18 Weeks Pregnant

By now you're feeling your baby-to-be wiggle inside you. The excitement surrounding her first movements will probably help you forget some of your pregnancy woes. As your tummy continues to expand you may notice stretch marks and have itchy skin. Your back will ache as your body adjusts to its new alignment. Thankfully, nausea is mostly likely a distant memory. (Need some ideas on things to do this week? Visit our sister site at Babble.com.)

Understanding Pregnancy Tests

Between your 16th and 20th weeks of pregnancy you'll be offered a series of tests to determine if your baby-to-be is at risk for certain genetic problems. Notice that these tests are offered but not required. Before you submit to the tests, it's important to understand what each entails and what the results will tell you about your developing baby.

Risk Assessments, Not Tests
Rather than view genetic tests as pass/fail, physicians see them as risk assessments, looking at whether your baby-to-be has a greater risk for certain genetic problems. For instance, a "positive" result doesn't necessarily indicate a problem with your baby, but that you might consider additional—and more definitive testing—to determine your baby's health. "The way I describe it to my patients is that these tests are like casting a wide net," explains Dr. Joanne Motino Bailey, a certified nurse midwife and a professor of women's studies at the University of Michigan. "You have plenty of healthy babies that are caught in the net" along with a handful of babies with genetic concerns.

Non-Invasive Tests
The main reason for so many false positives, according to Dr. Motino Bailey, is that physicians want to offer the safest, most non-invasive way to test your unborn baby for certain genetic disorders. These tests involve a simple blood test with virtually no risk to your baby-to-be.

• Quad test/triple screen: Your healthcare provider will draw your blood and test it for elevated levels of certain hormones and proteins, which may indicate your unborn baby has a genetic problem. The quad tests for Down syndrome, spina bifida, and other genetic problems. Keep in mind that this test has a high false positive and a high false negative rate.

• Depending on your healthcare provider, you may be offered the triple screen, which includes looking at two hormone levels and one protein, or the quad test (quickly becoming the norm), which tests for an additional protein that may indicate genetic problems.

• Nuchal translucency screening (NTS): A more recent test option combines a blood test with an ultrasound to look for risk factors that may indicate that your unborn baby has Down syndrome. The test is highly specialized, however, so it may not be available in your area.

• Cystic fibrosis: Your blood sample will also be tested for cystic fibrosis. The test will determine if you are a CF carrier. If you are, your partner may then be tested. Because this is a recessive disorder, both you and your partner must be carriers for your unborn baby to be at risk. If both parents are carriers, according to ACOG, your unborn baby has a one in four chance of developing the disease.

• Ethnicity-based blood tests: Certain genetic disorders tend to appear in certain ethnicities. Your physician will ask a series of questions about your background to determine whether these tests are necessary for you.

Invasive Tests
If your first round of screening comes back with positive results (again, not meaning that your baby has a problem, but that she might be at a risk) you will be offered additional tests to give you a definitive answer about your unborn baby's genetic makeup. While these tests offer the yes/no answers you may be looking for, they do carry a risk of miscarriage.

• Amniocentesis: With an amnio, your healthcare provider inserts a long needle through your abdomen and into the uterus to collect a small sample of amniotic fluid. The fluid is then tested for genetic problems, such as Down syndrome, neural tube defects, and other disorders. The test is done between pregnancy weeks 16 and 20.

• Chorionic villus sampling (CVS): In CVS, a needle placed through your abdomen or through your vagina withdraws a tiny portion of the placenta. The sample can then be tested for Down syndrome, sickle-cell anemia, and cystic fibrosis. The test is performed between pregnancy weeks 10 and 12.

Note: CVS, a first-trimester test, provides parents with an earlier diagnosis than amniocentesis, a second-trimester test. Until recently, CVS had been associated with a slightly higher rate of miscarriage, but findings published in the September 2006 issue of Obstetrics & Gynecology show that the miscarriage rate is actually the same for both CVS and amnio, below two percent.

These tests are not to be taken lightly, advises Dr. Bailey. Take time to ask your healthcare provider about each test.