The sonographer first confirms your baby's gestational age by measuring him from crown to rump to see whether he's about the size he should be for his age.Then she positions the sensor, called the transducer, over your abdomen so that your baby's nuchal translucency shows up on the monitor and measures the thickness of it on the screen with calipers.(They may know that they're at high risk for a baby with a chromosomal problem or a problem that can't be detected by screening.Or they may just want to know as much as possible about their baby's condition and are willing to live with the small risk of miscarriage to find out.) Some women decide to have no screening or testing at all.A risk of 1 in 4,000 means that for every 4,000 women with this result, one baby will have it and 3,999 will not. You may also be told that your results are "normal" or "abnormal," depending on whether they are below or above a specified cutoff. So a result of 1 in 4,000 would be considered normal because the risk that there's a problem is lower than 1 in 250.A ratio of 1 in 30 would be considered abnormal because that risk is higher than 1 in 250.

Your practitioner should discuss the pros and cons of various approaches with you.You can also wait and combine the results with second trimester screening results for more information about your baby's risk before you decide whether to proceed with an amnio.The NT scan is offered to all pregnant women in the first trimester.Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger than average. The NT scan must be done when you're between 11 and 14 weeks pregnant, because this is when the base of your baby's neck is still transparent.(The last day you can have it is the day you turn 13 weeks and 6 days pregnant.) It's usually offered along with a blood test in first-trimester combined screening options.(Some practitioners also look for the presence of the fetal nasal bone in the first trimester.If the risk is low, the results will offer you some reassurance.If the risk is high, you may want to consider CVS, which will reveal whether your baby has a problem while you're still in your first trimester.If no nasal bone is seen in the developing baby, some experts believe this increases the likelihood of chromosomal abnormalities.)Like other screening tests, an NT scan won't give you a diagnosis.The scan can only assess your baby's risk for certain problems and help you decide whether to have chorionic villus sampling (CVS) or amniocentesis to find out whether your baby is actually affected.If you need further information, consider meeting with a genetic counselor.But ultimately, whether to test and which tests to have is a personal decision.You may get the results right away, or you may have to wait up to 10 days if the doctor has to send the data to a processing center.You'll want to talk with your practitioner or a genetic counselor about how to interpret the results because unless you have a lot of experience with statistical analysis and evaluating the results of medical tests, the results can be confusing.And even if you're a wizard with statistics, you'll need help understanding your options.You'll get your results in the form of a ratio that expresses your baby's chances of having a chromosomal defect.