Ultraschall in Med
DOI: 10.1055/a-0770-2850
Letter to the Editor
© Georg Thieme Verlag KG Stuttgart · New York

True Cord Knot Means True Fetal Risk – Comment on the Title Image of Ultraschall in Med 2018; 39(02):127–128

Cathrine Ebbing
1  Department of Obstetrics and Gynecology, Haukeland Universitetssjukehus, Bergen, Norway
Jörg Kessler
1  Department of Obstetrics and Gynecology, Haukeland Universitetssjukehus, Bergen, Norway
Svein Rasmussen
2  Department of Clinical Science, University of Bergen, Norway
› Author Affiliations
Further Information

Publication History

Publication Date:
12 April 2019 (eFirst)

Dear Editor,

Thank you for the title page with comments by Merz and Pashaj [1] showing an ultrasound image of a true umbilical cord knot in a single umbilical artery cord. We would like to add some comments and new findings from our nationwide population-based study on the umbilical cord including cord knots [2] and have utilized our dataset to explore the occurrence and risk in cases with the combination of a single umbilical artery and a knot not previously published. We found that in our population of more than 900 000 singleton births, the occurrence of a combination of a cord knot and a single umbilical artery is a very rare condition (0.014 %). There was an increased risk of perinatal death in these cases, but the confidence intervals were wide, reflecting the rarity of the condition. For the total group of single umbilical artery with knot, the odds ratio for perinatal death was 5.26 (95 % CI 2.15 – 12.85). Since the condition shown on the title page (Ultraschall in Med 2018; 39(02)) is very rare, we are curious to learn how this pregnancy was handled and if the outcome was good.

In line with the reference in the comment to the title page, we found the occurrence of cord knots to be 1.32 % in Norway. Our study confirms that boys are at increased risk for cord knots compared to girls and that a long cord is a major risk factor. In fact, boys have longer umbilical cords than girls, and in our study this difference was evident from 28 weeks. However, as mentioned in the comment to the title page by Merz and Pahaj, knots are probably formed at a much earlier gestational age, as the incidence of cord knots does not vary with gestational age at birth [2]. A long cord and male sex are also associated with an increased risk of entanglement, and we find that the combination of knot and entanglement, (with a total occurrence of 0.43 %) has a more than additive effect on perinatal death and increases the risk to an odds ratio of 9.8 at term. Entanglement alone exhibited an increased risk of intrauterine death of 94 % and an increased risk of perinatal death of 70 %, but only in term births. We found that a cord knot alone increased the risk of perinatal death more than four times at term.

The mechanisms that regulate cord length are not understood, but the fact that boys have longer cords, more knots and entanglements than girls lends support to the “stretch hypothesis” suggesting that stretch and physical activity increases cord length [3]. Could this be an indication of the existence of early differences between the sexes in the CNS regulation of physical activity and the development of the musculoskeletal system?

Our study finds a more than doubled risk of a cord knot in a subsequent pregnancy of the same mother, regardless of the fetal sex in the second pregnancy, which suggests that genetic and/or persisting environmental factors also influence cord length and the formation of knots. Whether or not this finding justifies targeted examination in the following pregnancy needs to be answered by studies, taking both maternal (parental) and societal aspects into account.

Findings of cord knot and entanglement may induce anxiety in the mother, her family and the midwife or physician involved [4]. It is of paramount importance that the finding of a cord knot (with or without cord entanglement) does not lead to iatrogenic harm due to preterm and/or caesarean delivery without a sound indication, for example. As in medicine in general, informing the patient about risk and risk assessment, diagnostic uncertainty and lack of solid evidence demands communication skills, and the physician should not impose her or his own anxiety on the mother. This should, however, not keep us from developing our insight and diagnostic abilities, since umbilical cord accidents are responsible for a significant percentage of fetal deaths and adverse outcomes [5].

The diagnosis of a cord knot by ultrasound has proven to be hampered by difficulties, while entanglements are easy to diagnose by ultrasound. Thus, the choice of title page in your journal is highly valued as it raises awareness of and attention to the umbilical cord. The difficulties of examining the umbilical cord for length and knots aside, “There is nothing like looking, if you want to find something” (J.R.R Tolkien, The Hobbit).