First Trimester Pregnancy Screening
First trimester screening is an effective way to check for any chromosomal abnormalities, prior to the second trimester (approx. 15 weeks). The screening involve two steps:
- You will receive an ultrasound by a certified professional to measure an area at the back of your baby’s neck, called the nuchal translucency.
- We will obtain a blood sample through a finger stick to analyze the amount of two chemicals present: free Beta-hCG and PAPP-A.
You must be pregnant between 11 and 13 weeks, six days counting from the first day of the most recent menstrual cycle, to receive a first trimester screening. Until recently, prenatal screening was not possible prior to the second trimester. This screening provides you an opportunity to find out early on if your baby is at an increased risk of having a chromosome problem.
Specific risks for Down syndrome and trisomy 18 are calculated by combining the ultrasound measurement and the results of your blood test. Both conditions are caused by the presence of an extra chromosome, which results in mental retardation and various birth defects.
There are some instances where first trimester screenings are not possible. This can happen if your baby is not in a position that allows the nuchal measurement to be obtained, or if the pregnancy dating is over 13 weeks, six days. If you fall into this category, you do have the option of a second trimester screening, also known as the Quad screen.
How Accurate Is a First Trimester Screening?
Approximately 80% of all babies with Down syndrome and 90-97% of babies with Trisomy 18 will have an abnormal first trimester screening result. These results are an indicator of further testing.
What Is My Screening Test Shows an Increased Risk?
If your test results show an increased risk it does not mean that your baby has a chromosome problem. It only indicates that the risk for Down syndrome, Trisomy 18, or other birth defects is increased. In most cases of an abnormality though, your baby will not have a chromosome abnormality. You may choose to have additional testing, like amniocentesis or chorionic villus sampling (CVS), which can accurately determine if a chromosome abnormality is present.
If you have additional questions about your results and testing options, speak to your health care provider. You may also request a referral to speak with a genetic counselor.
What If My Screening Test Does Not Show an Increased Risk?
Although this is good news, first trimester screenings cannot remove the risk for Down syndrome or Trisomy 18. It does not estimate risks for other chromosome abnormalities.
Am I at Risk for Having a Baby With a Chromosome Abnormality?
All pregnancies are at risk of having an extra chromosome abnormality, like Down syndrome. However, the risk does increase as your age increases.
Should I Have the Screen If I Am Already Older Than 35?
First trimester screening is an option for all pregnant women, regardless of age. Women who are 35 or older at delivery, or have a positive family history of chromosome abnormalities, may consider CVS or amniocentesis. These diagnostic procedures can detect chromosome abnormalities with greater than 99% certainty. However, CVS and amniocentesis are associated with a risk for pregnancy complications of 0.5-1%.
What Is the Difference Between Screening & Diagnostic Tests?
Screening tests, like first trimester screening, the Quad screen or ultrasound, can adjust the risk for a chromosome problem in the pregnancy. By themselves, they cannot rule out or confirm the presence of a chromosome problem. You may feel that the screening option is a better option because these are typically non-invasive and associated with a low risk for pregnancy complications.
Diagnostic tests, such as CVS or amniocentesis, can determine with greater than 99% accuracy if a chromosome problem is present in the pregnancy. If you are interested in knowing with certainty about the presence of a chromosome problem, you may prefer this option. However, there is a risk (less than 1%) for pregnancy complications, such as miscarriage, associated with these procedures. First trimester screenings cannot replace CVS or amniocentesis because:
- The screening test alone cannot tell you whether your baby has a problem.
- The screening estimates a risk for only two chromosome problems.
- Screenings may false reassure some patients who still have an increased risk for abnormalities due to age.
Are There Other Prenatal Tests I Should Consider After First Trimester Screening?
You will be offered an alpha-fetoprotein (AFP) blood test between 15-20 weeks to screen for open neural tube defects, like spina bifida. Neural tube defects occur at rate of one to two per 1,000 babies. Approximately 80% of babies diagnosed with neural tube defects are detected by the AFP blood test.
Your health care provider may recommend an ultrasound in your second trimester, at approximately 20 weeks. Although many physical abnormalities can be visualized during an ultrasound, the ability to detect problems is highly dependent the technician, radiologist, position of the baby and your gestational age and size. Ultrasounds can assess the growth and structures of your baby, but it cannot guarantee normal function.
Should I Have a Quad Screen, Too?
Quad screens are not always necessary. First trimester screening is equally as accurate and may be less likely to give a false risk result. An additional screening for chromosome abnormalities is unlikely to improve detection, since most affected babies will have unusual results based on first trimester screening.
The information above is purely information. For a more detailed explanation and scheduling call your OB/GYN and set up an appointment. They’ll walk you through the screen in more detail and discuss your next steps and any concerns you may have.