First Trimester Prenatal Screening
Although most babies are born healthy, the risk of producing a child with a birth defect concerns many parents. Risks for certain types of birth defects, such as open neural tube defects and Down syndrome, can be assessed by screening tests. Approximately 5 percent of those undergoing screening are determined to be at increased risk and further testing can be performed to achieve a definitive diagnosis.
First trimester screening (Combined screening)
First trimester screening is performed between 11-13 weeks of pregnancy, and involves both an ultrasound and a blood test.
Ultrasound measures an area of fluid accumulation at the back of the baby’s neck called nuchal translucency (NT). This accumulation of fluid is a normal finding. Increased NT measurements, however, may indicate increased risk for chromosome abnormalities, congenital heart defects, and other genetic syndromes.
- Blood test
The blood test by a fingerstick measures the levels of two proteins, freeBeta-hCG and PAPP-A, which are normally found in the blood of pregnant women. The levels of these two chemicals are combined with the NT measurement and the mother’s age to provide a risk assessment for Down syndrome and trisomy 18.
The combined testing detects approximately 85 percent of Down syndrome and 97 percent of trisomy 18. Patients determined to be at increased risk for either of these conditions can then be offered diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis. If the screening test does not predict an increased risk, it is recommended that the patient proceed with maternal serum AFP screening in the second trimester to identify increased risks for open neural tube defects and abdominal wall defects.
It is important to remember that a normal screening test does not guarantee a normal baby, nor do abnormal test results definitely mean a baby with a birth defect.
Learn about second trimester screenings