During the first trimester of pregnancy (between 11 to 14 weeks), nuchal translucency (NT), a kind of ultrasound, assesses the amount of fluid behind the babys neck. The amount of fluid measured can be used to determine the probability that the baby will have a chromosomal or genetic mutation. A modest amount of fluid is typical. A problem is not diagnosed by NT ultrasonography; it is a screening test. It assists the physician in deciding whether or not the infant is at danger and whether more testing is necessary.