An obstetrician's most important duty is to help couples deliver the healthiest baby possible. Of course, not all babies are created perfectly. Imperfection is a human characteristic, so both parents and physicians should appreciate the fact that reproducing does not come with any guarantees.

In our attempt to identify babies with certain genetic conditions, obstetricians rely on a patient's medical history, her age, her family history and screening tests. For women under age 35 at the time the baby will be born, there are a series of blood tests to help focus our attention on those with the greatest risk of problems. The most commonly employed screening test is known as the quad screen.

The quad screen consists of four blood tests drawn when a woman is between 16 and 20 weeks pregnant—the earlier in this period, the better. Combining the four tests allows doctors to identify nearly 75 percent of Down Syndrome babies as well as newborns with a lethal chromosomal disorder called Trisomy 18. Remember, these tests, which have a 5 percent false positive reading, do not diagnose problems—they screen for them. Knowing the results helps us determine who should undergo further testing, in the form of an amniocentesis. Considered the gold standard of tests, the amniocentesis can predict the aforementioned problems with near perfect accuracy. Even so, there are many DNA conditions that amniocentesis can't detect. If you're a candidate for this test, you should discuss its advantages and disadvantages with your doctor.

As for the quad screen, one of its components, the alpha fetoprotein, has a second screening purpose: it helps detect nervous system and intestinal disorders in the baby. The test picks up nearly 85 percent of babies affected with spinal cord disorders (spina bifida). If an elevated level of AFP (alpha fetoprotein) is detected, the test is repeated before suggesting an amniocentesis. There are instances of false positive results with this test. Certain circumstances can affect the results, including miscalculation of the due date or twin pregnancies. Other fetal conditions and fetal demise can also trigger an abnormal result.

Over the past 20 years we have come a long way in blood screening for fetal disorders and defects. The AFP test alone predicted just 60 percent of Down Syndrome babies. The addition of two more screening tests improved our detection to 75 percent. Adding the fourth marker brings our detection rate to 80 percent.

Although most labs now offer the quad screen, you should know that it's not universally available. If it's available in your area, I would suggest it as your screening test of choice in most circumstances (an amniocentesis is the appropriate choice in certain situations). Check with your practitioner about the best screening test for you.

The future holds even more promises in the field of prenatal screening. The Nuchal screening, a first trimester test, is presently being evaluated across the country. With nearly 90 percent accuracy this test provides an individual specific risk for having a child with Down syndrome, and also detects other problems. Down the road we will be able to check the mother's blood for fetal cells, which will be used to determine the baby's genetic blueprint and help us avoid amniocentesis and blood screening tests.

Please talk with your doctor about the benefits of prenatal testing for you.