasound measurements are then sent to NTD labs to be analyzed. The results of a traditional first trimester screening tests are typically available in 5-7 days.
- Instant Risk Assessment
This is a way to provide first trimester screening results on the same day as the patient’s ultrasound. The patient has her blood work done prior to her ultrasound appointment. At her ultrasound visit, her blood results are combined with the ultrasound measurements to obtain the results of the first trimester screen and the patient receives her results that same day. To do an instant risk assessment, you will need to get a kit from your obstetrician’s office.
As with all screening tests, the first trimester screening test can have both false positive and false negative results. A negative test provides reassurance that the risk for these conditions is low, but it can not entirely exclude the possibility that the pregnancy is affected with one of these conditions. A positive first trimester screen result does not mean that the baby is affected, but rather indicates that the risk for one of these conditions is increased and that further testing should be considered.