A true knot is seen next to the cord clamp.
Knots develop early in gestation due to increased fetal movement, which creates the knot. True knots are not thought to form late in gestation when the fetus is larger and its movement more restricted. The frequency of true knots range from 0.4% to 0.5% of pregnancies. True knots can be loose or tight and obviously, it is the tight knots that have grave implications. Loose knots can maintain fetal circulation, however, tight knots leads to compression of umbilical vessels and potentially fetal demise. The knot may not tighten until the onset of labor with fetal descent into the birth canal.1
A true knot must be distinguished from false knot. A false knot is formed when one of the umbilical vessels (usually the vein) is longer than the other vessel and thus forms a loop.
Commonly associated with long cords, multigravidas, and male fetuses. Monoamniotic twins are associated with complex tangles of cord which may involve multiple true knots1 .
Typically knots are findings noted on delivery as antepartum diagnosis is difficult. In pregnancies with high rates of true knots (MoMo twins), continuous monitoring may be instituted in order to detect any "tightening" events as they occur.
Overall fetal mortality from true knots is 10%. Other sequelae include hypoxia with neurologic damage to the infant.1
1 Baergen RN. Manual of Benirschke and Kaufmann's Plathology of the Human Placenta. New York, NY: Springer; 2005: 260-2.