Frequently Asked Questions
When is the first trimester ultrasound evaluation performed?
For this evaluation, an ultrasound is performed between 11 and 13 weeks. Evaluation of the developing fetal nuchal translucency thickness (skin thickness at the back of the fetal neck), nasal bone and blood flow measurements can be used to calculate a risk for common chromosome abnormalities. Information regarding the results of the ultrasound are calculated, and patients are counseled regarding the results at the time of the appointment.
What are my options of screening for common chromosome abnormalities?
First trimester ultrasound
First trimester ultrasound screening is designed to evaluate for several chromosome problems, including Down syndrome. The ultra-sonographer will look at several different structures on your baby, including nuchal translucency (measurement of the amount of fluid underneath the skin at the back of the baby's neck), presence of the bone in the baby's nose and blood flow through a vessel called the ductus arteriosis. They also look at blood flow through the tricuspid valve in the baby's heart. Abnormalities in these specific areas can be associated with chromosome problems like Down syndrome.
First trimester biochemical
This is additional blood testing that looks at several pregnancy hormones in your blood. This blood testing may detect additional cases of Down syndrome, trisomy 13 or trisomy 18. This is an optional screen that can be performed in the first trimester.
Second trimester maternal serum
Second trimester maternal serum screening is designed to evaluate the risk for several chromosome problems, such as Down syndrome. It can also assess the risk for neural tube defects, such as spina bifida. This blood test can be performed between 15 and 22 weeks of pregnancy. This test in not recommended if a patient has already had screening in the first trimester of pregnancy.
Additional screening evaluating cell-free DNA from the placenta in a mother's blood is available in certain circumstances.
Is the maternal serum alpha fetoprotein screening required?
In the second trimester, between 16 and 20 weeks of pregnancy, a blood test to measure the amount of alpha fetoprotein in the mother’s blood may be offered. This test screens for neural tube defects, such as spina bifida. Women who have normal first trimester screening may opt for this test only. This screening is included in the second trimester serum screening.
What does having an amniocentesis involve?
If family history or screening tests indicate an increased risk for a disorder, amniocentesis may be used to provide diagnosis for the condition. This involves the removal of a small amount of amniotic fluid from the amniotic sac under the guidance of ultrasound. There is a small risk to the pregnancy, but it is often much less than the risk of the condition of concern. Prior to such testing, genetic counseling is offered to understand the risks, benefits and alternatives to invasive testing.
What about testing for women over 35?
In the past, all women over 35 were offered invasive prenatal testing such as amniocentesis or chorionic villus sampling, also known as CVS, to diagnose chromosome abnormalities like Down syndrome. While it is true that the risk for chromosome abnormalities increases with maternal age, most babies with chromosome abnormalities are born to women under 35. This is because women under 35 are having many more babies, and chromosome abnormalities can happen at any age. Therefore, maternal age is a poor predictor for fetal chromosome abnormality.
Recent advances in prenatal testing can evaluate the risk for common chromosome abnormalities without any risk to the pregnancy. Invasive prenatal testing should be based on the results of these screening tests, parental concern and other personal factors rather than just age alone.