Nuchal cord bubble CGI image.  Artistic representation of a human fetus within the nuchal cord bubble.  Nuchal cord, also known as nuchal translucency, is a fluid-filled space visible on early pregnancy ultrasounds. It sits at the back of the fetus's neck and usually indicates a normal pregnancy. Amniotic fluid, mostly water and electrolytes, fills the nuchal cord, surrounds the fetus to protect and support growth. The fetus and mother produce and replenish the amniotic fluid, which contains cells, proteins and other substances vital for fetal development.