If you're trying to decide whether to risk diagnostic testing, it all depends on what makes you more nervous—being unsure about baby's condition, or (very slightly) increasing miscarriage risk.
For some parents, the test results will determine the next action—if the results show a serious birth defect, you may decide to terminate the pregnancy. Even if you plan on having the baby no matter what, though, there are reasons to test (though some parents feel that if they’re not considering termination, there’s no reason to increase the risk of miscarriage just for the sake of knowing). If the test comes up positive, you’ll have time to learn about your child’s specific disorder, prepare emotionally, and start arranging any special care and resources that baby may need after birth. Your doctor will be prepared to deliver a baby with a special condition, and can make sure any necessary resources, including a neonatal intensive care unit, are available. The disorder may also affect the safest way to deliver baby (vaginal birth vs. c-section). And, if the test results show up negative, you’ll be able to take a major worry off your mind. (Though no test will detect every problem, it can definitely rule out enough for a big sigh of relief.)
Expert source: American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.