First Trimester Pregnancy Screening
What is first trimester screening?
First trimester screening is a way to check for chromosomal abnormalities prior to the second trimester (15 weeks). The screening involves two steps:
1. An ultrasound is performed by a certified professional to measure an area at the back of the baby's neck called the nuchal translucency.
2. A blood sample is obtained by fingerstick to analyze the amount of two chemicals present: free Beta-hCG and PAPP-A.
To have a first trimester screening, a woman must be pregnant between 11 weeks and 13 weeks, 6 days counting from the first day of the most recent menstrual cycle. Until recently, prenatal screening was not possible prior to the second trimester. First trimester screening provides an opportunity to find out earlier if a baby is at 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 the blood test. Both Down syndrome and trisomy 18 are caused by the presence of an extra chromosome, which results in mental retardation and various birth defects.
On some occasions, it may not be possible to perform first trimester screening. This can happen if the baby is not in a position that allows the nuchal measurement to be obtained, or if the pregnancy dating is over 13 weeks, 6 days. These patients have the option of second trimester screening, also known as the Quad screen, at 15-20 weeks.
How accurate is the test?
Approximately 80 percent of babies with Down syndrome and 90-97 percent of babies with trisomy 18 will have an abnormal first trimester screen result, indicating that further testing should be considered.
What if my screening test shows an increased risk?
Test results showing an increased risk do not mean that your baby has a chromosome problem; only that the risk for Down syndrome, trisomy 18, or other birth defects is increased. In most cases the baby does not have a chromosome problem. Patients may choose the option of 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 screen results and testing options, speak to your health care provider. You may also request a referral to speak with a Dean Genetic Counselor.
What if my screening test does not show an increased risk?
Although this is good news, first trimester screening cannot remove the risk for Down syndrome or trisomy 18, and does not estimate risks for other chromosome abnormalities.
Am I at risk to have a baby with a chromosome abnormality?
All pregnancies are at risk to have an extra chromosome abnormality, like Down syndrome. However, the risk does increase as the age of the mother increases.
|Age of Mother
||Risk for Chromosome Abnormality in 1st trimester
||1 in 486
||1 in 384
||1 in 254
||1 in 183
||1 in 122
||1 in 77
||1 in 36
||1 in 21
||1 in 12
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 are diagnostic procedures that can detect chromosome abnormalities with greater than 99 percent certainty. However, CVS and amniocentesis are associated with a risk for pregnancy complications of 0.5-1 percent.
What is the difference between screening and 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 nor confirm that a chromosome problem is present. Some patients may feel that the screening option is more appropriate for them, since typically these tests are non-invasive and associated with a low risk for pregnancy complications.
Diagnostic tests, such as CVS or amniocentesis, can determine with greater than 99 percent accuracy if a chromosome problem is present in the pregnancy. Patients who are interested in knowing with certainty about the presence of a chromosome problem may prefer this option. However, there is a risk (<1 percent) for pregnancy complications, such as miscarriage, associated with these procedures.First trimester screening cannot replace CVS or amniocentesis because:
1. The screening test alone cannot tell you whether the baby has a problem.
2. Screening estimates a risk for only two chromosome problems.
3. Screening may falsely reassure some patients who still have an increased risk for chromosome 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 a rate of 1-2 per 1,000 babies. Approximately 80 percent of babies with neural tube defects are detected by the AFP blood test.
Your healthcare provider may recommend you have an ultrasound in the second trimester, at approximately 20 weeks. Although many physical abnormalities can be visualized during an ultrasound, the ability to detect problems is highly dependent on the technician, radiologist, position of the baby and the gestational age and size of the mother. Ultrasound can assess the growth and structures of the baby; it can not guarantee normal function.
Should I have a quad screen, too?
No. First trimester screening is equally 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.
Contact your specific insurance company to verify that first trimester screening is covered. Dean Health Plan has agreed to cover the cost; however other insurance plans may have different policies. There are additional charges for the ultrasound, genetic counseling or physician consultation.
What is a genetic counselor?
A genetic counselor provides information and support to families, individuals and pregnant couples. The genetic counselor may review your medical, family or pregnancy history and address your specific risk factors. You can discuss genetic testing options, any test results, and the risks or benefits of testing with a genetic counselor. Dean Clinic has several genetic counselors available to address your questions or concerns. You can reach the Dean Genetics Department at (608) 252-7458.
First trimester screening summary
- The results of first trimester screening are not diagnostic. If the results show an increased risk, additional tests will be offered.
- Test results within the normal range do not guarantee a healthy baby.
- First trimester screening is an alternative to the Quad screen and is generally followed by an AFP blood test at 15-18 weeks to screen for neural tube defects. A Quad screen following first trimester screening is not indicated.
- This test is voluntary. You may decline testing at any point.