Your doctor will most likely speak to you about prenatal diagnostic testing during your first prenatal visit. The goal of prenatal testing is to provide expectant parents with information about their pregnancy. Testing helps to evaluate some of the inherited and environmental risks that can occur. Since 2007, the American College of Obstetricians and Gynecologists has recommended that all pregnant women be offered prenatal testing. It is no longer just for women above a certain age or of a particular ethnic group.
A number of hormone tests that make up various screening tools are described below.
Currently, screening tools are most useful in identifying pregnancies with Down syndrome (trisomy 21, which is when an individual has three copies of chromosome 21, rather than two), the most common chromosomal abnormality found in live births. However, the range of tests available can also shed light on other abnormalities that may be affecting the pregnancy. Whereas screening tools help to identify pregnancies with a higher likelihood of abnormality, diagnostic tests more definitively confirm the suspected diagnosis.
Early Risk Assessment
Screening tests available today involve blood testing and ultrasound. In the first trimester, a combined test including the mother’s blood sample and ultrasound measurement of the fetal nuchal fold — which measures the amount of fluid behind the neck — can estimate the risk of a pregnancy affected by Down syndrome, as well as other trisomies such as 18 and 13. This test, often called the early risk assessment, can be performed between 10 and 14 weeks. This combined test picks up 85 percent of pregnancies affected by Down syndrome. The result is reported as low risk (or negative) when the risk of Down syndrome is less than 1 in 250. This does not mean that the pregnancy is definitely not affected by Down syndrome, only that the risk is low. In five percent of cases, a high risk (or positive) result is found to be false on further testing.
To reduce the possibility of such a false positive result, this first-trimester assessment can be integrated with a second-trimester blood test that measures four hormones produced by the pregnancy between 15 and 18 weeks. This is called the integrated screen. These four hormone levels can identify pregnancies with chromosomal abnormalities or neural tube defects.
When integrated with the first-trimester result, this test picks up 85 percent of all pregnancies affected by Down syndrome, and only 1 percent of positive results are false. One drawback to this integrated test is that results are only available in the second trimester. One option is to report these results in each trimester step by step. This is called the sequential screen and approaches the detection ability of the integrated screen. While both these strategies are recommended, one thing to avoid is doing these tests independently, as that can raise the false positive result up to 17 percent.
For women who are too late, or who elect not to undergo screening in the first trimester, a quad screen can be performed between 15 and 22 weeks. On its own, this blood test picks up 81 percent of pregnancies affected by Down syndrome with a false positive rate of 5 percent. Additionally, whether done as part of the quad screen or independently, measurement of alpha-fetoprotein (AFP) in the second trimester picks up 75 to 90 percent of pregnancies with a neural tube defect.
For women whose screening tests show an increased risk, diagnostic testing is usually recommended with chorionic villus sampling (after ten weeks) or amniocentesis (after 15 weeks). Some women choose to go directly to these more definitive tests, and bypass the screening tests. These tests provide definitive genetic information about the pregnancy through sampling of the placenta or the amniotic fluid. Both of these tests carry a small chance of miscarriage.
Finally, as ultrasound technology continues to enhance our vision in utero, the fetal anatomic survey (after 15 weeks) provides detailed information that can influence the predictive value of the screening tests and identify nongenetic abnormalities. For most parents-to-be, visualizing the fetus enables a feeling of intimacy with the pregnancy.
Prenatal screenings are designed to rule out concerns. The odds of having a healthy baby are in your favor, and, in most cases, screening will give you peace of mind.
Astrid Christoffersen-Deb, MDCM, MPhil, is a Clinical Instructor in Obstetrics & Gynecology at Harvard University. She is also completing her doctorate in anthropology from Oxford University.