Prenatal Testing

(November 2008) - I’m a mother and a certified genetic counselor at Memorial Health University Physicians – Savannah Perinatology Associates. I remember the excitement I felt when I became pregnant and had my first prenatal visit. What I wasn’t excited about was the seven tubes of blood the nurse took from me that day. I was so relieved to survive the blood draw that I didn’t even think to ask what all that blood was being tested for.

In most cases, patients are routinely tested for blood type, anemia, and immunity to certain infections, such as rubella. This is often referred to as the “prenatal profile.” These tests typically are not optional because the results are essential in pregnancy management. However, there is an ever-expanding list of optional tests available. Based on a woman’s ethnic background, screening for hereditary diseases such as cystic fibrosis, sickle cell anemia, and thalassemias (genetic blood diseases) may be offered. It is estimated that 1 in every 30 Caucasians is a carrier of cystic fibrosis and 1 in every 10 African Americans is a carrier of sickle cell anemia. Even though you may not know of anyone in your family with these conditions, your children could be at increased risk just based on your race.

Screening is also offered in pregnancy for conditions that are typically not hereditary. In 2007, the American College of Obstetrics and Gynecology (ACOG) stated that all women should be offered prenatal screening or testing for Down syndrome, trisomy 18, and spina bifida. In most cases, these conditions don’t “run in the family” and are usually just a random occurrence.

All pregnant women have some degree of risk for these conditions. Also, with advancing maternal age, the risk for Down syndrome and trisomy 18 increases.
Although obstetricians have a responsibility to offer screening to their patients, it is ultimately each woman’s choice. Some women choose testing because they would rather have a few months during the pregnancy to learn more about the condition and prepare for a child with special needs. This information may also be helpful to the physician for pregnancy management or delivery recommendations. Some couples may not continue the pregnancy if a severe problem is found. In the case of trisomy 18, most babies pass away before their first birthday. On the other hand, if testing or screening is normal, it may provide reassurance and reduce anxiety during the pregnancy. Here are summaries of a few common testing techniques.

Sequential First and Second Trimester Screening
One of the newest and most accurate screens available is the sequential screen. At 11 to 14 weeks gestation, patients have part one of the screening, which involves a brief ultrasound and blood test. Results are available in a few days and patients are given a preliminary risk for Down syndrome and trisomy 18. If a woman is considered high risk, amniocentesis is offered. If a woman is considered low risk, part two of the sequential screen is performed at 16 to 18 weeks gestation, and involves another blood test. By combining the results of both parts, the sequential screen can detect approximately 90 percent of babies with Down syndrome and trisomy 18 and approximately 80 percent of babies with spina bifida. Compared with older tests such as the AFP or triple screen, the sequential screen is more accurate and creates fewer false alarms.

The Difference Between a Screening Test and a Diagnostic Test
A screening test gives you a risk estimate, basically telling you whether your risks are higher or lower than before screening. For example, the sequential screening may give you a risk of 1 percent for having a child with Down syndrome. Although this is considered an abnormal result, there is a 99 percent chance the baby does not have Down syndrome. A screening test does not detect all cases of a particular condition and can produce false alarms (referred to as a false positive result). The purpose of screening is to narrow down which pregnancies are at greater risk. If your risks are determined to be high, then a diagnostic test is offered, which is generally able to determine whether a problem truly exists. For example, amniocentesis is greater than 99.9 percent accurate in detecting chromosome conditions such as Down syndrome.

Although amniocentesis is extremely accurate and can be used to test for a variety of conditions, there is a small risk for complications. Studies published in 2006 indicate that when done in the second trimester of pregnancy (after 15 weeks gestation), amniocentesis has a risk for miscarriage estimated at 1 in 1,600 (0.06 percent). This means that 99.94 percent of women who have the procedure will not miscarry. Amniocentesis was traditionally offered to all women over the age of 35. However, due to the lower risk for miscarriage now quoted with amniocentesis, the procedure is available to all women, regardless of age.

Many women don’t think of ultrasound as a “test” but rather a fun way to see their baby and determine the baby’s sex. However, ultrasound is a very important screening test to make sure the baby is growing and developing appropriately. A detailed ultrasound at 18 to 20 weeks gestation can detect most major birth defects and screens for physical traits or characteristics of Down syndrome, spina bifida, and many other conditions.

There are many factors to consider when undergoing testing during pregnancy. As technology improves, we will no doubt be able to detect more and more conditions before birth. It is important for each woman to understand her options and make the best decision for herself, her family, and her pregnancy. For some women, this means declining screening. For others, it may mean taking every available screening. Women who have questions should talk with their physician or a genetic counselor.

Written by Libby H. Malphrus, M.S., a certified genetic counselor at Memorial Health University Physicians -- Savannah Perinatology Associates. To learn more, call Savannah Perinatology Associates at 912-350-5970. This article first appeared in Mother Knows Best magazine's November/December 2008 issue.