The umbilical cord acts as the fetus’s lifeline, connecting the developing baby and the placenta. It is composed of two arteries and one vein, encased in a protective, gelatinous substance called Wharton’s jelly. The cord transports oxygen and nutrient-rich blood to the fetus while carrying waste products away. Although the cord is designed for resilience and flexibility, physical variations can occur, including the formation of a true knot.
Defining the True Knot
A true knot is an entanglement of the umbilical cord vessels, forming a complete loop that can potentially tighten. This is a relatively rare occurrence, affecting an estimated 0.3% to 1.2% of all pregnancies. The presence of a true knot is distinct from other cord variations that are sometimes confused with it.
A true knot must be differentiated from a “false knot” or pseudoknot, which is a localized bulge or swelling. False knots are formed by excessive Wharton’s jelly or looping blood vessels, and they carry no clinical significance or risk. A true knot is also not the same as a nuchal cord, which is the term for the umbilical cord wrapped around the baby’s neck.
Mechanisms of Formation
A true knot typically forms early in gestation, generally between 9 and 12 weeks. During this time, the fetus is small relative to the volume of amniotic fluid, allowing for excessive movement. A knot is formed when the fetus moves or swims through a loop of its own cord.
Several physical characteristics and pregnancy conditions increase the risk of a knot forming. An unusually long umbilical cord is a primary predisposing factor, as is polyhydramnios (an excess of amniotic fluid). Monoamniotic twin pregnancies also carry a significant risk because the twins share the same amniotic sac, increasing the potential for entanglement. Factors like multiparity and having a male fetus are sometimes associated with knot formation.
Clinical Implications and Fetal Risk
While many true knots remain loose and harmless, a tightened knot poses a risk by compressing the cord’s blood vessels. This compression restricts the flow of oxygenated blood and nutrients. Wharton’s jelly acts as a protective buffer, usually preventing a loose knot from tightening completely, but this protection can fail.
If the knot tightens, especially during active fetal movement or labor contractions, it can lead to acute or chronic deprivation of oxygen, known as hypoxia. Chronic, mild hypoxia can result in intrauterine growth restriction (IUGR), where the fetus does not grow as expected due to insufficient nutrient delivery. More severe tightening can cause significant fetal distress, often indicated by a non-reassuring heart rate pattern observed during monitoring.
True knots are associated with a four-to-tenfold increased risk of stillbirth compared to normal pregnancies, though the majority of babies with a knot are born healthy. Severe oxygen restriction can lead to neurological complications such as hypoxic-ischemic encephalopathy (HIE) or fetal demise. The risk is pronounced during the third trimester and labor, when uterine contractions and fetal descent can further constrict the loop.
Detection and Management
Prenatal detection of a true knot is challenging, as the umbilical cord is long and mobile, making it difficult to visualize the entire structure via standard ultrasound. The diagnosis is often made incidentally at delivery when the knot is visually observed. Advanced imaging techniques, such as color Doppler ultrasound, can provide better visualization of blood flow patterns and help differentiate a true knot from a false one.
When risk factors are present or a true knot is suspected, management focuses on increased fetal surveillance. This monitoring may involve frequent biophysical profiles (BPPs) and non-stress tests (NSTs) to assess fetal well-being and heart rate variability. Doppler studies are useful for evaluating blood flow resistance within the umbilical artery, which can indicate potential compression. If signs of fetal distress or compromised blood flow are confirmed, a healthcare provider may consider a planned or early delivery, often via cesarean section.